Optimization of the coupled grid connection of offshore wind farms

Transcription

1 Optimization of the coupled grid connection of offshore wind farms Dirk Schoenmakers Graduation project at Evelop Netherlands BV Technical University of Eindhoven September 2008 Supervisors: TU Eindhoven / TU Delft Evelop Prof. Dr. Gerard van Bussel Drs. Ruud van Leeuwen, M.Sc. Dr. Ernst van Zuylen

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3 Preface This thesis report is the result of a Bachelor study in Electrical engineering and a master study in Sustainable Energy Technology with the specialization in wind energy. About a year ago I started working at Evelop at the department of offshore wind energy. During this year I first did a three month internship in which I studied the possibilities for the grid connection of offshore wind farms and got insight in the processes and challenges of developing an offshore wind farm. Due to my background in Electrical engineering and the ambition to work in the field of wind energy, this thesis study covers both fields well and was a logical continuation of the work done during my internship. During the nine months of thesis work I learned very much about all aspects of the development of an offshore wind farm. I d like to thank all people within Evelop and Ecofys who supported me with my thesis work and gave very useful feedback which led to the results of my thesis project as you can read in this report. At my first working day at Evelop the news came in that a colleague, Jord Engel, had become very ill. Several weeks later the diagnosis of a brain tumor was set. Due to his passion for wind (energy) he wanted to remain involved with the company and ongoing projects as much as possible despite of the hard and tough medical treatment of his tumor. As part of his involvement he wanted to guide me during my thesis work. Unfortunately his condition got worse and he passed away at 17 April I want to dedicate this thesis project to him and his passion for wind Dirk Schoenmakers Raamsdonksveer, September 2008 iii

4 Abstract In this study a model has been made for the optimization of the grid connection of offshore wind farms. For the model the currently available technologies (i.e. 132 kv and 150 kv HVAC and ±150 kv HVDC VSC) are used as well as technologies which will become commercially available soon (i.e. 220 kv HVAC and ±300 kv HVDC VSC). With the model the feasibility of a coupled grid connection is studied. An overview of the components in a coupled grid connection is given in the figure below. Overview of components in grid connection system of multiple offshore wind farms. The model is divided into two parts: the calculation of the losses and the development of a cost model for all components of the grid connection for all technologies. The calculation of the losses for the cables and the other components in the HVAC systems is based on standard calculations methods. For the calculation of the losses in the HVDC VSC converter station the model is based on published data of two commercial HVDC VSC projects and data published by ABB. For the cost model data found in literature and data known within Evelop has been used. Based on the data found cost models have been made for all components in the grid connection system for all technologies. For some components of the technologies which are currently not commercially available (i.e. the 220 kv HVAC and ±300 kv HVDC VSC systems), not much or no data at all could be found. The cost model of these components was based on assumptions and with reference to the same components for the other HVAC and HVDC VSC systems. With the model for the losses and the economical model made for all five technologies, the optimization is being done on basis of the investment costs and the losses over 20 years of operation of the offshore wind farms. Availability data and data about failure rates and mean repair times and costs have not been taken into account, because for the 220 kv HVAC and for the offshore HVDC VSC systems too little or no data was available at all. In order not to reduce the iv

5 reliability of the outcome of the model, these variables have not been taken into account. As was mentioned the optimization of the model is based on the investment costs and the costs of losses over the 20 years of operation of an offshore wind farm. In order to model the costs for the losses the net present value (NPV) is used which models the costs of the losses over 20 years to the present value. For the calculation of the NPV two variables are of high importance: the price of electricity for the power loss and the interest rate r which determine together the importance of future losses with respect to the investment costs done before operation of the offshore wind farm. With the model made the optimization process for individual and coupled grid connection is tested with a case scenario of three future offshore wind farms which are located relatively close to each other approximately 30 km off the Dutch coast. These three offshore wind farms are West Rijn (developed by Airtricity), Scheveningen Buiten (developed by Evelop) and Katwijk (developed by WEOM) and they are referred to as the Trident offshore wind farms. For these offshore wind farms different scenarios are studied: the individual grid connection and the coupled grid connection of two or three of the Trident offshore wind farms. When the results of the optimized individual and coupled grid connections are obtained, it turns out that the total costs for the coupled grid connection is in most cases similar to the summation of the total costs of the optimized individual grid connections. In some cases the total costs of the coupled grid connection were 1% to 4% lower as compared to the individual grid connections. Based on this slight advantage combined with the fact that a coupled grid connection of offshore wind farms of multiple developers is very complex and the development of the offshore wind farms become dependent on each other, can be concluded that it is from an offshore wind farm developer s point of view not feasible to start cooperating and develop a coupled grid connection. In order to test the sensitivity of the parameters used in the optimization process, a sensitivity analysis has been performed in order to see if the results change. For the sensitivity analysis five parameters have been studied: the price of electricity, the price of offshore HVDC VSC converters, the interest rate r, the cable price and the length of the offshore grid connection cables. With an increase in the price of electricity and the interest rate r, the importance of the losses is adapted. When the importance of the losses is being increased, it turns out that the outcome of the optimization process hardly changes. The main reason for this is that the total losses are higher for the coupled grid connection as compared to the individual grid connections due to the added losses in the interconnection cables required for the coupled grid connection. The sensitivity analysis of the price of an offshore HVDC VSC converter was to study the impact of a decrease of the price for the difference between HVAC and HVDC VSC systems. This combined with the sensitivity analysis of the length of the offshore v

6 cable showed that the difference in total costs between the HVAC and HVDC VSC systems is so high, that the HVDC VSC systems are not suited for the coupled grid connection of offshore wind farms up to a total grid connection distance of approximately 90 km. Also from the sensitivity analysis of the cable price it was concluded that with an increase in the cable prices (e.g. because of the increase in the metal prices) this is more beneficial for the individual grid connection as compared to the coupled grid connections. This is mainly due to the fact that for the coupled grid connection the most feasible system in all cases was the 220 kv HVAC system of which the cable price is already higher as compared to the other HVAC systems which are the most feasible for the individual grid connections. This higher cable price per km for the 220 kv HVAC system also results in large break even distances from shore at which point the coupled grid connections become more feasible as the individual grid connections. This was studied by increasing the offshore cable length of the grid connection cables. It turned out that for the economical parameters as given in the economical model, the break even point for the coupled and individual grid connections was around an offshore cable length of 85 to 100 km. For this analysis the onshore cable lengths of the individual and coupled grid connections were used. With the optimization model for the grid connection made and the sensitivity analysis done, one can conclude that for an offshore wind farm developer s point of view it is economically not feasible to arrange a coupled grid connection in case the interconnection distances between the offshore wind farms are significant compared to the grid connection distance (e.g. around 30% of the total main grid connection distance), as it is the case for most planned offshore wind farms in the Netherlands As was already indicated a coupled grid connection also increases the complexity of the grid connections and it make the offshore wind farms dependent on each other. It is therefore recommended for governments, in this case the Dutch government, to assign an offshore TSO which becomes responsible for the grid connection of offshore wind farms in order to get a structured development of offshore wind energy towards the targets set for offshore wind power, as for instance has been done in Germany. vi

7 Contents Preface... iii Abstract... iv 1 Introduction Problem description Research question Research method Project outlining Model description System losses System investment costs Case study: Trident offshore wind farms Case study: scenarios Poseidon vision Offshore wind energy in the Netherlands Dutch policy on offshore wind energy Grid connection arrangement in the Netherlands Financial support of wind energy Dutch transmission system Randstad 380kV project Overview of grid connection technologies Current situation of offshore wind farms Possible technologies Technical overview of HVAC Type of HVAC cables used Grid connection Modularity and redundancy Technical overview of HVDC VSC Principle of HVDC VSC Current status of HVDC VSC technology Type of HVDC VSC cable systems used Grid connection Modularity and redundancy Reliability Summary: comparison of HVAC and HVDC VSC technology Technical model Modeling of power production Infield cable loss Transformer loss model Cable model Electrical model of a power cable Ohmic losses Dielectric losses Induced losses...50 vii

8 Maximum transmission capacity and current distribution of AC cable Thermal model of cable Thermal resistance T Thermal resistance T Thermal resistance T External thermal resistance T External thermal resistance T 4 for AC cables External thermal resistance T 4 for DC cables Soil moisture migration Cable loss calculation Calculation of total thermal resistance of cable Influence of multiple onshore cable systems Reactive power compensation Loss HVDC converter station Loss of ±300 kv HVDC VSC converter Summary of system loss calculation Economical model Modeling of losses Subsidy system for renewable energy in the UK Price of losses Modeling investment costs Cable cost components Cost components submarine cables Costs components land cables Cost model cables Conclusions cable cost model Cost model cable installation Installation costs three phase submarine HVAC cables Installation costs single phase submarine HVDC cables Installation costs single phase land cables Cost model onshore and offshore substations Costs onshore HVAC substation Costs offshore HVAC substation Costs onshore HVDC converter station Costs offshore HVDC converter station Cost model reactive compensation for HVAC systems Verification cost model HVDC VSC system Verification cost model HVAC system Conclusion cost model for HVAC and HVDC VSC systems Financial optimization of grid connection Structure of developing an offshore wind farm Summary of economical model Application of model Optimization process Optimization of cable system viii

9 7.1.2 Development of offshore wind farms over the years Cable length and operating conditions Metal prices and currency exchange rate Scenario 1: Individual grid connections Individual grid connection Scheveningen Buiten Individual grid connection West Rijn Individual grid connection Katwijk Summary of individual grid connections Scenario 2: Coupled grid connections Scenario 2a: West Rijn and Scheveningen Buiten Scenario 2b: West Rijn and Katwijk Scenario 2c: Scheveningen Buiten and Katwijk Scenario 2d: West Rijn, Scheveningen Buiten and Katwijk Summary of coupled grid connections Sensitivity analysis of cost components Sensitivity analysis: cable price Sensitivity analysis: price offshore converter Sensitivity analysis: price of electricity Sensitivity analysis: interest rate r Sensitivity analysis: grid connection length Conclusions sensitivity analysis Results and conclusions Recommendations Appendices Appendix A: Trident offshore wind farms Appendix B: Cable construction and cable systems B.1 Submarine and land cable design B.2 HVAC land cable systems B.3 HVDC Cable systems Appendix C: Cable characteristics C1: Submarine three phase HVAC cables C2: Onshore single phase HVAC cables C3: Submarine single phase HVDC cables C4: Onshore single phase HVDC cables C5: Correction factors C5.1 Correction factors: submarine HVAC cables C5.3 Correction factors: submarine HVDC cables C5.4 Correction factors: HVDC land cables Appendix D: Power curves Appendix E: Cost figures of HVAC and HVDC cables E.1 Submarine AC cables E.2 Submarine DC cables E.3 AC and DC land cables References Glossary of Terms ix

10 1 Introduction The offshore wind farm industry has been and still is developing rapidly. In the beginning the offshore wind farm size was relatively small (e.g. less than 20 MW) and the distance to shore was up to several kilometers. Currently the development of offshore wind farms goes to wind farms of several hundred MW and distances to shore of 50 km or even more than 100 km. With this increase in wind farm capacity and distance to shore it might become economically attractive for wind farm developers to cooperate and create a coupled grid connection with the onshore grid of several offshore wind farms. 1.1 Problem description With the increase in rated capacity of the offshore wind farms currently under development, the cost price per generated kwh is being decreased. Due to several reasons this growth in rated capacity implicates that the offshore wind farms need to be developed further offshore which increases the costs for the grid connection: due to legislation it is in many countries not allowed to built offshore wind farms within 12 nautical miles from the shore due to other offshore activities development of offshore wind farms is not allowed at each location, so most locations are relatively far from offshore due to the costly foundations and the goal to keep the price of generated electricity as low as possible, the water depths need to be relatively low (e.g. preferably up to 30 m) With both the increase in distance to shore and the increase in rated capacity, the question rises which technology (e.g. HVAC or HVDC VSC) and transmission voltage is the most suited for the grid connection of offshore wind farms in order to minimize the costs. Also with the larger distance to shore, the relative distance between many offshore wind farms under development decreases. Therefore the question rises whether it would be economically feasible to have a coupled grid connection of several offshore wind farms. In some countries in Western Europe (e.g. Denmark and Germany) the grid connection has to be paid by the transmission system operator (TSO) which owns the offshore grid up to the point of the offshore substation in the offshore wind farm. In that case the investment is earned back by means of a transport tariff for the power transmission over the cable or the investment is spread out over the kwh price for the consumer. In both situations the observation of the different scenarios is not of importance for the offshore wind farm developer, because the coupled grid connection will be developed and paid by the TSO. In many countries though, the grid connection costs have to be paid by the offshore wind farm developer. This is also the case for offshore wind farm developers in 10

11 the Netherlands. The grid connection can take up to 10% to 20% of the total investment costs of an offshore wind farm, depending on the distance to shore. Several studies have been performed to analyze the development of offshore wind in the Netherlands. The focus of one of these studies, the Connect II study [14] was mainly to study what topology of the grid connection of offshore wind farms should be preferred. From this study the conclusion was drawn that for the nearby future it would be most optimal for offshore wind farms to have an individual grid connection (i.e. each offshore wind farm has its own cable connection with the onshore grid). Later on a coupled grid connection of several offshore wind farms would be preferable to minimize the number of grid connection cables and accordingly the number of cable landings through the dunes which would minimize the environmental impact. A coupled grid connection might be economically feasible, but a coupled grid connection of several offshore wind farms of different developing companies increases the complexity of realizing a grid connection. When there is an economical advantage and this is known in an early stage in the development though, this might be a serious option for the wind farm developer to take into consideration. 1.2 Research question As was outlined in the paragraph above, a coupled grid connection can have several benefits (i.e. minimizing the environmental impact, possibly more economically than individual grid connections, synergy with grid integration/support, etc.). In order to be able to get insight when a coupled grid connection might be a feasible or attractive solution, a model needs to be developed which is suited for investigating al possible situations of offshore wind farms and which compares all suited technologies available. Creating such a model is the goal of this study. Therefore the main research question of this thesis is as follows: How can the grid connection of multiple offshore wind farms of different power ratings and distances to shore be optimized on basis of economical and technical criteria? As can be seen in the research question above, the optimization of a coupled grid connection is done by looking at technical and economical criteria. The technology used must be able to transmit the total power of all the offshore wind farms to shore and with as low losses as possible, because all losses over the entire lifetime of an offshore wind farm are a reduction of income. The economical criteria is to keep the total investment costs for the coupled grid connection as low as possible. These technical and economical criteria together determine what the optimized coupled grid connection is. Besides the investment costs and losses, also availability due to failure rates and repair time (e.g. down time creates a loss of income) are important to get a more detailed insight in the 11

12 optimized grid connection. Because these data are not fully known for both technologies used in this optimization study (e.g. the HVDC VSC technology has only been used since the last decade and has up till now never been used in the offshore wind industry and can therefore not be compared to the HVAC system on availability), these are out of the scope of this research project. The goal of this research project, as mentioned before, is to develop a grid connection optimization model based on a technical model (e.g. to calculate the losses of a transmission system) and an economical model (e.g. to calculate what the investment costs of a certain transmission technology are) to determine how the optimized coupled grid connection should look like. With this tool the optimization can be done for all offshore wind farms of different power ratings and distances to shore. In order to test the model it will be applied to a real situation with three offshore wind farms which are under development off the Dutch coast indicated as the Trident offshore wind farms. 1.3 Research method With the goal of this study in mind to optimize the coupled grid connection of offshore wind farms, the research is divided into four parts. The first part of the research consists of obtaining the basic information, determining the scope of this research project and developing different scenarios of the development of multiple offshore wind farms. This includes a description of all the components which are taken into account for the grid connection, the calculation of the power production of an offshore wind farm, a description of the three Trident offshore wind farms, the possible scenarios of their development and finally the possibilities for connecting offshore wind farms to the onshore grid in the Netherlands (Chapters 2 and 3). Also a short overview of the current situation for the grid connection of offshore wind farms and the characteristics of the HVAC and HVDC VSC technology are given (Chapter 4). The second phase consists of the development of a technical and economical model. The technical model includes the power production and losses of all the components in the grid connection system for both the HVAC and HVDC VSC technology (Chapter 5). The economical model includes a model for determining the total investment costs of all the components in the system and a model for determining the economically optimized grid connection based on the investment costs and costs of losses (Chapter 6). The third phase is to combine the technical and economical model into a single grid connection optimization model. This model is then applied on a real case scenario with the three Trident offshore wind farms for all possible development scenarios. In order to test the outcome of the optimization a sensitivity analysis is performed on several parameters to see how sensitive the model is to changes of parameters (Chapter 7). 12

13 The fourth and last phase of this research project is to come to conclusions of this research based on the case study with the Trident offshore wind farms and to give recommendations for future research (Chapters 8 and 9). 13

14 2 Project outlining In this chapter the project outlining is given which starts with a description of what components are taken into account in the model and what are the variables of the model. Also a short description is given of the three Trident offshore wind farms, a description of the scenarios which are taken into account and a short overview of the Poseidon vision (i.e. a vision of Econcern 1 about the sea/ocean as energy source) of which this study is part of. 2.1 Model description In the figure below an overview of all the components in the model is given. The power produced in each wind farm is collected with infield cables which are connected to the offshore substation in the wind farm. Several offshore wind farms are then interconnected to a main offshore platform and from this main offshore platform all the power of all the offshore wind farms is transmitted via a grid connection system consisting of submarine and land cables. The land cables are connected to an onshore station which is finally connected to the onshore grid. The power collection inside the wind farm and the power transmission to the main offshore station is in this model always an AC system. A DC transmission system is only suited for longer transmission distances, which will be shown later on. Therefore the grid connection system, consisting of the main offshore station, the submarine cables, the land cables and the onshore station which is connected to the onshore grid, can either be a HVAC or a HVDC system. For an HVAC grid connection system also reactive compensation has to be applied, which will be explained later on. Figure 2.1: Overview of all the components in the model. For the optimization process of the coupled grid connection system two parts are being optimized which together determine the final most optimized grid connection system: 1 Econcern is a holding of 5 sustainable energy companies of which Evelop is the project developing company. 14

15 Losses in the system Investment costs of the electrical system Both parts are shown in more detail in the following paragraphs System losses An overview of the losses in the system is given in the figure below. Figure 2.2: Overview of losses in the grid connection system. Of all the wind farms in the model the power production is calculated, based on the type of turbine used, the number of turbines and the wind characteristics at the site. For calculating the power production of a wind farm the availability of the turbines need to be taken into account (i.e. not all turbines are always available for power production due to component failures) and also the loss in power production due to wake operation of turbines. From the infield cables to collect the power produced up to the point where the power is fed into the onshore grid, there are electrical losses: Infield cable loss Offshore substation loss Wind farm interconnection cable loss Main offshore substation or converter loss (including reactive compensation loss in case of an HVAC grid connection system) Submarine cable loss Land cable loss Onshore substation or converter loss (including reactive compensation loss in case of an HVAC grid connection system) All the losses as mentioned above are a loss of income and for wind farms with an operational lifetime of 20 years this is significant. In chapter 5 the power production and losses in all components of the system are given in more detail System investment costs For the investment costs only the power transmission system is taken into account starting at the offshore substations inside the offshore wind farms towards the connection with the onshore grid. The infield cables are not taken into account due to several reasons. The optimization process for the infield cables is mostly not directly driven by means of minimizing losses or investment 15

16 costs. For minimizing the investment costs for the infield cables, one should use an increasing conductor cross section from each end of a string interconnecting turbines towards the offshore substation inside the wind farm. This is in practice never done due to the fact that this makes the installation more difficult (i.e. differences in cable diameter, each cable section is specially made for a certain section so installation of each cable section is very specific, etc.). For minimizing the losses a large conductor cross section should be chosen for each interconnection in a string which is not economically due to the fact that for the power transmission near the end of the string the cables would be over rated. Therefore in practice one chooses either to have 2 or 3 different conductor cross section for each string, increasing from the end of the string towards the offshore substation, or one chooses to have 2 cables with an equal conductor cross section but one with a copper conductor and the other with an aluminum conductor. This has the advantages that the installation vessels and burial machinery can handle equally sized cables. Due to this daily practice the optimization of the infield cables is left out of the scope of this study. The investment costs calculated consist of the following components: Offshore substation inside each wind farm Interconnection cables between wind farms Main offshore substation or converter station (including costs for the offshore reactive power compensation in case of a HVAC system) Submarine grid connection cables Land cables Onshore substation or converter station (including costs for the onshore reactive power compensation in case of a HVAC system) Besides the investment costs for the total transmission system also the availability of all components due to failure rates and accordingly the repair time of the components could be taken into account. In this study these two variables are not taken into account. The reason for this is that for the power transmission system the HVAC and HVDC VSC system are compared for all the possible voltage levels of these systems. For the HVAC system 132 kv and 150 kv are now commonly used, but the 220 kv HVAC system has not yet been applied in large commercial offshore projects. The HVDC VSC technology exists only for about a decade and only a few commercial ±150 kv projects and no commercial ±300 kv projects have been developed. Therefore not enough data is known about the reliability of all components in these systems in order to compare them and not reducing the accuracy of the model. In the economical model therefore only the investment costs and costs due to losses (i.e. loss of income) of all these systems are taken into account. Due to the fact that the investment costs are paid before the wind farms become operational and that the losses occur during the total 20 year lifetime of the wind farms, the value of costs over time needs to be taken into account. In chapter 6 16

17 the economical model will be given in more detail which contains the investment costs of all components and the economical optimization process. 2.2 Case study: Trident offshore wind farms For testing the coupled grid connection optimization model, the model will be applied on a case study with three offshore wind farms which are planned relatively close to each other approximately 30 km off the Dutch coast. These three offshore wind farms are further mentioned as the Trident offshore wind farms. The three offshore wind farms are the following: Table 2.1: 'Trident' offshore wind farms. Offshore wind farm Developer Scheveningen Buiten Evelop Katwijk WEOM West Rijn Airtricity Detailed information about the wind farms is given in appendix A. With the characteristics of all three Trident offshore wind farms known, the coordinates of the wind farms, the OHVS and coastal landing points can be plotted to get an impression of the individual designs and the relative distances between the offshore wind farms as can be seen in the figure 2.3 on the next page. Though none of the three offshore wind farms has a building permit yet (i.e. offshore wind farm Katwijk of WEOM even has been rejected and will thus not be further developed [8]) and it is uncertain whether they will be fully developed in the near future, they are used for the case study to study the possibility of a coupled grid connection. In figure 2.3 the overview is given of the three offshore wind farms, their offshore high voltage stations (OHVS) and individual grid connection landing points. The individual grid connection points are chosen based on the location with respect to the wind farm, the possibility to feed in the maximal wind farm power into the onshore grid and the possibility for laying the cable offshore and onshore. The reasoning for choosing a certain grid connection point is made by the individual wind farm developers based on the possibility to feed in their wind farm power at the selected location and their expected possibilities for getting a permit for the cable route offshore and onshore. The grid connection points used are stated in the Environmental Impact Assessment of their wind farms. These chosen landing points will thus be used as a reference for the individual grid connection options. As can be seen in figure 2.3 the Trident wind farms are planned relatively close to each other, which indicates that a coupled grid connection might be economically feasible. More details about the Dutch grid and possibilities for feeding in the coupled power of the Trident wind farms will be given in Chapter 3. 17

18 Figure 2.3: Overview of location of 'Trident' offshore wind farms and their individual grid connection points onshore. 2.3 Case study: scenarios As was mentioned before none of the Trident wind farms has a building permit and it is still uncertain which of these wind farms will be fully developed in the future. This uncertainty about the development of the offshore wind farms makes it important to study the different possibilities in order to see if a coupled grid connection is suitable and economically a feasible option. In order to see what the influence of development will be with respect to the combined grid connection of the offshore wind farms, different scenarios can be observed which need to be reflected to the case business as usual in which all offshore wind farms have their own optimized grid connection. The following scenarios will be observed in this study: 1. Base case: Business as usual all three offshore wind farms will develop with an own optimized grid connection (this scenario is the one reflected to in the other scenarios) 18

19 2. Market development in this case the government takes no lead in developing offshore wind farms but leaves the development to the market. In this case two or three of the Trident offshore wind farms will actually be built in the future (if only 1 wind farm would be developed it would be scenario 1). Therefore four sub scenarios can be observed: o 2a. WR&SB: development of West Rijn and Scheveningen Buiten offshore wind farms o 2b. WR&K: development of West Rijn and Katwijk offshore wind farms o 2c. SB&K: development of Scheveningen Buiten and Katwijk offshore wind farms. o 2d. WR&SB&K: all three offshore wind farms West Rijn, Scheveningen Buiten en Katwijk will be developed. 3. Governmental coordinated development the government coordinates the development of offshore wind farms (can be seen as a tendering of (a) certain location(s) where offshore wind farms can be developed). In this case the government decides where the offshore wind farms will be developed and in this case they are located relatively close to each other. The government will therefore stimulate the combined grid connection of the three offshore wind farms. For the financial impact of the development of a coupled grid connection it is of high importance to know the status of the different offshore wind farms. If one offshore wind farm turns out not to be built, this can have a large impact on the financial situation of another offshore wind farms which arranged the grid connection which could be used by multiple offshore wind farms. Therefore it is important to observe scenarios 2a, 2b, 2c and 2d in the case when the offshore wind farms will be developed (at the same time or one after another when they are at different stages of development and not all completely certain if they will actually be built). For scenario 1 it doesn t matter if the other offshore wind farms will be developed and for scenario 3 the government assures that all three offshore wind farms will be built (which is currently not the case in reality for the Netherlands because only 2 offshore wind farms will be built in the coming round in the Netherlands and offshore wind farm Katwijk of WEOM has been rejected [2], [8]). In this study scenarios 2a - 2d (i.e. the coupled grid connection scenarios) will be compared to scenario 1 in which the wind farms will have an individual grid connection. In these scenarios also the years of development of the wind farms will be varied to see the impact if this variation. 19

20 2.4 Poseidon vision The POSEIDON vision is the integral vision of Econcern 2. The POSEIDON concept is an integral vision on the world s seas as a key factor to the development of a sustainable energy supply. The key element of the POSEIDON concept is the infrastructure for transport of electricity. The concept consists of several sustainable technologies at sea which will generate the energy required onshore [11]: o clean usage of fossil fuels (100% CO 2 capture and storage) o offshore wind energy o wave energy o tidal energy o osmotic power (based on pressure difference between salt and fresh water) o bio energy (biomass of for example algae or seaweed) o Ocean Thermal Energy Conversion (OTEC, makes use of temperature difference of surface water and water at deep depths). Due to the variability of most of the sustainable sources as mentioned above, well designed energy storage systems are required for a sustainable supply of energy. Figure 2.4: Overview of available sustainable technologies available at the world's seas indicated in the POSEIDON concept [10]. All the possible sustainable technologies as indicated above show the enormous energy potential of the world s seas. With a well designed offshore infrastructure the energy can be transported to shore where the demand is. Of the indicated 2 Econcern is a holding of 5 sustainable energy companies on project development, development of innovative products and services for a sustainable energy supply. Evelop is the project developing company on the field of wind energy, biomass and solar energy. 20

21 sustainable technologies, nowadays offshore wind energy is the most developed. With an increase in the use of offshore wind energy, the electrical infrastructure becomes of more and more importance. In order to come to a reliable electricity supply, interconnection of generation sites is important. In the figure below the POSEIDON approach is indicated for the shared use of the offshore electricity grid for offshore wind energy. Figure 2.5: Example of the creation of a POSEIDON segment over time. Left: conventional approach with no system optimization. Right: POSEIDON approach with system optimization [10]. As can be seen in the figure above, the idea of the POSEIDON approach for offshore wind farms is first to share part of the offshore grid (i.e. connection multiple offshore wind farms together to the onshore grid) and later on extend 21

22 these offshore local grids to one large connected offshore grid. In this way the reliability of power supply can be enlarged and the offshore grid might be used to enforce the onshore grid. The idea for the POSEIDON approach for the grid connection of offshore wind farms can be taken into account in this research. The connection of multiple offshore wind farms with a single grid connection is the first step. If one wants to come to a fully connected offshore grid, this should be taken into account when designing the grid connection of multiple offshore wind farms with respect to the dimensions of the connection and the technology used. Because this process takes many years or even decades and is more a development at international and/or governmental level, the optimization of the coupled grid connection of offshore wind farms is done from the offshore wind farm developer s perspective. This optimization process is done for offshore wind farm developers which need to pay for their grid connection themselves and therefore the grid connection will be rated only for the Trident offshore wind farms under consideration. A quote of the POSEIDON vision about the development of offshore wind energy [12]: Increased penetration of wind is in itself a driver for more interconnection, since interconnectors allow the diversity in wind between different areas to offset each other. 22

23 3 Offshore wind energy in the Netherlands In this chapter more insight is given in the offshore wind climate in the Netherlands. First the renewable targets and policy is given, then the grid arrangement and the subsidy system in the Netherlands and finally the current layout of the Dutch electricity transmission system and the planned reinforcements of the onshore grid. 3.1 Dutch policy on offshore wind energy Due to the location near the North Sea the Netherlands has an excellent wind climate. Therefore a very large part of the Dutch electricity demand can be covered by means of wind power. The Dutch government has clear targets for the share of renewables in the production of electricity. The target for 2020 is set at 20%. Wind energy will have the major role in achieving these targets [3]. The available space on land for exploiting wind energy in the Netherlands is limited. Therefore the expansion of wind energy will mainly be offshore. In order to reach the target of 20% renewable electricity generation in 2020 a total of 6000 MW of wind capacity is planned to be installed offshore. This will take approximately 1.5% of the Dutch part of the North Sea to give an indication of the enormous potential of wind energy [3]. Currently the total installed capacity of offshore wind power in the Netherlands is 228 MW in two offshore wind farms [4]. In the table below the characteristics of these two offshore wind farms are given. Table 3.1: Specifications of operational offshore wind farms in the Netherlands [4]. Name Online Turbines and capacity Turbine Distance Offshore Grid Connection to shore Substation OWEZ Nov * 3 MW = 108 MW Vestas V km 3 x single phase AC 34 kv No Princess Amalia Wind Farm (former Q7) May * 2 MW = 120 MW Vestas V80 23 km 1 x three phase AC 150 kv Yes Total = 228 MW According to a recently published report of the Transition Group Offshore Wind in the Netherlands [5], the target of 6000 MW installed offshore wind capacity can be achieved if a stable regime is created for the development of offshore wind in the Netherlands. Financial support by the government will be crucial for this. With increasing knowledge over time and the increase in the price of electricity this dependency will reduce significantly over the years. By the year 2020 it must be possible to develop offshore wind farms on a commercial basis without any subsidy required. They state that in order to reach the offshore wind target of 6000 MW it is essential to upgrade the onshore transmission grid to enable the connection of all the offshore wind farms to the onshore grid and have enough capacity to transport the offshore wind power. Also the principle of a socket at sea where offshore wind farms can feed in their power, is discussed which should be arranged by TenneT, the Dutch transmission system operator (TSO). 23

24 More insight in the Dutch electricity grid and planned reinforcements are given in paragraph Grid connection arrangement in the Netherlands In the Netherlands all costs for the grid connection up to the point of coupling to the onshore grid are for the offshore wind farm developer. Any additional costs for reinforcements of the grid due to the connection of wind energy to the grid will be paid by the Dutch TSO. Because the grid connection costs can take up to 10% to 20% of the initial investment costs, offshore wind farm development in the Netherlands is quite costly without a good financial support mechanism. In several studies performed about the target of having 6000 MW installed offshore wind power in the Netherlands, the need for an offshore regulator and grid owner (preferably the onshore TSO) is indicated [3],[5]. With the high target of total capacity of offshore wind, the geographical location of the planned offshore wind farms, the limited possibilities for grid connection (i.e. limited number of cables and number of connection points possible) and the costs for grid connection which are now paid by the developer, they indicate that an offshore TSO is required for reaching the target of 6000 MW offshore wind power. The offshore TSO should then have to pay for the (grouped) grid connection of multiple offshore wind farms or arrange an offshore grid with connection points where the offshore wind power can be fed into. Recently the Dutch government intended to do research to this request to study if this is indeed required. 3.3 Financial support of wind energy In the Netherlands until recently there was a fixed premium subsidy. This subsidy had to cover the unprofitable top (i.e. the difference in production price and the price of electricity on the market) calculated by Dutch research institutes. Since the start of the subsidy scheme in 2003 the height of the subsidy would be evaluated every year. In November 2004 the subsidy for offshore wind energy was set to per kwh for an operational period of 10 years. Due to the many initiatives for renewable projects and too little budget the subsidy was set to zero in May 2005 for a period of six months and this zero subsidy level has been repeated since. In November 2006 when it became clear that the government would reach the target of 9% renewable electricity in 2010 without new renewable projects, the subsidy scheme was set to zero indefinitely. Therefore no new large scale renewable projects will be supported by this subsidy scheme. In April 2008 a new subsidy scheme has become effective, the SDE (i.e. Stimulering Duurzame Energie Stimulation of Renewable Energy). This subsidy scheme does not yet include a subsidy system for offshore wind though. This is due to the fact that the Dutch government is currently developing a new development mechanism for offshore wind in which the Dutch government allocates suited area for the deployment of offshore wind energy. Offshore wind 24

25 farm developers can take part in a tender for these areas and the winner(s) get a building permit and a subsidy contract [6]. The new subsidy system also works as a compensation for the unprofitable top. The subsidy will consist of a base price which is valid for the entire period in which the subsidy will be given, based on the total costs of the installation per kwh. Due to the income of the electricity sold by the producer of renewable electricity, a varying amount of money will be subtracted from the base price based on the mean electricity price. Due to this variation the subsidy will change every year. When the electricity price decreases below the level used for the base price, the subsidy for that year will be increased [7]. The base price for the subsidy of offshore wind energy still needs to be determined by the Dutch government Dutch transmission system As was indicated previously the development of the Dutch transmission grid is of high importance for the development of offshore wind in the Netherlands. In order to get insight in what reinforcements are required, the planned offshore wind farms in the Netherlands and the Dutch transmission system as it is now are shown in figure 3.1 and figure 3.2. In figure 3.1 below all the possible locations for offshore wind farms in the Netherlands are shown. The total capacity of all these initiatives are more than the target of 6000 MW of offshore wind energy, but it gives an indication where the offshore wind farms in the Netherlands will be built to realize this target. Figure 3.1: Overview of possible locations for offshore wind farms in the Netherlands [9]. 3 Up till August 2008 no new subsidy level for offshore wind was determined for the SDE subsidy system in the Netherlands. 25

26 Figure 3.2: Overview of Dutch transmission grid per July 2007 [9]. In figure 3.2 above an overview is given of the Dutch transmission grid. As one can see together with figure 3.1 all possible offshore wind farm locations are such that for the grid connection mainly the 380 kv substations Maasvlakte and Beverwijk are suited. As can be read in appendix A the Trident offshore wind farms are planned to be connected to 150 kv substations onshore. The power 26

27 transmission capacity of these stations is just enough to handle the power of one offshore wind farm of the size of the Trident offshore wind farms. The choice for the grid connection landing point depends on the location of the offshore wind farm with respect to the onshore grid and the possibility to connection to a substation which has enough transmission capacity to safely transport the offshore wind power into the onshore transmission grid. When a large capacity of offshore wind power is fed into the onshore grid it has to be connection to the 380 kv grid. As was mentioned before the main challenge for the development of 6000 MW offshore wind power in the Netherlands is the integration into the onshore transmission system. Several studies have been done to extensively analyze the possibility of the integration of 6000 MW of offshore wind power into the Dutch transmission grid [13], [14], [15], [16]. As was concluded in these studies and can be seen in figure 3.2, the onshore 380 kv substations Maasvlakte and Beverwijk are the most suited for the large scale integration of offshore wind power in the Netherlands. For the connection of a large amount of offshore wind power to these substations, the infrastructure at these substations must be extended to make this possible. Also a suited and feasible cable route both offshore and onshore is required. Especially for the connection to substation Maasvlakte the cable routes offshore and onshore are complex. In the figure below 3 possible cable routes are given as were given by the Connect 6000 MW study of Royal Haskoning [17]. Figure 3.3: Overview of possible cable routes for landing points at substation 'Maasvlakte' [15]. 27

28 The landing at substation Maasvlakte is complex. This is due the fact that an actively used shipping area of the North Sea has to be crossed. Also a second industrial area in the North Sea next to the existing Maasvlakte is planned. Option 3 has the main difficulty for the onshore cable trajectory which is an extensively used area. Option 2 has the major technical difficulty that the deep shipping lanes towards the harbor of Rotterdam need to be crossed by means of a directional drilling which gives very high risks for investment and insurance. Option 1 has the main difficulty that the offshore cable route has to cross a nature reserve which is a protected area and thus causes difficulties for determining a cable route which has the least environmental impact and difficulties with getting a permit. The landing at IJmuiden for the connection with substation Beverwijk gives fewer difficulties. For the connection with substation Maasvlakte either option 1 or option 3 is the most feasible. For this study landing option 3 at substation Maasvlakte will be taken into account, though it is at this point in time uncertain if this is the most feasible solution for the grid connection of the Trident offshore wind farms to the Maasvlakte substation. With the upgrading of substation Westerlee near substation Maasvlakte for the Randstad 380kV project as will be given in the next paragraph, it might be possible to connect to that substation once it is upgraded. Up to this point this is still uncertain and therefore the connection will be assumed to substation Maasvlakte. Also a landing at substation Beverwijk will be taken into account. From the studies as mentioned before, the conclusion could be drawn that a total of 3000 MW of offshore wind power can be integrated into the onshore transmission grid without any problems when planned grid reinforcements are taken into account. The most important grid reinforcements are a 700 MW HVDC connection with Norway (i.e. the NorNed cable connection at substation Eemshaven in the north of the Netherlands which became operational in May 2008), a 1320 MW HVDC connection with the UK (i.e. the BritNed cable connection at substation Maasvlakte ) and the extension of the 380 kv transmission grid with project Randstad 380kV. For the integration of 6000 MW offshore wind power extra grid reinforcements further inland are required Randstad 380kV project The Randstad 380kV project is very important for the stable and safe operation of the Dutch transmission system now and in the future. Due to the growth in electricity use (i.e. mainly in the western coastal area) and the large scale development of offshore wind energy, the 380 kv transmission grid needs to be reinforced to guarantee the delivery of electricity now and in the future in this coastal region. The project consists of 2 new 380 kv ring connections as is shown in figure 3.4. The Southern ring connects substations Maasvlakte, Westerlee, Wateringen and Bleiswijk. The Northern ring connects substations Bleiswijk, Beverwijk, Oostzaan and Diemen. For the Southern ring connection 2 new 380 kv substations need to be built at substations Wateringen and Westerlee. Also in the Northern ring connection several substations need to be adapted to the 380 kv connection. With these 2 new 380 kv connections the 28

29 supply of electricity in this region will be secured for the coming decennia. The planning is to determine the final trajectory and obtain permits at the end of 2008 for the Southern ring and at April 2009 for the Northern ring and finalize construction at the end of 2010 and beginning of 2011 accordingly [19]. The substations Maasvlakte and Beverwijk will then be suited for the integration of the largest part of the 6000 MW offshore wind power. Also substations Wateringen and Westerlee might become feasible to connect several offshore wind farms. As can be seen in figure 3.4 part of the trajectory of the connection between substations Maasvlakte and Westerlee will be similar to the landing trajectory of option 3 at substation Maasvlakte (i.e. as can be seen in figure 3.3). Therefore option 3 for the connection of offshore wind farms with substation Maasvlakte becomes more feasible if this trajectory for the Randstad 380kV project is made suited for other future connections for the integration of offshore wind. Figure 3.4: Overview of the Randstad 380kV project with 2 new 380 kv ring connections [18]. 29

30 4 Overview of grid connection technologies In this chapter a short overview is given about the possible technologies (i.e. HVAC and HVDC VSC systems) for the grid connection of offshore wind farms which will be compared in this study. First the current status of grid connections of offshore wind farms is given, then a short overview of the possible technologies and finally more detailed information about the current status of these technologies. 4.1 Current situation of offshore wind farms Currently there are 27 offshore wind farms operational in 10 different countries 4 [20]. Of these 27 offshore wind farms, 22 are located close enough to shore or nearby a gas or oil platform (i.e. up to about 10 km) and/or have a total rated power small enough (i.e. below 100 MW) that they have a direct AC connection up to 36 kv to the grid or nearby located gas or oil platform. For these connections no additional offshore transformer station is required. Currently there are 5 large offshore wind farms operational which have an offshore HVAC substation [20]. The characteristics of these offshore wind farms are given in table 4.1 below. Table 4.1: Overview of characteristics of current large offshore wind farms operational with an offshore HVAC substation [20]. Name Location Online Turbines and capacity Distance to shore Horns Rev Denmark Dec * 2 MW = 160 MW 8 km Nysted Denmark Nov * 2.3 MW = MW 10 km Barrow UK May * 3 MW = 90 MW 7 km Lillgrund Sweden Nov * 2.3 MW = MW 10 km Princess Amalia Wind Farm Netherlands May * 2 MW = 120 MW 23 km (former Q7) All 5 offshore wind farms use a single three phase submarine cable at 132 kv or 150 kv for the grid connection. 4.2 Possible technologies As was indicated above all current existing large scale offshore wind farms use the HVAC technology for the grid connection. This technology is the standard in transmission systems on land and has proven itself for a long time during the past decades. The technology is relatively cheap which makes it in many cases the most feasible solution. With the current trends in the design of offshore wind farms (i.e. increase in required transmittable power over a longer distance), the question rises up to which requirements HVAC is still the preferred technology for 4 Up to August

31 offshore grid connection and when the HVDC technology becomes more favorable. Currently there exist 2 HVDC technologies: HVDC LCC ( classic HVDC ) and HVDC VSC (new HVDC technology, commercially named as HVDC Light by ABB or HVDC Plus by Siemens). Traditional HVDC systems (HVDC LCC) are also already used for decades and have their main usage in bulk power transmission over long distances or in the connection of different grids when no synchronous and/or equal grid frequency and voltage is required. Worldwide there are quite some examples of HVDC systems which have proven to be reliable, but none of these examples have an offshore substation which will be required for offshore wind farms. This classic HVDC technology has two main drawbacks: the enormous size of the converter required and the need for an additional power source when an offshore substation is used. These two drawbacks make the system not suited for the offshore wind farm industry. In 1990 an improved design of the HVDC system has been developed: HVDC VSC. This technology uses sophisticated power electronics which reduces the required area for the converters and gives a highly improved controllability of the system. The size of a converter station is much smaller as compared to the classic HVDC system, but still much larger as compared to a HVAC system (e.g. the size of the 400 MW HVDC VSC offshore converter station design for the Nord E.On project is 50 x 30 x 20 m [77] and the size of the offshore HVAC substation of 140 MVA for the Princess Amalia Wind Farm (former Q7 ) is 20 x 11 x 15 m). This technology also doesn t require an additional power source and therefore the characteristics of this technology make it suited for the offshore wind industry. Both HVAC and HVDC VSC have different characteristics. In the following paragraphs both technologies will be observed in more detail Technical overview of HVAC HVAC is the standard for power transmission onshore. It has overall low losses, the voltage can easily be transformed up or down to the required voltage level, the technology is cheap and it has proved itself during the past decades which makes it a reliable technology. These advantages make the technology suited for the grid connection of offshore wind farms. The HVAC technology has also been used to connect several large scale offshore wind farms to the onshore grid and has also shown to be reliable under offshore conditions (i.e. salty and moist conditions). The main drawback of the technology is the reactive power generated by cables which limits the maximal transmission distance (i.e. this will be explained in more detail in Chapter 5). In the following sections some additional characteristics of the HVAC system are given. 31

32 Type of HVAC cables used For the grid connection of offshore wind farms always cables are used. Offshore HVAC cables are mainly three phase cables and onshore mainly single phase cables. The state of the art in cable design is with cross linked polyethylene (XLPE) insulation. This type of cable has good thermal characteristics (i.e. a maximal operational temperature of 90 C), can withstand high voltages, has significantly lower dielectric losses (i.e. losses in the insulation) then other technologies and requires no oil supply system. Therefore they are more environmental friendly and require less maintenance than other types of cable. The materials used and the design of the single and three phase cables is similar as will be shown in Chapter 5 and can be read in Appendix B, but they have a few different characteristics. First of all three phase cables have a much larger diameter which makes them require a much larger bending radius than single phase cables. Therefore onshore single phase cables are more feasible with respect to the installation. Offshore large vessels are available which can install three phase cables. Secondly cables and cable laying offshore is very expensive which makes a three phase cable the most cost effective solution for large scale offshore HVAC power connections with respect to three single phase cables, though the single phase cables are cheaper. The third difference is the power transmission capacity of the cables. The total power (i.e. active and reactive) which can be transmitted through a cable is thermally limited. Because in three phase cables the three phases are bundled in one cable, they have a lower power transmission capacity per phase due to the thermal influence on each other. Because of these reasons further on all submarine HVAC cables mentioned will be three phase cables and all onshore HVAC cables will be single phase cables. The most used submarine HVAC cables in the offshore wind farm industry nowadays are 132 kv or 150 kv. Now also 220 kv submarine cables are available, but they have not yet been applied in any commercial project due to the fact that the submarine cable joints for these 220 kv cables are not yet certified [21], which will be expected soon. Therefore all three cable voltages will be taken into account in this study. The maximal power rating per three phase cable is approximately 190 MW, 215 MW and 310 MW for the 132 kv, 150 kv and 220 kv cables correspondingly for a single three phase cable with conductor cross sections of 1000 mm 2 per phase (i.e. the maximum power rating depends on the operational conditions of the cables and the total cable length, as will be explained in Chapter 5) Grid connection As indicated earlier, a HVAC cable generates reactive power. Therefore a compensating system is required. These compensation systems can be simple or complex. The simplest and cheapest compensation systems can only be switched on or off and the total reactive power compensated is fixed. The more complex and more expensive compensation systems are controlled which are 32

33 capable of continuously adapting the compensation, also with respect to the reactive power balance at the grid connection point. Depending on the grid connection point, the type of reactive compensation used should be determined (i.e. the TSO can demand for a controlled reactive power compensation system in case the connection of the offshore wind farm would create problems in the reactive power balance at the grid connection point). The HVAC system works without primary power input of the grid. Therefore the system can work as a standalone system and in case of a complete shut down of the grid, it has black start capability (i.e. the ability of a power station to commence generation and liven the bus after being completely shut down, with no electrical input from the power system). A HVAC system adds up to the short circuit power in the grid. The short circuit power can be several times the rated power of the cable. The grid is characterized by a design short circuit capacity (i.e. a maximum fault current to be never exceeded), related with the rating of switchgear and circuit breakers and the thermal and mechanical endurance of all safety equipment. Therefore special care must be taken for the fault protection when adding offshore wind farms with HVAC cables to the onshore grid Modularity and redundancy Modularity is an important property of a HVAC system. The system can be used to connect multiple offshore wind farms via HVAC connections with a single HVAC connection to the onshore grid. Depending on the total power which needs to be transmitted to shore, multiple submarine cables might be required which must be laid in separate trenches which gives the system more redundancy. Due to the fact that the submarine cables are buried the chance of a cable failure due to damage by an anchor is very rare and can be excluded [21]. Due to higher costs for more redundancy and the little chance of a cable failure when buried, adding redundancy to a cable system by adding another cable is not economically feasible and will thus not be a goal in the system design Technical overview of HVDC VSC HVDC VSC is an improved HVDC technology based on voltage source converters (VSC). The HVDC classic technology has some drawbacks with respect to the size of converter stations and the need for an auxiliary power supply. Also the controllability of active and reactive power is limited. These are the main points were HVDC VSC has improved the classic HVDC technology and which make this technology suited for the offshore wind farm industry. When this technology is observed in more detail (as will be done in the following chapters) it turns out that both the investment costs and the electrical losses are higher for relative short connections (i.e. up to 50 to 100 km from shore) as compared to a HVAC system. This is mainly caused by the high costs and losses of the converter stations. The HVDC VSC technology becomes an economically 33

34 feasible alternative with an increase in transmission distance and capacity, which will be shown later on. In the following sections first the principle of the HVDC VSC technology is given and afterwards the most important characteristics of the technology Principle of HVDC VSC The HVDC VSC technology was introduced by ABB in the late 1990s as HVDC Light and later also Siemens developed this technology and called this technology HVDC plus. Up to now these are the only companies which manufacture HVDC VSC systems. HVDC VSC is a HVDC technology based on voltage source converters (VSC). The VSC technology uses Insulated Gate Bipolar Transistor (IGBT) power semiconductors which are switched using Pulse Width Modulation (PWM). With this technology both active and reactive power can be controlled rapidly, continuously and independent of each other. In this way the transmitted power through the system can be controlled fast and continuously. The classic HVDC technology uses thyristors as switching elements which can only be switch on and therefore this system has less controllability of the power flow through the system. For the HVDC VSC system the control of the reactive power can be done at both ends of a cable independently and this control is also independent of the DC transmission voltage level, so no communication link is required between terminals. The controllability of active and reactive power makes the system suited for every possible grid connection (e.g. weak or strong does not matter), because active and reactive power can be varied in order to control the voltage and frequency variations in the AC network. Also the polarity of the connection is kept constant regardless of the power direction. This makes the system suited for a multi terminal transmission system. The usage of power semiconductors make the system also much more compact than the HVDC classic technology, which makes the system more suited for offshore applications, but the system is still quite much larger than for the HVAC system though Current status of HVDC VSC technology At the moment only ABB has several HVDC VSC systems operational. Currently they have 9 projects operational and 1 under construction [22]. The first system was developed in 1997 at Hällsjön and transmitted a power of 3 MW. Systems for a power transmission of 350 MW have now already been built and are operational for several years already (see table 4.2). Of all the projects which are developed up till now, several are submarine systems, but HVDC VSC has not yet been used for the offshore wind farm industry. In 2005 the first and only offshore system with an offshore converter station has been developed for an offshore oil platform at Troll A in Norway. 34

35 This is the only experience with offshore converter stations so far. There are some developments going on though. In Germany a pilot project will be built next year which connects a 400 MW offshore wind farm with the HVDC VSC technology to the onshore grid. The wind farm and the HVDC VSC system are currently under construction and they are expected to become operational mid 2009 [23]. This offshore wind farm is located 128 km offshore and will also have an onshore trajectory of 75 km to the inland substation Diele in Germany. This large distance makes the HVDC VSC technology more suited as the HVAC technology. Because of the sophisticated active and reactive power control capability of the HVDC VSC system, this is also beneficial for the grid stability in the Northern part of Germany, where currently a weak grid exists. In the table below the characteristics of the most recent and largest HVDC VSC projects are given as well as the characteristics of planned projects. Table 4.2: Characteristics of largest existing and planned HVDC VSC projects [22]. Project Location Year of commissioning Power Length System Type of cable Directlink Australia MW 65 km 3x bipolar ±80 kv, land Cross Sound USA MW 40 km bipolar ±150 kv, submarine Murraylink Australia MW 176 km bipolar ±150 kv, land Troll A Norway MW 70 km 2x bipolar ±60 kv, submarine Estlink Estonia & Finland MW 105 km bipolar ±150 kv, submarine & land NORD E.ON 1 Germany 2009 (expected) 400 MW 203 km bipolar ±150 kv, submarine & land As can be seen in the table above the largest HVDC VSC systems operational at the moment, use a ±150 kv cable system. Also ±300 kv cable systems have been developed, but no commercial ±300 kv project has been built or is planned at the moment [22] Type of HVDC VSC cable systems used As is the case for HVAC cables, also for HVDC XLPE cables are used. For HVDC VSC systems nowadays three different types of systems have been developed: ±80 kv, ±150 kv and ±300 kv systems. Due to the power transmission capacity for the offshore wind farm industry mainly the ±150 kv and ±300 kv are suited and will thus be taken into account in this study. The maximal transmission capacities per bipolar cable system are approximately 740 MW and 1480 MW for ±150 kv and ±300 kv HVDC VSC systems correspondingly for a conductor cross section of 3000 mm 2 (i.e. the maximum power transmission depends on the operational conditions as is the case for the HVAC systems, as will be shown in Chapter 5). For HVDC systems always single phase cables are used. The main difference in cable design between HVDC and HVAC single phase cables is the thickness of the insulation layer (e.g. for AC systems always the RMS value of the voltage is used and not the actual peak value of the voltage which is 2 higher, therefore the insulation for a 150 kv AC cables must be thicker as for a 150 kv DC cable). Due to costs and power capacity limits of cables offshore mainly copper 35

36 conductors are used (i.e. high power transmission capacity but also higher costs) and onshore mainly aluminum cables (i.e. lower transmission capacity compared to copper conductors but with lower investment costs). Due to the fact that cables under DC operation generate no reactive power and have low losses, the transmission distance for DC cables is practically unlimited. The transmission capacity depends on the conductor cross section, the transmission voltage and the type of system (i.e. monopolar or bipolar, see Appendix B.3). Monopolar systems use one cable and the earth or sea as a return conductor. Bipolar systems have 2 conductors at apposite polarity. Due to the high steady electromagnetic field of a single cable under DC operation, monopolar systems are in many countries not allowed. When a bipolar system is used and the cables are laid in the same trench, the electromagnetic fields of both cables with opposite polarity will nearly fully cancel out. Therefore mainly bipolar systems are used throughout the world with the cables buried in the same trench. Burying the cables in the same trench does reduce the power transmission capacity of the cable due to thermal influence of the cables on each other. Both onshore and offshore the cables can be installed at the same time and be buried in the same trench as was shown for the installation of the submarine cables for the Estlink project [22]. For the HVDC VSC systems in this study therefore only bipolar systems will be used of either ±150 kv or ±300 kv Grid connection As was indicated previously the basic principle of the HVDC VSC system is the usage of the Voltage Source Converter (VSC). With the semiconductor IGBT s used as switching element, the system can control the active and reactive power flow independently and continuously. Due to this separately controlled flow, the controllability of the grid voltage and frequency at the grid connection point is high and gives many possibilities for supporting the grid at the point of coupling. This is one of the main benefits of the system when it is used in the offshore wind farm industry. The system does not require an auxiliary power source, so it can start up the grid after a severe fault. Therefore the system has black start capability. In case of a fault on the AC onshore grid, the contribution with respect to the short circuit power is limited to the rated power of the converters in the DC system due to the decoupled operation of the DC and AC system. Therefore the connection of a HVDC VSC system to the onshore grid has less impact on the short circuit capacity of the onshore grid compared to a HVAC connection Modularity and redundancy A HVDC VSC system is suited for modular design. This is also due to the IGBT s used as switching element. Because the active and reactive power supply can be controlled separately, the polarity of the system does not have to change in case of a change in power flow direction. Therefore it will give no problems when 36

37 multiple offshore wind farms are connected in series/parallel. A modular system has not yet been built though. Due to the fact that no reactive power is generated in cables under DC operation, the power transmission per cable system is much higher for DC cables than for AC cables. For wind farms with a high power rating in general fewer cables will be required in a DC system as compared to an AC system. Therefore the redundancy is in general less than for the HVAC system, but as was indicated for the HVAC system, cable failure in offshore cables which are buried are rare [21]. In case one of the cables in a bipolar HVDC VSC system is damaged the total bipolar system is shut down. Therefore in general a HVAC system has more redundancy than a HVDC system when multiple HVAC cables are used in the system Reliability The HVDC VSC system has only been used for 10 years now. For offshore wind farms is has not yet been used, but it has one offshore substation already operational for the offshore oil industry (i.e. the Troll A HVDC VSC system). Due to this relative short time of experience, the amount of information available is little. Some information is available though about two projects of ABB [22]. The two projects of ABB (i.e. Cross Sound Cable and Murray Link ) had an availability above 98% during 1.5 year of operation. This availability level is very high, but both of the mentioned projects are based onshore. This means that the system is designed very well for the onshore conditions, but does not automatically indicate that the reliability of an offshore converter station will also be that high. The first and only operational offshore converter station had the first half year an availability of nearly 100% [29]. The reliability for the offshore usage of the system is with a single project not proven and needs to be proven with more offshore projects. The HVDC VSC system is much more complex than the standard HVAC system which makes it more vulnerable in offshore conditions. With the pilot project which will be developed in Germany, more experience will be obtained with the HVDC VSC system under offshore conditions. 4.3 Summary: comparison of HVAC and HVDC VSC technology In the previous paragraphs an overview has been given of the HVAC and HVDC VSC technologies and their application for the offshore wind farm industry. Both the HVAC and HVDC VSC technologies have their advantages and disadvantages. In table 4.3 an overview is given with a summary of the characteristics of both technologies. 37

38 Table 4.3: Comparison of HVAC and HVDC VSC technology for use in offshore wind farm industry. HVAC HVDC VSC Voltage levels 132 kv, 150 kv, 220 kv ±150 kv, ±300 kv Maximum transmission capacity per cable system 190 MW at 132 kv 215 MW at 150 kv 310 MW at 220 kv 740 MW at ±150 kv 1480 MW at ±300 kv Type of cable used XLPE Three phase offshore Single phase onshore Yes, reactive power compensation required XLPE Single phase both offshore and onshore in bipolar systems No Transmission capacity distance depending Black start capability Yes Yes Fault contribution Yes Low compared to HVAC Technical capability for network Limited, additional systems Large range of possibilities for support required for voltage and voltage and frequency support frequency support Offshore substations in operation Yes, many Yes, only 1 Space requirement offshore Small Large substation Relative volume 1 Relative volume Modularity possible Yes Yes Redundancy High in case of multiple cables Low compared to HVAC due to high transmission power of single bipolar system 38

39 5 Technical model In this chapter the technical model is given in detail. With the technical model the power production and all loss components in the system are calculated for both the HVAC and HVDC VSC system. In order to keep the overview of losses in the system, once again the overview of the loss components is given in the figure below. Figure 5.1: Overview of all loss components in the system. In the following paragraphs the methods for power production calculation and calculation of the losses of all components in the system will be given. 5.1 Modelling of power production The power production of an offshore wind farm can be divided into a theoretical possible power production and the actual power production. The theoretical power production is without availability or wind farm array effects taken into account. For the model the actual power production will be calculated with the power production loss due to availability and wake operation taken into account. For the calculation of the power production, several characteristics of the wind farm need to be known: Number of turbines used Type of turbine used (and accordingly the power curve) Wind characteristics (i.e. mean wind speed at hub height and Weibull shape factor for modeling the wind speed distribution over a year) The wind speed distribution at a site over a year is modeled by means of the Weibull distribution. For the Weibull distribution 2 parameters are of importance: the yearly mean wind speed at hub height and the shape factor. The Weibull distribution calculates the chance of occurrence of a wind speed V over a year. The Weibull distribution is given as follows: With: (k 1) k k V V f(v) = exp - (1) A A A 39

40 f(v): chance of occurrence of wind speed V [-] V: wind speed under consideration [m/s] k: Weibull shape factor [-] A: Weibull scale factor [-] A = 2 V π With a varying mean wind speed and Weibull shape factor, the distribution changes. A typical value for the Weibull shape factor for offshore wind farms 2.1. In the figure below the Weibull distribution is given for various mean wind speeds and a Weibull shape factor of 2.1. Weibull distribution for different average wind speeds (k = 2.1) Probability Density [-] wind speed [m/s] 8 [m/s] 9 [m/s] 10 [m/s] 11 [m/s] Figure 5.2: Weibull distribution for different average wind speeds and a Weibull shape factor of k = 2.1. In Appendix D the power curves of the currently available offshore wind turbines and some future offshore wind turbines are given. In figure 5.3 the power curves of these turbines are given in a single figure. As can be seen in figure 5.3 the differences between turbines with a similar power rating with respect to their power curve, are quite small. When the type of turbine and number of turbines used in an offshore wind farm are known together with the wind characteristics, the theoretical power production of a wind farm can be calculated as a summation of the operational range of the offshore wind turbine (i.e. from the cut in wind speed to the cut out wind speed): With: cut out f V = cut-in ( V) Hr/Yr p( V) P = # turbines (2) theoretical 40

41 P theoretical : theoretical yearly power production of wind farm [MWh] f(v): chance of occurrence of wind speed V (Weibull distr.) [-] p(v): power production at wind speed V [MW] Hr/Yr: hours per year, 8760 [-] Offshore Wind Turbine Power Curves Power [MW] Wind Speed [m/s] Vestas V80 Vestas V90 GE 3.6 Siemens 3.6 Vestas V120 Multibrid M5000 REpower 5M Bard VM Figure 5.3: Overview of power curves of offshore wind turbines. As was mentioned before the theoretical power production is the power production if the availability and wake operation in a wind farm are not taken into account. Both the availability and the wake operation are very dynamic and complex. The availability of a wind farm depends on many aspects: type of turbine: not all turbines are even reliable weather situation: in order to go for a repair the weather must be suited for the ships and personnel to visit the offshore wind farm availability of ships and personnel: in order to perform a repair ships and personnel are required type of maintenance strategy: different types of maintenance strategies are possible (e.g. repair every failure directly or wait and repair several turbines at the same time) type of maintenance contract: for the maintenance of offshore wind farms a contract can be signed with a maintenance company. In the contract a certain wind farm availability target can be settled which determines if the 41

42 maintenance company is forces to repair a turbine or if they can wait till another turbine fails distance to shore: the further the wind farm is located offshore, the larger the weather window must be (i.e. weather period with suited conditions to go offshore) in order to be able to perform a repair Also the wake operation depends on many aspects: mean wind direction: the wind direction and the layout of the offshore wind farm determine how many turbines operate in the wakes of other turbines turbulence intensity: the higher the turbulence intensity, the more power exchange between different wind layers of air and the shorter the wake distance behind a wind turbine wind farm layout: the larger the distance between turbines in a row and between rows, the lower the wake losses in a wind farm As can be read above, both the availability and wake operation depend on many factors. Due to the dependency on the wind farm layout and development strategy, both these components are modeled by a percentage of loss of the total theoretical power production in order to use them in this general model. Both percentages are based on experience with currently operational offshore wind farms and general values [30], [31]. The mean percentages for the availability and wake losses are set at: Availability: 92% Wake loss: 10% These values are also used in the EIA of the Trident offshore wind farms (see Appendix A). With these values the actual power production becomes: With: P actual = P Availability (1 Wake Loss) (3) theoretical P actual : actual power production of wind farm [MWh] P theoretical : theoretical power production of wind farm [MWh] Availability: mean availability of wind farm set at: 92% [-] Wake loss: mean wake loss of wind farm set at: 10% [-] The actual power production of the wind farm is used as a reference for the power loss of all the electrical components in the system. In the following paragraphs the loss models for the electrical components are given 5.2 Infield cable loss As was the case for the availability and wake losses, also the losses in the infield cables for the power collection of the offshore wind farm depend on many factors: 42

43 Length of cables: the longer the infield cables (i.e. which depend on the layout of the wind farm), the higher the conductive losses Infield cable voltage: the higher the infield cable voltage, the lower the conductive losses, infield cable voltages are mostly between 24 and 36 kv Conductor cross section: the larger the conductor cross section, the lower the conductive losses Type of conductor: a copper conductor has a lower resistance than an aluminum conductor with the same cross sectional area Operating conditions: the burial depth, the sea bed temperature and the type of soil have their impact on the losses of a cable as will be shown in more detail later on Number of cross sections: if multiple conductor cross sections are used, the conductor cross sections can better fit the actual power transmission between two turbines in a string (i.e. dependant if it is the connection of the last or first turbines in a row) As can be seen the losses in the infield cables are very case specific, depending on the choices made during the design of the offshore wind farm. Therefore also for the infield losses a percentage of loss is used. Because cable losses are dependant on the current squared through the cable, the percentage used will be the infield cable loss for the wind farm operating at rated power. The infield cable loss can then be calculated as a summation of the operational range of the turbines which is then given as: With: P loss, infield cut-out = V= cut in I(V) I rated 2 infield loss P rated f(v) Hr/Yr P loss,infield : infield cable loss [MWh] I(V): total wind farm current for wind speed V [A] I rated : total rated current of wind farm [A] Infield loss: infield loss percentage at rated power [%] P rated : rated power of the wind farm [MW] f(v): chance of occurrence of wind speed V [-] Hr/Yr: hours per year, 8760 [-] (4) In order to get an indication of the total yearly infield cable loss, an example is given for a 300 MW offshore wind farm with 100 Vestas V90 3MW turbines. The wind characteristics are set at a mean wind speed of 9 m/s at hub height and a Weibull shape factor of k = 2.1. The availability and wake loss are set at 92% and 10% correspondingly. This gives a total theoretical power production of GWh per year and accordingly an actual power production of GWh when the availability and wake loss are taken into account. For various values for the infield loss percentage the total yearly infield cable power loss can than be calculated as can be seen in table

44 Table 5.1: Overview of infield cable loss for various infield loss percentages. Infield loss percentage [%] Yearly Infield Loss [MWh] Yearly Infield Loss [%] For the North Hoyle offshore wind farm in Wales the infield cable loss was approximated at 0.5% of the total yearly power production [31]. The North Hoyle offshore wind farm has approximately 18 km of infield cables at 33 kv connected in 4 strings which are interconnected in a ring topology [32]. In order to get an estimation for the infield cable loss of the Trident offshore wind farms, the wind farms are compared to the North Hoyle offshore wind farm as is done in the table below with data obtained from the EIA of the Trident wind farms. Table 5.2: Characteristics of infield cables of North Hoyle and 'Trident' offshore wind farms [1], [32]. Wind Farm Infield Cable Voltage Total Infield Cable Length Strings Power per String Conductor Cross Section Mean String Length North Hoyle 33 kv 18 km MW 185 mm km Katwijk 34 kv 103 km MW 500 mm km West Rijn 33 kv 57 km MW 400 mm km Scheveningen Buiten 33 kv 59 km MW 400 mm km As can be seen in the table above the infield cable voltages which will be used for the Trident offshore wind farms will be (nearly) the same as is the case for the North Hoyle offshore wind farm. Whether multiple conductor cross sections will be used for the Trident offshore wind farms was not yet indicated in their EIA, but it is expected for reducing the investment costs in the infield cables. As can be seen the Trident offshore wind farms will have a larger total power per string, but also cables with a larger conductor cross section will be used. The larger power per string causes a larger current through the cables and accordingly higher losses, but due to the larger conductor cross section the resistance is lower which mitigates the increase in losses. As an estimation for the infield cable loss for the Trident offshore wind farms therefore mainly the mean string length is compared with the North Hoyle offshore wind farm. The resistance of the cable depends linearly on the cable length and therefore also the losses depend linearly on the cable length for relative short cable lengths. Therefore the infield cable loss percentages at rated power for the Trident offshore wind farms are set at: Katwijk: 1.1% Yearly infield loss appr.: 1.00% West Rijn: 0.6% Yearly infield loss appr.: 0.55% Scheveningen Buiten: 0.6% Yearly infield loss appr.: 0.55% The reason that offshore wind farms West Rijn and Scheveningen Buiten will probably have similar infield cable losses though offshore wind farm Scheveningen Buiten has 2 offshore high voltage stations, is because of the more suited area for the West Rijn offshore wind farm. This more suited area makes it possible to have 11 strings of approximately 5 km length each, while for 44

45 the Scheveningen Buiten offshore wind farm 2 offshore high voltages substations are needed in order to minimize the infield cable length and losses to get to a mean string length of approximately 5 km each. 5.3 Transformer loss model In the figure below the electrical equivalent model of a transformer is given. Figure 5.4: Electrical equivalent model of a transformer. In a transformer 2 parts can be distinguished: Primary and secondary windings Magnetic core The losses in a transformer are therefore also divided into conductive losses in the windings and magnetizing losses of the magnetic core. The conductive losses in the windings depend on the loading of the transformer. The magnetizing losses are independent of the loading and are therefore fixed. The losses of the transformer can then be model as follows: With P loss,transformer cut-out = V= cut-in P loss,no load 2 I(V) P loss,full load f(v) Hr/Yr I + (5) rated P loss,transformer : transformer loss [MWh] P loss,no load : static magnetizing losses [MW] P loss,full load : conductive losses at full load [MW] I(V): wind farm current at wind speed V [A] I rated : rated current of wind farm [A] f(v): chance of occurrence of wind speed V [-] Hr/Yr: hours per year, 8760 [-] The efficiency of large scale transformers is very high. In a paper about the Lillgrund offshore wind farm in Sweden the characteristics of the 120 MVA offshore transformer of that offshore wind farm are mentioned [33]. The no load losses are given as 55 kw and the full load losses as 310 kw which correspond 45

46 to efficiencies of % and 0.258% correspondingly. These values are similar to the efficiencies of the offshore substation of the Princess Amalia Wind Farm (former Q7 ) [34]. Therefore the no load and full load losses of the transformer will be given as: With: P loss,no load = P rated [MW] P loss,full load = P rated [MW] P rated : rated power of the transformer [MVA] 5.4 Cable model Calculation of the losses in a cable is more complex and will thus be explained in more detail. For a proper understanding of the behavior of a power transmission cable, both the electrical and thermal behavior of a cable must be known. The electric behavior differs between AC and DC cables and also the thermal model is different because of the difference in the number of conductors in a cable (e.g. for submarine cables). For a proper understanding of the behavior of a cable it is sufficient to look in detail at the AC power transmission cable only. On basis of the model for the AC cable simple adaptations can be made to get the model for a power cable under DC operation. In the following paragraphs the electrical model, the thermal model and the loss calculation model for power cables are given Electrical model of a power cable The electric behavior of a cable section can be modeled by a very simple π model as is shown in the figure below for a single phase of a cable. Figure 5.5: Electric cable model for a single phase [35]. As can be seen in the figure above, a cable has a resistance, an inductance and a capacitance. A cable has three sources of losses: 46

47 Ohmic losses: Dielectric losses: Induced losses: caused by the cable resistance losses in the insulation material ohmic losses due to induced currents in metallic sheaths and armoring of the cable Induced losses only occur in a cable under AC operation due to the constant fluctuating electromagnetic field which causes induced currents in conducting materials in the cable (i.e. the sheath and armor for a submarine cable). Also the ohmic losses differ between a cable operational under AC or DC. In the following paragraphs all three types of losses will be treated in depth Ohmic losses Ohmic losses are caused by the resistance of the cable and are the largest source of losses in a cable. For AC the resistance is always higher than the resistance for DC, mainly because of the presence of skin and proximity effect which appear for AC. The skin effect is the non-evenly distribution of current over the cross section of the conductor which has effect on the increase of the effective resistance of the conductor. The proximity effect is about circulating eddy currents within a conductor caused by the changing magnetic field due to AC operation which has effect on surrounding conductors. Due to the influence on the current distribution over the conductor cross section, this also has the effect of an increase in the effective resistance of the conductor. Therefore the AC resistance should be kept as low as possible by a careful design of the construction of the conductor. Normally both the AC and DC resistance of a cable are given. The resistance of a cable is temperature dependant which is given by the following formula [36]: With: R θ = R 20 (1+α θ (θ-20)) (6) R θ : resistance of cable at temperature θ [ km Ω ] R 20 : AC or DC resistance at 20 C [ km Ω ] (i.e. 20 C is the standard reference temperature) α θ : temperature coefficient of conductor [ 1 C (i.e. for copper: α θ = , for aluminum: α θ = ) θ: operating temperature of the cable [ C] The ohmic losses are then given as: P Ω = n I(x) 2 R θ (7) o ] 47

48 With: P Ω : ohmic losses in the cable [ km W ] n: number of phases [-] I(x): the conductor current which is distributed along the [A] cable length x as will be shown later on R θ : resistance of conductor at operating temperature θ [ km Ω ] Dielectric losses Dielectric losses are losses in the dielectric insulation material. The dielectric insulation acts as a capacitor when it is subjected to alternating current. Therefore a charging current will flow into the cable to charge this capacitance. The work required to effect the realignment of electrons in the insulation material each time the voltage direction changes (i.e. 50 or 60 times per second depending on the frequency) by means of the alternating electromagnetic field, produces heat and results in a loss of real power which is called dielectric loss. The capacitive behavior (charging current) and resistive behavior (leakage current) of the insulation material is modeled by the capacitor and parallel resistor (see figure 5.6). When a voltage is applied over the insulation material a current I charge will flow which consists of two parts: I charge = j I c + I r (8) With: I charge : charging current of cable [A] I c : capacitive charging current given as I c = 2 π f C L V L [A] I r : VL leakage current given as I r = Ri [A] With: f:: frequency [Hz] C : capacitance per unit length F [ ] km V L : RMS line voltage [V] R i : insulation resistance [Ω] L: length of the cable [km] The representation of the cable insulation is given in figure

49 Figure 5.6: Representation of cable insulation. As can be seen the current I charge will have a phase angle φ with respect to the applied voltage V. An ideal insulator would have no resistive leakage current and correspondingly φ = 90 and R i = 0 Ω. Due to the fact the insulator is not ideal, R i 0 Ω and there will be an angle δ between I charge and I c which is representative for the dielectric loss. Since, in the case of good insulating material, the magnitude of the leakage current vector is much smaller than that of the capacitance current vector, the loss angle δ is very small. These losses are modeled by tan(δ), the dielectric loss factor, which will be given later on. The capacitance C of the insulation per unit length depends on the dimensions of the cable and the relative permittivity ε r of the insulation material, also referred to as dielectric constant. The capacitance of the insulation is given per phase as: With: C = ε D 18ln( d i c 10 ) 9 (9) C : capacitance per unit length [ km F ] ε: relative permittivity of insulation [-] D i : external diameter of the insulation excluding screen [mm] d c : diameter of conductor including screen [mm] In practice the dielectric loss factor tan(δ) and relative permittivity ε are assumed constant for performing cable rating computations. In reality they are not constant but temperature dependant. For XLPE insulated high voltage power cables these variables are given as [36]: ε: = 2.3 [-] tan(δ) = [-] For a cable under DC operation the charging current will only flow once into the cable to charge the capacitance when the cable gets energized. Therefore the dielectric loss for a DC cable can be neglected. For a cable under AC operation the voltage and current alternate and therefore the cable capacitance will be 49

50 charged and discharged each cycle. This has two effects for cables in AC operation: Due to the alternating current and the according alternating electromagnetic field in the cable, the cable has a constant loss in the dielectric insulation: dielectric losses (W d ) A cable has a maximal current rating (I max ) for which the conductor temperature reaches its maximal operating temperature. The larger the cable capacitance, the larger the charging current and the smaller the available current for active power transmission which will be shown later on. The dielectric losses for a cable under AC operation can be modeled as [36]: With: W d 2 VL = n 2 π f L C' tan (δ) (10) 3 W d : dielectric losses [W] f: frequency [Hz] L: length of the cable [km] C : cable capacitance per phase [ km F ] V L : RMS line voltage [V] tan(δ): dielectric insulation loss factor [-] n: number of phases [-] The effect of the cable capacitance on the current rating will be shown later on Induced losses Cables under AC operation also have induced losses which occur due to induced currents in other metallic parts of the cable then the conductor. The currents are induced due to the alternating electromagnetic field. For a cable under DC operation there will be no alternating electromagnetic field and accordingly there will also be no induced losses. The induced losses are also ohmic and are mainly caused by induced currents in the metallic sheath and armor which depend on the type of material used and the type of bonding of the sheath and armor of the cable (see Appendix B: Cable construction and cable systems). Because of the ohmic character of these losses they are modeled as an increase in the cable resistance and will be given by the manufacturer of the cable as loss factors for the sheath and armor [36]: R' = R20 (1+ λ1 + λ 2 ) (11) 50

51 With: R : total AC cable resistance per unit length [ km Ω ] R 20 : AC resistance of the cable at 20 C [ km Ω ] λ 1 : sheath loss factor [-] λ 2 : armor loss factor [-] With the induced losses in the sheath and armor of the cable the ohmic losses as given in equation (6) will become: With: P 2 ' Ω = n L I(x) Rθ (12) P Ω : total ohmic losses of AC cable [W] n: number of phases [-] L: length of the cable [km] I(x): the conductor current which is distributed along the [A] cable length x as will be shown later on ' Ω R θ : total AC resistance of conductor at operating [ ] km temperature θ including the sheath and armor losses ' (e.g. R = R (1+ λ + λ )(1+ α (θ 20) ) θ θ Maximum transmission capacity and current distribution of AC cable As was mentioned before a cable under AC operation behaves as a large capacitor and accordingly a charging current will flow through the cable each cycle. Due to the fact that the cable is thermally limited and therefore has a maximum continuous operating current, the total active power which can be transmitted by an AC cable reduces with length due to this capacitive behavior. In this paragraph the maximal power versus transmission distance will be shown as well as the current distribution along the cable. The total power which can be transferred by a cable is given as: With: r S = P + jq (13) S v : apparent power [VA] P: active power, given as: VL P = n I cos( ϕ) 3 [W] Q: reactive power, given as: VL Q = n I sin( ϕ) 3 [VAr] 51

52 With: V L : RMS line voltage [V] I: RMS current per phase [A] φ: phase difference between voltage and current [ ] n: number of phases [-] The active power is the real power transferred (e.g. in this case power of the wind farm) and the reactive power is the power generated by the cable due to its capacitive behavior. As was mentioned before a cable only has capacitive behavior under AC operation. For a cable under DC operation the total power which can be transferred will thus be: v S = P A cable under AC operation has a capacitive behavior. A capacitive load (e.g. the cable capacitance) has a leading current (e.g. φ < 0) and an inductive load (e.g. an overhead line) has a lagging current (e.g. φ > 0). Therefore a cable generates reactive power (e.g. Q cable < 0). The difference between an inductive and capacitive load is shown in the figure below: [W] Figure 5.7: Power triangles for inductive and capacitive load. As can be seen in the figure above, an increase in reactive power generated in a cable will decrease the amount of active power which can be transferred (e.g. the total maximal power S is fixed because V is constant and I max is constant). The capacitive behavior of a cable is much larger than the inductive behavior. Therefore the reactive behavior is mainly capacitive (e.g. φ < 0 and Q < 0). The charging current per phase can be calculated with [37]: With: I charge VL = 2 π f C' L (14) 3 I charge : charging current per phase [A] f: frequency [Hz] C : cable capacitance per phase [ km F ] 52

53 L: cable length [km] V L : RMS line voltage [V] This gives for the reactive power generated in a cable: With: 2 VL VL Q = n Icharge = n 2 π f C' L [VAr] 3 3 n: number of phases [-] With an increase in length, the capacitance of a cable increases and accordingly the reactive power generated. As was mentioned before the total apparent power (i.e. S) is fixed, so with an increase in reactive power (i.e. Q) the total available active power (i.e. P) which can be transferred will reduce. Therefore the transmission distance for AC cables is limited. The maximal transmission length of an AC cable is the point when the whole current through the cable is used to charge the cable, meaning no active power can be transferred. The reactive power produced by a cable needs to be compensated. In general two types of compensation configuration are used: 100% : 0%: 50% : 50%: full compensation onshore at the grid connection point 50% offshore and 50% onshore compensation Onshore the compensation is installed in order to keep the grid working at unity power factor (i.e. cos(φ) = 1). The type of compensation configuration used determines the maximal transmission distance due to the current distribution through the cable. Therefore first insight must be given in the current distribution along the cable for the two mentioned reactive compensation configurations. For the 100% : 0% configuration all the reactive power is compensated onshore. This means that, seen from the offshore point of the cable, the current increases along the cable towards the onshore end of the cable. For the 50% : 50% configuration the reactive power is evenly distributed to both ends of the cable. Therefore the charging current at each end of the cable is only half of the total charging current onshore as in the 100% : 0% configuration. This principle becomes more obvious when looking at the power triangle for a cable under AC operation as is shown in figure 5.8. As was given before, the maximal cable current per phase is thermally limited and is given as I max. When it is assumed that the wind farm only generates active power and the cable losses are neglected, the current in the cable is given as: With: I (x) = I (x) + I (15) cable 2 charge 2 windfarm I cable (x): total current distribution along the cable [A] I charge (x): charging current distribution along the cable [A] 53

54 I windfarm : wind farm current [A] Figure 5.8: Power triangle for AC cable. As an example the current distribution along a regular 150 kv three phase submarine cable will be shown. The characteristics of the cable used are given in the table below. Table 5.3: Characteristics of a 150 kv three phase HVAC submarine cable. Total length L: 100 [km] RMS line voltage V L : 150 [kv] Rated current I max : 856 [A] Conductor cross section Θ: 800 [mm²] Rated capacity: 222 [MVA] Capacitance per phase C [µf/km] Capacitive charging current per phase I charge [A/km] For different wind farm powers the current distribution is given along the cable in figure 5.9 with 0 the offshore cable end. The dotted lines are for 100% onshore compensation and the full lines are for 50% onshore and 50% offshore compensation. Figure 5.9: Current distribution along a 100 km 150 kv three phase cable. Full lines: compensation at both cable ends. Dotted lines: compensation only onshore. 54

55 As can be seen in figure 5.9 there is quite a difference in current along the cable for the two different configurations of reactive power compensation. For 50% onshore and 50% offshore compensation more than 210 MW of wind power can be transmitted along the cable, while for 100% onshore compensation only 185 MW of wind power can be transmitted over a length of 100 km. This example shows that for long cable lengths (i.e. up to or over 100 km) and large wind farm powers it is certainly beneficial to have 50% onshore and 50% offshore compensation instead of 100% onshore compensation. Reactive compensation offshore is more expensive though, but due to the effect on the current distribution along a cable, this can save costs on the submarine cable due to a smaller conductor cross section of the cable which might be required. Now the current distribution along the cable is known, the cable length L can be varied. With this variation in cable length, also the maximal transmittable wind farm power will vary. As can be seen in equation (14) the charging current of a cable also depends on the cable voltage. The difference in transmission distance is significant for different cable voltages. The total active power which can be transmitted by an AC cable is given by: With: P V L = n Imax,WF (16) 3 P: total active power transmission through a cable [W] V L : RMS line voltage [V] I max,wf : maximal wind farm current per phase [A] given as: I max, WF = I 2 rated I 2 charge,total n: number of phases [-] As an example for the maximal transmittable power versus the transmission distance, 3 three phase HVAC submarine cables with a conductor cross section of 800 mm 2 are used for cable voltages of 132 kv, 150 kv and 220 kv. The characteristics of the three cables are given in the table below. Table 5.4: Characteristics of three submarine cables of 132 kv, 150 kv and 220 kv. RMS line voltage V L : 132 kv 150 kv 220 kv Rated current I max : 854 A 854 A 846 A Conductor cross section Θ: 800 mm mm mm 2 Rated capacity: 195 MVA 222 MVA 322 MVA Capacitance per phase C µf/km µf/km µf/km Capacitive charging current per phase I charge A/km A/km A/km In figure 5.10 the maximal transmittable power versus the transmission distance is shown for the three different voltage levels. 55

56 Figure 5.10: Maximal transmittable power versus transmission distance for three different cable voltages. As can be seen in the figure above the transmission distance roughly doubles when reactive compensation is applied both onshore and offshore for 50%. Also the influence of the cable voltage on the transmission distance is obvious when the results for the 220 kv cable are compared to the 132 kv and 150 kv cables. Based on the observation of the current distribution along the cable and the maximal transmission distance of AC cables, in this study it is assumed that 50% onshore and 50% offshore reactive compensation is used. This type of reactive power compensation configuration gives lower losses (i.e. based on the current distribution along the cable) and a larger possible power transmission over a longer distance compared to a system with 100% reactive compensation onshore Thermal model of cable As was mentioned before the maximum allowable cable loading, the maximum current through the cable, depends on the thermal limits of the cable. When a high voltage cable is energized and operational, part of the electrical energy is converted into heat: the losses. The higher the loading of the cable the higher the losses will be and accordingly the more heat is generated in the cable. The maximum operating current of the cable is thermally limited, in most cases by the insulation material. For XLPE cables which are mostly used nowadays, the maximal operating temperature is 90 C. For polyethylene (PE) cables the thermal limit is 70 C. The maximum operating current then depends on the thermal resistance of the cable and the cable surroundings, which determine the amount of heat which can be transported from the cable to the surroundings per unit of length. The thermal characteristics of the cable can be designed and are 56

57 well known. Both submarine and land cables will be buried. The thermal characteristics of the surroundings will then depend on: Thermal resistance of soil/seabed [ W Km ] Temperature at burial depth Burial depth Diameter of the cable Parallel heat sources Soil moisture migration [ C] [m] [m] As is shown above, the thermal behavior of the surroundings depend on many factors. A general maximal current rating of a cable can therefore not be given, but only for certain test conditions. The thermal model of a three phase AC cable is given in figure 5.11 and figure More details about the different layers and their function is given in Appendix B. Figure 5.11: Cross section of heat model of three-phase HVAC cable [35]. In the figure above the important cable parameters of the thicknesses and outer diameters of the most important layers are given. In figure 5.12 the following thermal resistances are shown: T 1 : T 2 : T 3 : T 4 : thermal resistance between the conductor and sheath thermal resistance between the sheath and armor thermal resistance between armor and cable surface external thermal resistance (thermal resistance of surroundings) 57

58 Figure 5.12: Heat model for a three-phase cable [35]. The definition of the indicated thermal resistances T 1 to T 4 is shown in the following paragraphs Thermal resistance T 1 The thermal resistance between one conductor and the sheath is defined as [36]: With: ρi 2t1 Single phase cable: T1 = ln(1+ ) (17) 2π d c ρ Three phase cable: T = i 1 G 2π (18) Km T 1 : thermal resistance between conductor and sheath [ ] W ρ i : thermal resistivity of insulation [ W Km ] d c : diameter of conductor [mm] t 1 : thickness of insulation between conductor and sheath [mm] G: geometrical factor which takes the dimension of a [-] three phase cable into account As is shown above, the thermal resistance between one conductor and the sheath differs between a single phase and a three phase cable. For different types of three phase cables relations are given for the geometrical factor based 58

59 on empirical measurements, but these will not be treated in this study. The thermal resistivity of the insulation is for XLPE and PE typically ρ i = 3.33 W Km Thermal resistance T 2 The thermal resistance between the sheath and armor is defined as [36]: With: T ρ2 2t 2 = ln(1 ) (19) 2π D 2 + s T 2 : thermal resistance between the sheath and armor [ W Km ] ρ 2 : thermal resistivity of the armor bedding [ W Km ] D s : external sheath diameter [mm] t 2 : thickness of bedding between sheath and armor [mm] The equation for the thermal resistance between the sheath and armor is equal for single phase and three phase cables with a common metallic sheath around all phases, which is most common Thermal resistance T 3 The thermal resistance of the outer covering (serving) is defined as [36]: ρc 2t b T3 = ln(1+ ) (20) 2π Da With: T 3 : thermal resistance of the outer covering [ W Km ] ρ c : thermal resistivity of the covering [ W Km ] D a : external diameter of armor [mm] t 3 : thickness of covering [mm] The equation for the thermal resistance of the outer covering is equal for single phase and three phase cables. For unarmored cables (e.g. land cables normally don t have an armor protection layer) D a is taken as the external diameter of the component immediately beneath it (e.g. the sheath, screen or bedding) External thermal resistance T 4 The current carrying capability of cables depends to a large extend on the thermal resistance of the medium surrounding the cable. The temperature rise of a conductor can depend for more than 70% on the external thermal resistance when the cable is buried. As was mentioned before the external thermal resistance of a cable depends on the thermal resistance of the soil, the burial 59

60 depth, the temperature of the soil, the cable external diameter, influence of neighboring cables and moisture migration. Therefore the cable ratings given by a manufacturer are always given for certain test conditions. The external thermal resistance will be shown in more detail for AC and DC cables in the following sections External thermal resistance T 4 for AC cables Parallel three phase submarine AC cables are normally laid in separate trenches with distances of about 50 to 100 m between them. Therefore they don t have thermal influence on each other and it gives a higher reliability. This distance is required in order to be able to repair a cable failure in a save way, without damaging neighboring cables [27]. Also the chance of two separate cables being damaged by an anchor is extremely rare. In the figure below an indication of the cable path is shown when a submarine cable needs to be repaired. Figure 5.13: Indication of cable path required for a repair, depending on the local water depth d. The additional bend in the cable depends on the local water depth, because of the fact that the repair joint will be made on a ship. Individual cables which are laid in the same trench can be repaired though, but it is much more difficult. Therefore three phase cables are usually laid in separate trenches [21]. For onshore single phase AC cables which are laid relative close to each other, either at flat or trefoil formation, the thermal influence on each other is relevant. This will be treated later on though. The general equation for the external thermal resistance of an AC cable is defined as [36]: 60

61 With: T ρs 2 = ln(µ + µ 1) (21) 2π 4 + T 4 : external thermal resistance of a cable [ W Km ] 2L µ = [-] D e ρ s : thermal resistivity of the soil [ W Km ] D e : external diameter of the cable [m] L: depth of burial of the centre of the cable [m] As can be seen above, the external thermal resistance of the cable surroundings depends on the external diameter of the cable, the burial depth and the thermal resistivity of the soil. The diameter of a cable is fixed for a certain design. In order to protect a submarine cable from crawling anchors, trawling gear and in some cases against high water currents (e.g. in case the cable will not be covered by soil at some spots), the cable needs to be buried. The depth of burial is in general between 1 and 2 m, most often 1.5 m. As can be seen the external thermal resistance will be lower if the burial depth is smaller which increases the maximal allowable cable loading (e.g. due to the fact that the soil will contain more moisture near the seabed surface which gives a lower thermal resistance). On the other hand the chance of the cable becoming uncovered by soil increases which is unwanted. Therefore the burial depth will be a compromise and depends on the local conditions (e.g. is there a chance for ships anchoring, how strong is the water current, are there sand waves, etc.). The thermal resistivity of the soil plays a very important role in the external thermal resistance of the cable and thus on the maximum current rating of a cable. Different types of soil have different thermal resistances. In the table below an overview is given of typical thermal resistivity of different types of soil [38]. Table 5.5: Typical thermal resistivity values for different types of soil [38]. Thermal resistivity [ W Km ] Gravel layers 0.05 Coarse sand 0.10 Wide graded dense sand 0.50 Fine silt sand 0.80 Clay 0.90 For a large part of the Dutch part of the North Sea a thermal resistivity of 0.50 Km Km is valid [34], [38]. In general a thermal resistivity of 0.8 is used for the W W North Sea seabed [37]. A submarine cable needs to be designed for the worst 61

62 case though. This is in general at the landing of the cable where the conditions are the worst. Typical cable conditions at the landing of a submarine cable are given in the table below as well as the typical conditions offshore (i.e. more than 3 km from shore) and onshore. The design of the cable is such that the thermal conductivity is optimized. Therefore the heat produced in hot spots such as at the landing is not only transported to the surroundings at the location itself, but also through the cable to colder parts of the cable. The soil around the hot spot can also be filled up with soil which has better/improved thermal characteristics in order to mitigate the heating effect locally. Table 5.6: Typical conditions of cables at the landing point, offshore and onshore [34]. Typical cable: landing conditions offshore onshore Soil temperature 15 C 12 C 15 C Thermal resistivity 1.1 Km/W 0.8 Km/W 1.0 Km/W Burial depth 3 m 1.5 m 1.0 m At the landing point the soil contains less moisture and heats up more. Therefore the soil temperature and thermal resistivity are higher as (far) offshore. Due to the washing of the waves the cable needs to be buried around 3 m deep from 3 km offshore to the shore in order to prevent the cable from being exposed. With the thermal model as shown in figure 5.12 and the electrical parameters of the cable, the permissible current rating of a cable can be calculated for a varying burial depth, varying thermal resistivity of the soil and a varying soil temperature. The permissible current rating per phase can be calculated when the total losses at 100% are known for a cable (e.g. losses generated in conductors (I 2 R), in insulation (W d ) and in the sheath and armor (λi 2 R)). Each of the sources of heat is multiplied with the thermal resistance of the layers it has to pass (e.g. T 1, T 2, T 3 and/or T 4 ). This gives the following equation for the conductor temperature rise [36]: With: θ = (I R AC + Wd )T1 + (I R AC(1+ λ1) + Wd )nt (22) (I R AC(1+ λ1 + λ 2 ) + Wd )n(t3 + T4 ) θ: temperature difference between soil and conductor [ C] I: current flowing in one phase [A] R AC : AC resistance per phase per unit length at operating [ m Ω ] temperature W d : dielectric loss per phase per unit length for the [ m W ] insulation 62

63 T x : thermal resistance per unit length for various cable [ W Km ] layers n: number of conductors [-] λ 1 : sheath loss factor [-] λ 2 : armor loss factor [-] Rewriting formula (22) above gives the following formula for the permissible current rating at the maximal operating temperature per phase [36]: 1 θ W d( T1 + n(t2 + T3 + T4 )) I = 2 (23) R T + nr (1+ λ )T + nr (1+ λ + λ )(T + T ) AC 1 AC 1 2 AC In formula (23) θ is given as the temperature difference between the soil and the maximum operating temperature of the cable (e.g. 90 C for XLPE cables). When the characteristics of the surroundings of a cable are known for a certain rating, the influence of a change in the thermal resistance of the seabed, a change in the temperature of the seabed and a change in the burial depth can be calculated. Based on these calculations correction factors for the rated current can be calculated for the cable operational for different characteristics. As an example the FXBTV 150 kv 400mm 2 three phase submarine AC cable of ABB [26] is taken. The characteristics of the cable are given in the table below. Table 5.7: Characteristics of FXBTV 150 kv 400 mm 2 HVAC three phase cable of ABB [26]. Cable Type ABB: FXBTV 150 kv 400 mm² Thermal Resistance Seabed: 1.1 [Km/W] Temperature of Seabed: 15 [ C] Burial Depth: 3.0 [m] AC resistance at 90 C: [Ω/km] Sheath loss factor λ 1 : [-] Armor loss factor λ 2 : [-] Thermal resistance of insulation T 1 : Thermal resistance of bedding T 2 : Thermal resistance of outer covering T 3 : Thermal resistance of surrounding T 4 : [Km/W] [Km/W] [Km/W] [Km/W] Dielectric loss per phase W d : [W/m] Number of phases n: 3 [-] Maximal operating temperature θ max : 90 [ C] External Diameter Cable D ex : [m] Rated current base conditions I rated : [A] 63

64 When the thermal resistance of the seabed or burial depth is changed, the external thermal resistance T 4 will change and accordingly the current rating of the cable. Also with a change of the temperature of the seabed the current rating will change. In the following three figures (i.e. figure 5.14, figure 5.15 and figure 5.16) the changes for the current rating is shown. Figure 5.14: Current rating for various soil thermal resistivity. Figure 5.15: Current rating for various burial depths. 64

65 Figure 5.16: Current rating for various soil temperatures. As can be seen in figures 5.14, 5.15 and 5.16 the external operational characteristics for a cable have a large influence on the maximal current rating of the cable. The correction factors for a change in soil thermal resistance, burial depth or soil temperature are given in the tables below for three phase submarine cables. Table 5.8: Rating factor for ground thermal resistivity of example. Thermal resistivity [ W Km ] Correction factor [-] Table 5.9: Rating factor for burial depth of example. Laying depth [m] Correction factor [-] Table 5.10: Rating factor for soil temperature of example. Temperature [ C] Correction factor [-] The adjusted current rating per phase of a cable depending on the cable characteristics can then be calculated as: I = I CF CF CF (24) adj ref T4 L Temp 65

66 With: I adj : adjusted rated current for cable operating conditions [A] I ref : reference rated current under specified conditions [A] CF T4 : correction factor for soil thermal resistivity [-] CF L : correction factor for burial depth [-] CF Temp : correction factor for soil temperature [-] For single phase land cables the correction factors are used as given by ABB [26], [42]. In Appendix C the correction factors are given for three phase and single phase HVAC cables with reference to the cable conditions offshore as shown in table External thermal resistance T 4 for DC cables For DC cables the current rating is normally given for closed and spaced laying. Closed laying means that the cables are laid in the same trench. The cables will then have thermal influence on each other. The current rating is therefore lower then for DC cables under spaced laying (e.g. in separate trenches of at least 2 meters apart). Due to the high static electromagnetic field of a HVDC cable, normally DC cables are buried in the same trench. In Appendix C the characteristics are given for XLPE HVDC cables of ABB [26], [42]. ABB gives the current rating for the cables for closed and spaced laying for the given test conditions. For cables operating at different conditions as specified in table 5.6, the correction factors for calculating the allowable cable rating are given in Appendix C Soil moisture migration One aspect of the thermal rating of cables which has not been observed yet is the possibility of soil moisture migration. With the losses in a cable heat is produced which has to be transferred to its surroundings. Due to the losses in a cable the cable heats up and accordingly the soil at the cable surface. This causes the moisture in the soil to vaporize and accordingly the vapor pressure of the surrounding soil will increase. Due to the natural effect of convection this water vapor extends to the colder surrounding soil where there is a lower vapor pressure, condensates and will flow back as water towards the cable due to capillary forces. As long as this circulation is not disturbed, the soil remains moist and the soil thermal resistivity remains constant. If the temperature difference between the cable surface and the surrounding soil exceeds a certain value, the soil surrounding the cable starts to dry out until the temperature difference between the surface of the dry zone and the surrounding soil falls below its limit [37]. Km The specific thermal resistivity of the dry zone is typically around W [37]. This will have a significant impact on the thermal rating of a cable and therefore needs to be prevented. Typical sea bed temperatures are between 5-66

67 18 C and it depends on the water depth (e.g. at smaller water depths the water temperature above the sea bed will rise more in the summer than for higher water depths and accordingly the soil temperature) [40]. Cables used for the grid connection of offshore wind farms will in general never reach the state where the surrounding soil starts to dry out. Due to the variation in wind and the fact that largest part of the time the cable will only be loaded between 30% and 80% (e.g. based on the Weibull distribution of the wind speeds over a year), the cable temperature will hardly ever reach its maximum operational temperature. Due to the fact that the soil has a certain thermal capacitance, the actual operating temperature at a certain current depends on the history of the loading of the cable and the accordingly generated amount of heat. This thermal capacitance can cause the cable to reach its maximal operating temperature only after days of strong winds when the offshore wind farm would produce full power. Because of the fact that a cable for offshore wind farms is in general always rated for maximal power production and the fact that most of the time the cable will only be loaded between 30% and 80%, it is assumed that drying out of the soil will not occur [41] Cable loss calculation Now the electrical and thermal behavior of a cable are known, the total losses in a cable for a certain current I can be calculated based on the different loss components as given in the previous paragraphs. As was mentioned before the resistance of a cable is temperature dependant and therefore also the losses. In a paper written by George J. Anders and Heinrich Brakelmann [41] a calculation method is presented by which the cable losses for a certain current I can be calculated and where the temperature dependence of the losses is taken into account. In their model they also distinguish the two modes of operation of a cable in moist and dry soil. As was already explained, for cables used for transferring power from offshore wind farms it is assumed that moisture migration does not take place and thus the cable will always operate in moist soil. The total losses in a cable for a certain current I can be calculated by [41]: With: W total 2 I(x,V) (V) = Wd + Wtotal,Ω vθ I (25) rated W total (V): total cable losses at certain wind speed V [W] W d : total dielectric losses of the cable, defined as: [W] 2 VL Wd = n 2 π f C' L tan(δ) 3 W total,ω : total ohmic losses of the cable at rated current [W] defined as: W = n R 2 L I [1+ α (θ 20)] total, Ω total,20 rated θ max 67

68 n: number of phases [-] f: frequency of operation (e.g. 50 or 60 Hz) [Hz] C : capacitance per phase per unit length [ km F ] V L : RMS line voltage [V] tan(δ): dielectric loss factor [-] R total,20 : total AC or DC resistance (including sheath and [ km Ω ] armor losses for AC) at 20 C per unit length L: cable length [km] I(x,V): current distribution per phase for wind speed V [A] I rated : rated current of the cable per phase [A] α θ : temperature coefficient of conductor [ 1 C θ max : maximum operating temperature of cable [ C] ν θ : temperature coefficient [-] c1 defined as: v θ = 2 I c + 1 c 2 1 Irated c 1, c 2 : constants defined as: c = 1+ α (θ 20 θ ) [-] c 1 θ amb + 2 αθ( θmax θd = ) [-] θ amb : ambient temperature [ C] θ max : (maximum operating temperature - ambient temperature) [ C] θ d : temperature increase of conductor due to dielectric [ C] losses, defined as: θ = n W T d d total d o ] T total : total thermal resistance of cable incl. surroundings [ W Km ] With equation (25) and all the variables as given above, the total losses of a cable can be calculated in Watt. When this calculation is performed over the total operational range of the wind turbine used, the total cable losses in MWh can be calculated as is given in the equation below. With: cut out -6 P = W (V) 10 Hr/Yr f(v) (26) loss, cable = total V cut in P loss,cable : total cable loss per cable system [MWh] W total (V): total cable loss for wind speed V [W] Hr/Yr: hours per year, 8760 [-] f(v): chance of occurrence of wind speed V [-] 68

69 As can be seen, the total losses depend on quite some variables which are either cable characteristics or characteristics of the surroundings. Therefore many details of the cables used and the surroundings where the cables will be laid are needed in order to perform the cable loss calculation. In the tables below the variables are specified which need to be known of the cable characteristics and the surroundings. Table 5.11: Variables of the cable surroundings needed in order to be able to perform the cable loss calculation. Variable Unit Burial depth Usually 1.5 m offshore, 3.0 m near shore and 1.0 m onshore [m] Soil temperature θ soil [ C] Soil thermal resistance T 4 [Km/W] Presence of more cable systems Mainly onshore issue [-] Table 5.12: Variables which are needed of the cable characteristics in order to be able to perform the cable loss calculation. Variable Unit Type of cable AC or DC [-] Number of phases 1, 2 or 3 [-] Length of Cable L [km] Cable capacitance per unit length C [F/km] RMS Line voltage V L [kv] tan(δ) for XLPE cables [-] Total AC or DC resistance at 20 C per unit length R total,20 [Ω/km] Rated current I rated [A] Temperature coefficient of conductor α 20, for copper, for aluminum Maximum operating temperature θ max, 90 C for XLPE cables [ C] Operating frequency 50 or 60 [Hz] Total thermal resistance of cable incl. surroundings T total [Km/W] For the calculation of the cable losses two remarks remain. One is about the total thermal resistance of the cable including its surroundings and the other is about the thermal effect of multiple cable systems on each other, especially for onshore cables. These are mentioned in the following paragraphs Calculation of total thermal resistance of cable As can be seen in Appendix C for none of the cables the internal thermal resistance of the cable is given. In order to be able to calculate the total thermal resistance of a cable including its surroundings, the following equation is given by Anders and Brakelmann [41] in order to estimate the total thermal resistance of the cable. [ 1 C o ] With: T total (θ max amb (27) W θ total,ω ) 69

70 T total : total thermal resistance of a cable incl. surroundings [ W Km ] θ max : maximal operating temperature of the cable [ C] i.e. 90 C for an XLPE cable θ amb : ambient temperature of the surrounding soil [ C] W total,ω : total ohmic losses at rated current [W] The approximation given by equation (27) results in a very small error for cables with small dielectric losses, which is normally the case for XLPE cables Influence of multiple onshore cable systems As was mentioned before offshore HVAC cable systems are normally installed relatively far apart in separate trenches. Therefore these cables will not have a thermal influence on each other. For submarine HVDC cables it was already mentioned that per bipolar cable system they are laid in the same trench and therefore have a thermal influence on each other. Multiple HVDC submarine cable systems will be laid in separate trenches far enough separated that they will not have a thermal influence on each other, like is the case for submarine HVAC cables. Onshore the conditions are different. Due to the small space available for cable trenches in general, multiple cable systems need to be buried relatively close to each other (i.e. they most likely have a thermal influence on each other). For HVDC land cables this will most likely not be much of importance, due to the fact that an enormous amount of power can be transferred per bipolar cable system which causes a HVDC system to have only 1 or 2 bipolar cable systems. For HVAC land cables the power transmission capacity per three phase system is much lower which causes an onshore HVAC system to have multiple three phase cable systems. Therefore it is expected that these three phase cable systems will have a thermal influence on each other. In a paper written by Prof. Heinrich Brakelmann [43] this problem of the thermal influence of multiple three phase cable systems onshore is dealt with. A calculation method is given for the calculation of the temperature increase due to the thermal coupling of neighboring cable systems and accordingly the maximal loading possible of a cable system. The thermal coupling depends on the type of installation of the three phase system (i.e. in trefoil or flat formation) and the distance between the three phase cable systems. Also the type of soil is of importance (i.e. in order to improve the transmission of heat from the cable to its surroundings the cable trench can be filled with soil with a good thermal conductivity which decreases the temperature increase of cables due to losses). When the calculation is done for three phase cable systems used for the grid connection of offshore wind farms, the following two characteristics need to be taken into account: 70

71 Due to the wind speed distribution the loading of the cable systems is mainly between 30% and 80% of the rated power of the offshore wind farm. This together with the thermal capacity of the soil causes the soil to have a mean temperature which is much lower as compared to a cable system which is operated constantly close to the maximal power rating of the cable system. Due to the soil thermal capacitance it takes time for the soil to heat up by the heat produced due to the cable losses. The cable systems are always designed for the rated power of the offshore wind farms. Because of the fact that the soil heats up relatively slowly due to the thermal capacitance, it can take several days (i.e. up to 3 or 6 days or more, depending on the loading history of the cable system) of full load production of the offshore wind farm before the soil surrounding the cable systems gets heated up significantly. Accordingly the temperature in the cable system in general remains lower as would be expected for rated power (i.e. the cable temperature will remain far below the maximum operational temperature of 90 C as would be expected for rated power). With the two observations as made above and calculations done with sophisticated software, it turns out that the thermal influence of multiple cable systems is lower for cable systems used in the grid connection of offshore wind farms as is expected. Also the temperature of the individual cable systems will depend on the configuration of the multiple three phase cable systems (i.e. in a horizontal plane and or vertical plane). Due to the complexity of multiple cable systems and the thermal capacitance of the soil combined with the constantly fluctuating power production of offshore wind farms, the thermal influence of multiple three phase onshore cable systems is being neglected in this study. Another important factor for the cable rating of onshore HVAC cables is the type of formation the three phase cables are installed: in trefoil or flat formation. Installation in trefoil formation has the advantage that due to the symmetry the electromagnetic field is further reduced as is the case for a flat formation installation. A disadvantage of the trefoil formation installation is that due to the fact that the single phase cables are laid attached to each other, they have a thermal coupling between them which reduces the maximal power rating of the cable system in trefoil formation compared to a flat formation. As can be seen in table C.5, table C.6 and table C.7 in Appendix C two rated currents are therefore given for onshore single phase HVAC cables for both the trefoil and flat formation installation. It is assumed that in case there are multiple onshore three phase cable systems required, the type of installation will be in trefoil due to the limited space available for the cable trench in general. 71

72 5.5 Reactive power compensation The need for reactive power compensation for HVAC cable systems was already indicated in paragraph 5.4 where the cable model was treated. As was mentioned reactive power compensation can be done with several systems which are either passive or active. Passive reactive power compensation for a cable under AC operation is mainly by means of a shunt reactive power compensation system which consists of a coil which can be connected or disconnected. Therefore the reactive power compensation is fixed and can only be switched on or off. This is a simple and cheap solution for reactive power compensation. Active reactive power compensation can be done by using power electronics combined with capacitors and coils which together are capable of continuously adapting the reactive power compensation based on the reactive power produced by the cable and the offshore wind farm and the reactive power situation at the connection point with the onshore grid. These systems therefore have the capability to assist the grid voltage and frequency up to a certain extend (i.e. depending of the type of active system). Due to the fact that the active reactive power compensation systems are more advanced and use power electronics, they are also much more expensive as compared to the passive reactive power compensation. The type of reactive power compensation used, has to be discussed with the onshore transmission system operator (TSO) depending on the strength of the onshore grid where the offshore wind farm will be connected to. Because of the fact that the type of reactive power compensation is site specific and there are multiple active compensation systems available each with their own characteristics, the type of reactive compensation used in this study will be the passive shunt reactor which is the simplest and cheapest. In the figure below an overview is given of this type of reactive power compensation. Figure 5.17: Overview of shunt reactive power compensation. When multiple offshore wind farms have a coupled grid connection, the total reactive power which needs to be compensated can be significant which might requires an active compensation system. Also in this case a passive compensation system will be used in this study because of the fact that the 72

73 losses in the compensation system and the investment costs compared to other components in the grid connection system are relatively low. For this study only the reactive power generated in the cables will be taken into account. The total reactive power generated by a cable under AC operation which needs to be compensated, was given in paragraph 5.4 as: With: 2 VL VL Q = n Icharge = n 2 π f C' L [VAr] 3 3 n: number of phases [-] f: frequency of operation, 50 or 60 [Hz] C : cable capacitance per phase per unit length [F/m] L: length of the cable [km] V L : RMS line voltage of the cable [V] The losses in the reactive compensation system are not load dependant due to the continuous operation. Therefore the losses in the reactive compensation are given as follows. With: P compensation = η Q Hr/Yr (28) rated P compensation : loss in reactive compensation system [MWh] η: loss factor of compensation system [-] Q rated : net reactive compensation of system [MVA r ] Hr/Yr: hours per year, 8760 [-] A typical value for the loss factor of reactive power compensation systems is 0.15% (i.e. η = ) as given by Prof. Brakelmann [44]. As was already indicated, in this study 50% onshore and 50% offshore compensation will be assumed. 5.6 Loss HVDC converter station For a HVDC VSC system for the grid connection of offshore wind farms both offshore and onshore an AC/DC and DC/AC converter is required. In figure 5.18 a simplified scheme is given of the components in the converter and the total HVDC VSC system. 73

74 Figure 5.18: Simplified diagram of HVDC VSC system [45]. The different components in a converter station and their usage are as follows: Transformer: adapting the AC voltage level to the DC transmission voltage level Harmonic filter: due to the high frequency switching in the insulated gate bipolar transistor (IGBT) valves, harmonics are brought into the system which must be filtered out to prevent them from penetrating into the AC system, the main components of the filter are a capacitor and reactor Phase reactor: together with the capacitor in the harmonic filter a low pass filter is created to filter out the high frequency components caused by the switching in the valves, the phase reactor also limits short circuit currents on the AC side Valves and diodes: the IGBT valves and diodes form the main device of the converter because by means of PWM the IGBT s are switched on and off in order to generate DC from AC and vice versa DC capacitor: the DC capacitor reduces the harmonics ripple at the DC side of the converter, limits the voltage variations due to disturbances on the AC side which depends on the size of the DC capacitor, acts as energy storage in order to control the power flow and provides a low inductive path for the current when the system is switched off 74

75 In order to be able to calculate the losses in a converter the detailed electrical schematic of the converter should be available as well as the type of control of the system (e.g. the control of the switching of the valves, the switching frequency, etc.). In a paper written by Jacobson [45] an overview is given of the development of the HVDC VSC converter systems. Now HVDC VSC systems are available at ±300 kv which can transport more than 1000 MW per bipolar cable system. In the paper the development of the converter stations of several existing HVDC VSC systems is shown. Also the type of converter topology and switching frequency used are indicated which have improved over time. Because of the constant development of the HVDC VSC converter stations and the complexity of the system, a model for the losses in the converter is created based on given loss characteristics of two commercial HVDC VSC projects as was done in a study performed by Barberis Negra [46]. ABB has published two papers in which the losses of two large scale HVDC VSC projects are presented [48], [49]. The characteristics of the two projects are given in the table below. Table 5.13: Characteristics of the Cross Sound Cable and Murraylink HVDC VSC projects [47], [48], [49]. Project Cross Sound Cable Murraylink Power rating 330 MW 220 MW Commissioning year August 2002 August 2002 # cable systems 1 1 DC voltage ±150 kv ±150 kv Type of cable PE PE Maximal cable temperature 70 C 70 C Submarine cable length 2 x 40 km - Onshore cable length - 2 x 180 km Type of submarine cable 1300 mm 2, copper - Type of onshore cable and 1400 mm 2 (#) Rated current cable 1200 A 739 A Switching frequency converter 1260 Hz 1350 Hz (#): due to non uniformity of the soil with different thermal characteristics The losses as presented in the papers about both projects are given for a certain input power of the converter on one side of the system. The losses of both systems are shown in figures 5.19 and The estimated losses (red lines) are the losses which were expected for the improved converter designs. The actual losses (blue lines) are the losses which were actually measured during operation of the two converter stations. 75

76 Figure 5.19: Estimated and actual loss of Cross Sound Cable project [48]. Figure 5.20: Estimated and actual loss of Murraylink project [49]. The HVDC VSC system can be modeled as is given in figure Based on the cable characteristics given in table 5.13 and the model for calculating the losses in the cables given in paragraph 5.4, the losses in the converter stations can be calculated when is assumed that the converters have the same efficiency (i.e. which is actually not the case due to the losses in the cables the input power at the second converter is lower as for the first converter station). Figure 5.21: System block diagram of VSC system. 76

77 For the model given in the figure above the following relations can be obtained: P1 = (1 x) P in (29) 2 P1 2 Ploss, cable = P1 P2 = R I = R ( ) 2 V (30) L Pout = (1 x) P 2 (31) With: P in : the input power at the first converter station [MW] P 1 : power output after first converter station [MW] P loss,cable : power loss in the cables [MW] P 2 : power output at input of the second converter station [MW] R: total resistance of the cable system [Ω] given as: R = 2 R L for a single bipolar system DC R DC : DC resistance of cable per unit length [Ω/km] L: cable length [km] I: current per cable [A] V L : absolute line voltage of cable, i.e. in this case 150 kv [V] x: percentage of losses in the converter stations [-] P out : output power after the second converter station [MW] When equations (29), (30) and (31) are rewritten the following equation is obtained: Pin R 2 V L 2 (1 x) 3 P (1 x) in 2 + P out = 0 (32) In the equation above the only unknown in the system is the percentage loss x of the converter stations in the HVDC VSC system. As was mentioned in paragraph 5.4 the resistance of a cable is temperature dependant which must be taken into account in the equation above. Now the equation can be solved for both HVDC VSC systems as is given in the tables 5.14 and For the Murraylink project the mean conductor cross section of 1300 mm 2 is used, because the individual cable lengths of the 1200 mm 2 and 1400 mm 2 cables was not specified. For both projects PE cables were used and not XLPE cables, thus the maximal cable operating temperature is 70 C instead of 90 C as is the case for XLPE cables. 77

78 Table 5.14: Overview of calculated losses and efficiencies of Cross Sound Cable Project. Cross Sound Cable Project Input Power Output Power Total Losses Total Losses Loss per Converter Losses Converters Losses cables Losses cables [MW] [MW] [%] [MW] [%] [MW] [%] [MW] Table 5.15: Overview of calculated losses and efficiencies of Murraylink Project. Murraylink Project Input power Output power Total Losses Total Losses Loss per Converter Losses Converters Losses cables Losses cables [MW] [MW] [%] [MW] [%] [MW] [%] [MW]

79 An overview of the losses in the cables and converters of both projects is given in the figures below Losses Cross Sound Cable Project Losses [MW] AC Input Power [MW] Losses Cross Sound Cable Project Losses Converters Losses Cables Figure 5.22: Overview of the losses for the Cross Sound Cable project. 14 Losses Murraylink 12 Losses [MW] AC Input Power [MW] Losses Murraylink Losses converter Losses Cables Figure 5.23: Overview of the losses of the Murraylink project. As can be seen in the figures above the cable losses of the relatively short cable connection of the Cross Sound Cable project are small compared to the losses in 79

80 the converter. For the Murraylink project the losses in the cables become more significant when the power transmission increases due to the long cable length of 180 km. In the figure below the loss curve of the converters used in both projects is shown. Figure 5.24: Loss curves of the converters for the Cross Sound Cable and Murraylink projects. As can be seen in the figure above the loss curves of the converters for the Cross Sound Cable and the Murraylink projects are similar but the losses are slightly lower for the converters of the Murraylink project. This is due to the difference in the design of both converters (e.g. different switching frequency and control system). As can be seen in the figure the curve decreases towards the maximum input power of the converters and reaches a loss factor of 1.6% if one would extrapolate the losses. Over the years the design of the HVDC VSC converter stations has improved significantly. An overview of the improvements with respect to the complexity (e.g. the number of components used) and the losses is given in figure As can be seen the number of components has been brought down during the years to about 30% of the original design. The losses have been brought down to about 40% of the initial design. On their website ABB states that the current status of the converter design is such that the losses at rated power are about 1.6% per converter for high converter ratings up to 1000 MW. Of these losses the standby losses are about 0.2%. The main contributions to these losses are the IGBT valves (i.e. 1.1%), the converter transformers (i.e. 0.21%) and the converter reactors (i.e. 0.12%). The rest of the losses come from the AC filters, the station service power and the DC capacitor [22]. In order to model the losses in the converter a function is fitted on the loss curves of the Cross Sound Cable and the Murraylink and the improvement of the design over the years it taken into account (e.g. at rated power the loss will be approximately 1.6%). 80

81 Figure 5.25: Overview of the number of components and the relative losses of the HVDC VSC system over the years [47]. When the losses of the converter are fitted and the improvement in the design is taken into account, the following fitted function is obtained: 5 2 Ploss, conv = Pin (V) Pin (V) (33) With: P loss,conv : absolute losses in the HVDC VSC converter [MW] P in (V): input power of the converter depending on wind [MW] speed V In figure 5.26 the percentage of loss of the fitted function is compared to the Cross Sound Cable and Murraylink projects. The calculated loss per converter by equation (33) is slightly lower as compared to the Murraylink project and has at rated power a loss of approximately 1.6% at a rated power of 300 MW. Figure 5.26: Fitted loss function of HVDC VSC converter. 81

82 In order to be able to model converters with higher rated powers, more insight must be given about the main loss components in the converter. With an increase in the input power, the current depending losses start dominating. Therefore the percentage of loss reduces significantly and the curve flattens for higher input power as can be seen in figure 5.24 and figure As was given before the main loss components are the losses in the IGBT valves. In figure 5.27 and figure 5.28 the switching behavior of the IGBT s is shown with respect to the voltage and current curve of an IGBT. The losses in the IGBT s come from conduction and switching losses. As can be seen in figure 5.28 the conduction losses are relatively low compared to the switching losses. The conduction losses may be reduced by using a larger semiconducting area. The switching losses depend on the switching time, switching frequency and the voltage and current at the switching instant. Fast switching and a low switching frequency reduce the power dissipation. However the switching frequency affects the controllability of the converter so there is a trade-off between them. Figure 5.27: Example of single phase voltage signal of HVDC VSC converter with in blue the fundamental frequency [22]. In figure 5.28 on top the switching signal is given (e.g. which is part of the PWM control signal). When the control signal goes from Off to On, the IGBT starts conducting. During this transition period the current through the IGBT rises approximately linear and the blocking voltage decreases approximately linearly as can be seen in the middle figure. The main power loss is during these transition periods during the switching of the IGBT. As can be seen the loss is proportional to the current through the IGBT. This linear dependency of the loss in an IGBT causes also a linear dependency of the loss in a converter for higher input powers, when the current dependant losses start dominating and where the IGBT is the main source of power loss of the converter. Therefore it can be assumed that for higher power ratings (e.g. above 300 MW) the power loss remains 1.6%. Now the power loss in the converter is known, the total power loss of the converter for the whole operating range of an offshore wind farm can be calculated as: With: cut out P = P (V) f(v) Hr/Yr (34) converter = loss,conv V cut in P converter : power loss in HVDC VSC converter [MWh] 82

83 P loss,conv : absolute power loss of converter for wind speed V [MW] f(v): chance of occurrence of wind speed V [-] Hr/Yr: hours per year, 8760 [-] Figure 5.28: Overview of switching pattern of IGBT and the loss of energy in an IGBT [50] Loss of ±300 kv HVDC VSC converter The calculation of the losses as given previously was done based on ±150 kv HVDC VSC systems. The question is, based on these calculations, what the losses of a converter for a ±300 kv HVDC VSC system will be. In order to answer this question one has to look in more detail into the converter design for both operating voltages. The basic design of the converters as shown in figure 5.18 is similar for both operating voltages. As was given before the main loss components are the IGBT valves. Therefore these valves are observed more closely for the converter design for both voltage levels. A valve consists of many IGBT s as is shown in figure

84 Figure 5.29: Overview of number of IGBT's in HVDC VSC converter station [22]. Many IGBT s are connected in series in order to withstand the high voltage and many strings are connected in parallel to increase the current carrying capacity (i.e. an IGBT has a maximum current capacity and maximum blocking voltage). For a ±300 kv system the voltage level is twice as high as for the ±150 kv system, so double the amount of IGBT s need to be connected in series to withstand the higher voltage for the ±300 kv system. For the same power rating the current for the ±300 kv system is only half the current for the ±150 kv system though. Therefore only half the number of IGBT strings need to be connected in parallel. From this simple observation can be assumed that the losses for a converter in ±300 kv systems are similar as for ±150 kv systems with the same power rating. Therefore the equations given for the calculation of the losses of a converter as given in equations (33) and (34) will also be used for the ±300 kv systems. 5.7 Summary of system loss calculation In the previous paragraphs all the loss components in the system as given in figure 5.1 have been given in detail with formulas for calculating the losses. For a single wind farm or multiple offshore wind farms with a coupled grid connection the total power fed into the grid can now be calculated by the following formula: P fed into grid = P actual P loss infield P loss offshore transformers (35) P loss interconnection cables P loss main transformer or P loss offshore converter P loss, grid connection cables P loss onshore substation or P loss onshore converter With: P fed into grid : total power of all wind farms fed into the onshore grid [MWh] P actual : actual power production in all wind farms in the model [MWh] 84

85 with availability and wake losses included (equation 3) P loss,infield : total infield cable loss of all wind farms in the model [MWh] (equation 4) P loss offshore : total losses of offshore transformers of secondary [MWh] transformer offshore wind farms in case of a coupled grid connection system (equation 5) P loss inter : total losses in interconnection cables in case of a [MWh] connection cables coupled grid connection system (equation 26) P loss main : transformer total losses in main offshore transformer in case of a [MWh] HVAC grid connection system including the losses for the reactive power compensation (equations 5 and 28) P loss offshore : total losses of offshore converter in case of a HVDC [MWh] converter grid connection system (equation 34) P loss, grid : total losses in grid connection cables (equation 26) [MWh] connection cables P loss onshore : total losses in onshore substation in case of a HVAC [MWh] substation grid connection system including losses in the reactive power compensation system (equations 5 and 28) P loss onshore : total losses in onshore converter in case of a HVDC [MWh] converter grid connection system (equation 34) 85

86 6 Economical model In this chapter the economical model is given in detail. With the economical model the investment costs for all the components in the grid connection system of a single or for multiple offshore wind farms can be calculated for both the HVAC and HVDC VSC grid connection system. The model for the investment costs of both systems is verified on basis of existing commercial projects. Also a model for the costs of losses is given and the final model for the optimization of the grid connection system on basis of the losses in the system and the investment costs. In order to give more insight in the complexity of a coupled grid connection for multiple offshore wind farms of several different developing companies, more insight is given in financing an offshore wind farm. 6.1 Modelling of losses As was indicated before, the losses in the electrical system are modeled by means of the subsidy gained per MWh produced. In paragraph 3.3 the subsidy system for renewable energy in the Netherlands was already outlined. Offshore wind energy is not yet included in this subsidy system. Therefore the modeling of the losses will be based on the subsidy system in the UK which is already operational for several years now and works successful. In the UK the offshore wind farm industry is rapidly growing and the conditions (e.g. distance to shore, water depths, grid connection arrangements) are similar Subsidy system for renewable energy in the UK The support mechanism for renewable electricity production in the UK is by means of a banded Renewables Obligation Certificate (ROC) mechanism. This banded RO mechanism will provide differentiated support levels for different renewables based on the different cost prices of electricity generated by the different renewable sources. The certificates obtained per generated MWh can be traded. The banded RO mechanism is an adaptation to the existing RO mechanism with 1 ROC per generated MWh for all technologies. The banded RO mechanism will come fully operational from 1 April 2009 onwards. This new subsidy system gives additional certainty on long term Renewable Obligation Certificate prices and is better fitted to the current status of all the different renewable technologies [54]. With the new banded RO mechanism the electricity supplied by offshore wind farms will receive 1.5 ROC per generated MWh which will increase the stimulation for the development of offshore wind farms in the UK. An overview of the banded RO mechanism is given in table 6.1. The current average value per ROC is 53,27 per ROC 5 [55]. With the new RO system this would hold 79,91 6 / 5 53,27 is equivalent to 67,19 with a currency of 1,- = 1,2613 (July 2008) 6 79,91 is equivalent to 100,78 with a currency of 1,- = 1,2613 (July 2008) 86

87 100,78 per generated MWh of electricity by offshore wind farms which is more in line with the production costs for offshore wind energy nowadays. Table 6.1: Overview of banded ROC mechanism in the UK [54]. Band Renewable source ROC s per MWh 1: Established Sewage gas Landfill gas 0.25 Co-firing non-energy crop biomass 2: Reference Onshore wind Hydro 1 Co-firing of energy crops 3: Post-demonstration Offshore wind Dedicated regular biomass 1.5 4: Emerging Technologies Wave Tidal Solar PV 2 Geothermal Figure 6.1: Overview of average ROC prices in the UK from October 2002 to April 2008 [55]. In the figure above an overview is given of the average ROC price over the past years in the UK. The figure shows that the ROC price is quite stable and even steadily growing over the past years Price of losses Based on the overview of the subsidy system in the UK for renewable energy, the losses in this study will be modeled with a fixed price of 100,- per MWh for the total operational life time. This value is in line with subsidy systems in other European countries and also with the previous Dutch subsidy system (e.g. before the SDE subsidy system the MEP subsidy system was valid which had a subsidy of 97,- per MWh for offshore wind energy). The value of the losses of an offshore wind farm over its whole operational life time of 20 years is significant. Therefore the sensitivity of the value per MWh of losses needs to be studied. The 87

88 value per MWh of electricity lost will therefore be taken into account in the sensitivity analysis. 6.2 Modelling investment costs In order to be able to define a cost model for both the HVAC and HVDC systems, all the different cost components in both systems need to be defined. For HVAC systems the total costs for an offshore wind farm grid connection system consist of the following components: HVAC cables (land and submarine) Cable installation (on land and at sea) Onshore substation Offshore substation Reactive compensation (both onshore and offshore) For an offshore wind farm with a HVDC grid connection system the total costs consist of the following components: HVDC cables (land and submarine) Cable installation (on land and at sea) Onshore converter station Offshore converter station The investment cost model is made with reference to specified costs in literature and by internal tender bids from manufacturers known within Evelop. All the different cost components will be given in the following paragraphs Cable cost components In order to be able to make a cost model for the cables used, it must be clear what type of cables will be used on land and at sea for both the HVAC and HVDC systems. In Appendix B the type of cables used offshore and onshore for the HVAC and HVDC VSC system are given. The cost model for the submarine and land cables will be observed separately Cost components submarine cables As is outlined in Appendix B HVAC submarine cables are three phase cables and submarine HVDC cables are single phase cables. Both the single and three phase submarine cables will have a copper conductor and lead sheath. Some of the main cost drivers for submarine cables are the copper conductor, the high quality XLPE insulation and the lead sheath. During the past years the prices of copper and lead have increased significantly. From the period of January 2003 up to December 2007 the copper price has increased from about US$1800,- per ton to a maximum of about US$8800,- per ton. In the same period the lead price has increased from about US$400,- per ton to a maximum of about US$4000,- per ton [51]. As can be seen in figures 6.2 and 6.3 the prices of both metals fluctuate continuously and therefore this causes a resulting fluctuating 88

89 cable price. Typically around 8.5% to 9.1% more conductor material and around 10% more lead is used for the production of a cable than would be required in the ideal case for the total volume of conductor and lead sheath required. Figure 6.2: Copper price history between January 2003 and December 2007 in US Dollar per ton [51]. Figure 6.3: Lead price history between January 2003 and December 2007 in US Dollar per ton [51]. Besides the changing material prices also the currency exchange rate between the US $ and the has changed significantly over the past years. Due to the large decrease in the currency exchange rate from $ to, the increase in the material prices is mitigated up to a certain extend. In figure 6.4 the currency exchange rate is shown from January 2003 up to December The currency exchange rate from US $ to has decreased from [ /$] on the 1 st of January 2003 to [ /$] on the 1 st of January 2008 [52]. 89

90 Exchange Rate [EURO/US$] /01/2003 Currency Exchange Rate USD vs EURO 07/01/ /01/ /01/ /01/ /01/ /01/2006 Date [MDY] 07/01/ /01/ /01/ /01/2008 Figure 6.4: History of currency exchange rate from US $ to EURO [52]. The total price in of the copper conductor and lead sheath needed per km cable, can be calculated as follows: Copper price for a cable: P copper = nv ρ p Y (36) conductor copper copper dollar vs euro Lead price for a cable: Plead = nvleadρleadp leadydollar vs euro (37) With: P copper : total raw copper price needed for a cable [k /km] P lead : total raw lead price needed for a cable [k /km] n: number of conductors [-] V conductor: volume of conductor required per km cable,.[m 3 /km] given as: V = C conductor conductor 10 V lead-sheath : volume of lead required per km cable, [m 3 /km] 2 2 Douter D inner 6 given as: Vlead = π π C conductor : conductor cross section [mm 2 ] ρ copper : density of copper: 8960 [kg/m 3 ] ρ lead density of lead: [kg/m 3 ] p copper : actual copper price [$/ton] p lead : actual lead price [$/ton] Y dollar-vs-euro : actual currency exchange rate [ /$] D outer : outer diameter of lead sheath [mm] D inner : inner diameter of lead sheath [mm] 90

91 In formulas (36) and (37) the only cable parameters which are generally unknown are the inner and outer diameters of the lead sheath. From data of manufacturers a typical lead sheath thickness of 2.5 to 2.8 mm is obtained. In a report from ABB [53] the outer diameter of single phase submarine DC cables for both 150 kv and 300 kv DC cables is given. With the assumption of typical thicknesses of the outer serving of 4 mm, the armor wires of 6 mm, the bedding of 0.8 mm and the inner jacket of 1.8 mm (e.g. typical submarine cable design as given in Appendix B) this gives an outer diameter of the lead sheath which is approximately 25 mm smaller than the outer diameter of the cable. In the figure below the approximated outer sheath diameter of submarine single phase HVDC cables is shown. 140 Outer sheath diameter of single phase submarine DC cables Outer Sheath Diameter [mm] Conductor Cross Section [mm2] 150 kv 300 kv Figure 6.5: Approximation of the outer sheath diameter for submarine DC cables with various conductor cross sections for ±150 kv and ±300 kv cables. For the three phase submarine HVAC cables data given by ABB [42] for the outer diameter of the insulation of single phase cables of 132 kv, 150 kv and 220 kv has been used. With typical thicknesses of the insulation screen of 0.7 mm and the swelling tape of 0.6 mm (e.g. typical submarine cable design as given in Appendix B) this gives an inner diameter of the lead sheath which is approximately 2.6 mm larger than the outer diameter of the insulation layer. In figure 6.6 the approximated inner sheath diameter of the submarine AC cables is given. With the approximations of the inner and outer lead sheath diameters for both submarine HVAC and HVDC cables the total lead costs for the cables for different conductor cross sections can be calculated. 91

92 Inner Sheath Diameter [mm] Inner sheath diameter of three phase submarine AC cables Conductor Cross Section [mm2] 132 kv 150 kv 220 kv Figure 6.6: Approximation of the inner sheath diameter for submarine AC cables for various conductor cross sections for 132 kv, 150 kv and 220 kv cables Costs components land cables As is outlined in Appendix B normally for both HVAC and HVDC land systems single phase cables are used with an aluminum conductor. Land cables don t require a protective armor layer and mostly don t have a lead sheath but instead a copper wire screen. The only main difference in the cable design for HVAC and HVDC systems is the thickness of the insulation. For an AC cable the insulation thickness has to be designed for the amplitude of the voltage and not for the effective RMS value which is normally given (e.g. an 150 kv AC cable has an RMS line voltage of 150 kv, but the amplitude is 2 or 1.41 times higher). Typical insulation thicknesses for different cable voltages of single phase land cables are [42], [53]: 132 kv AC land cable: 15 mm 150 kv AC land cable: 17 mm 220 kv AC land cable: 23 mm ±150 kv DC land cable:12 mm ±300 kv DC land cable:24 mm The aluminum conductor, the XLPE insulation and the copper screen wires are the important cost drivers for a typical land cable. The 132 kv AC, 150 kv AC and ±150 kv DC land cables typically have a copper screen of 95 mm 2 and the 220 kv AC and ±300 kv DC land cables typically have a copper screen of 185 mm 2 [42], [53]. From the given insulation thicknesses and sizes of the copper wire screen one can conclude that the 132 kv and 150 kv AC land cables will be quite similar in price as well as the 220 kv AC and ±300 kv DC land cables. As was the case for copper and lead prices over the past years, also the aluminum price has increased significantly. From the period of January 2003 up 92

93 to December 2007 the aluminum price has increased from about US$1400,- per ton to a maximum of about US$3200,- per ton [51]. Also the price of aluminum fluctuates continuously which has its effect on the cable price of aluminum land cables. Figure 6.7: Aluminum price history between January 2003 and December 2007 in US Dollar per ton [51]. The total price in of the aluminum conductor can be calculated in a similar way as for the copper conductor. For single phase land cables the copper wire screen is always given as the sum of the total cross sections of all the wires. As was given before, 150 kv DC, 132 kv AC and 150 kv AC cables typically have a copper wire screen of 95 mm 2 and 300 kv DC and 220 kv AC cables have a copper wire screen of 185 mm 2. The price of the aluminum conductor and copper wire screen in a single phase land cable can than be calculated as given below: With: Aluminum price cable: P Copper screen price cable: aluminum P = V ρ p Y (38) screeen conductor copper aluminum copper aluminum copper dollar vs euro = V ρ p Y (39) dollar vs euro P aluminum : price of raw aluminum needed for a single [k /km] phase land cable P screen : price of raw copper needed for screen for a [k /km] single phase land cable V conductor: volume of conductor required per km cable, [m 3 /km] given as: V = C conductor conductor 10 V copper : volume of copper screen required per km [m 3 /km] 6 cable given as: V = C copper screen 10 C conductor : conductor cross section [mm 2 ] C screen : copper screen cross section [mm 2 ] ρ aluminum : density of aluminum: 2702 [kg/m 3 ] ρ copper density of copper: 8960 [kg/m 3 ] p aluminum : actual aluminum price [$/ton] 93

94 p copper : actual copper price [$/ton] Y dollar-vs-euro : actual currency exchange rate [ /$] Y dollar-vs-euro : actual currency exchange rate [ /$] Cost model cables In order to make a cost model for the HVAC and HVDC submarine and land cables, the cost prices given in literature and tender bid prices of manufacturers known within Evelop [56] have to be compared. Therefore the type of cables must be specified (land or submarine), the conductor material (copper or aluminum), the conductor cross section and the date of publication of the price because of the significant increase in copper, aluminum and lead prices during the past years and the continuous fluctuating prices. When all data is present, several steps have to be taken in order to compare the various cable prices: Calculate the amount of copper, aluminum and lead required per km cable Look up the copper, aluminum and lead prices in the period when the cable cost price is given [51]. Look up the currency conversion rate in the period when the cable cost price is given [52]. Calculate the total price of the copper, aluminum and lead which is required for the cable per km based on the given material prices and currency conversion rates (equations (36), (37), (38) and (39)). Calculate the updated total material price to the reference material prices and currency conversion rates of a fixed date. Update the cable price with respect to the difference in material price between the reference date and the date of publication of the price Compare the cost prices for the different type of cables and calculate trendline. In Appendix E the original costs and the updated costs for three phase submarine AC cables, single phase submarine DC Cables, single phase AC land cables and single phase DC land cables are given. When the material prices have been updated to December 2007 [51] and the cable cost prices for various conductor cross sections are compared, the results are obtained as given in the following sections. 132 kv three phase submarine HVAC cables As can be seen in Appendix E six prices for 132 kv submarine cables have been found in several sources. When all the cost prices are updated and plotted a linear price trend can be calculated. The approximated linear trend is given as: P 132 kv, 3 phase AC submarine : C (40) 94

95 In the formula above P 132 kv is given in [M /km] and C is the conductor cross section in [mm 2 ]. In the figure below the result is shown for the approximated linear cost price for the 132 kv three phase submarine HVAC cable and the cost prices as they were given in literature. 1.2 Calculated price 3-phase copper submarine AC cables kv Price [MEURO/km] Conductor Cross Section [mm2] Calculated Price Given Price Sources Figure 6.8: Calculated price trend 132 kv submarine HVAC cables. 150 kv three phase submarine AC cable For the 150 kv three phase AC submarine cable three prices were given by manufacturers. When those given prices are updated and plotted, the price for a 150 kv submarine HVAC cable can be approximated by a linear trend given as: P 150 kv, 3 phase AC submarine : C (41) In the formula above P 150 kv is given in [M /km] and C is the conductor cross section in [mm 2 ]. As can be seen in figure 6.9 the tender bid prices for 800 mm 2 three phase submarine HVAC cable of 150 kv a quite far apart. Therefore the mean of these prices is used for the linear price approximation. 95

96 1.2 Calculated price 3-phase copper submarine AC cables kv Price [MEURO/km] Conductor Cross Section [mm2] Calculated Price Price Manufacturer Figure 6.9: Calculated price trend 150 kv submarine AC cable. 220 kv three phase submarine AC cable A 220 kv three phase submarine AC cable has been developed several years ago, but until now no large scale commercial projects have used this type of cable thus far. This is because of the fact that the submarine joints for this type of cables still need to be certified (e.g. which is currently taking place). Therefore the prices given in sources vary much more as was the case for the 132 kv and 150 kv three phase submarine cables. Besides prices given in sources also one price was given by a manufacturer. When the prices found are plotted, two linear price trends can be obtained as is shown in figure Price trend A comes closer to the price given by the manufacturer, but due to the large slope the prices for the cables with a conductor cross section of 400 and 500 mm 2 become unrealistic low. Therefore price trend B is probably more realistic, also when one has in mind that when commercial projects start using 220 kv three phase submarine HVAC cables, the production of this type of cable goes up and accordingly the price will go down. The price as given by the manufacturer will therefore most likely drop the coming years. A reasonable linear approximation for the price of a 220 kv three phase submarine HVAC cable can therefore be given as: P 220 kv, three phase AC submarine : C (42) In the formula above P 220 kv is given in [M /km] and C is the conductor cross section in [mm 2 ]. 96

97 1.2 Calculated price 3-phase copper submarine AC cables kv Price [MEURO/km] Conductor Cross Section [mm2] Calculated Price A Calculated Price B Price Manufacturer Given Price Sources Figure 6.10: Two different calculated price trends for 220 kv submarine HVAC cables. ±150 kv single phase submarine DC cable For ±150 kv submarine DC cables four prices have been found. When those given prices are updated and plotted, the price for a ±150 kv single phase submarine HVDC cable can well be approximated by a linear trend given as: P ±150 kv, single phase DC submarine : C (43) In the formula above P ±150 kv is given in [M /km] and C is the conductor cross section in [mm 2 ]. Calculated price 1-phase copper DC submarine cable 150 kv Price [MEURO] Conductor Cross Section [mm2] Calculated Price Given Prices Sources Price Manufacturer Figure 6.11: Calculated price trend ±150 kv submarine DC cable. 97

98 ±300 kv single phase submarine DC cable Four prices have been found for ±300 kv submarine DC cables of which one is from a manufacturer. No commercial ±300 kv HVDC transmission system has been built thus far, but 380 kv single phase submarine HVAC cables have been used in several submarine cable connections. Therefore the production of this type of cable is already being done. When one compares the prices found in literature with the price given by the manufacturer, there is quite a significant difference in price. When a linear cable price is approximated on basis of the prices found in literature (e.g. price trend A in figure 6.12), it turns out that this approximation is quite unrealistic because the price would be even below the calculated price trend for the ±150 kv DC submarine cable. Price indications by manufacturers are not always in line with each other as can be seen in figure 6.9 for the 150 kv three phase HVAC cable. Therefore it is assumed that the price for ±300 kv submarine HVDC cables will be in between the price given by the manufacturer and the prices found in literature. The manufacturing process of a ±300 kv DC submarine cable is similar as for the ±150 kv DC submarine cable. The only difference is the roughly doubled insulation thickness for the ±300 kv cable. Therefore the linear approximated price for ±300 kv DC submarine cables is assumed to have the same slope as for ±150 kv DC submarine cables, but a higher base price. With an increase in the base price of 45 k /km with respect to the ±150 kv DC submarine cable, the price trend B is obtained as shown in figure The linear approximation for the price of a ±300 kv submarine DC cable can be given as: P ±300 kv, single phase DC submarine : C (44) In the formula above P ±300 kv is given in [M /km] and C is the conductor cross section in [mm 2 ]. 98

99 Calculated price 1-phase copper DC submarine cable 300 kv Price [MEURO] Conductor Cross Section [mm2] Calculated Price A Calculated Price B Given Prices Sources Price ABB Figure 6.12: Two different calculated price trends for ±300 kv submarine DC cables. Single phase HVAC and HVDC land cables For HVAC and HVDC land cables fewer prices have been found. As was mentioned before, there is not much difference between HVAC and HVDC land cables for the different voltages. The price for 132 kv and 150 kv HVAC land cables are assumed to be similar and also the price for 220 kv HVAC and ±300 kv HVDC land cables are assumed to be similar. As can be seen in Appendix E only four prices were found for single phase land cables. The price ratio of the aluminum conductor and copper wire screens is only 10% to 20% of the total cable price. The slope of the cost price will therefore be much lower as for submarine cables because of the lower price for the aluminum conductor and the easier manufacturing process of single phase land cables than for (three phase) submarine cables. Three prices have been found for 150 kv HVAC single phase land cables. Based on these prices found a linear price trend can be given for the 132 kv and 150 kv HVAC land cables as: P 132 kv & 150 kv, single phase AC land : C (45) In the formula above P 132 kv & 150 kv is given in [M /km] and C as the conductor cross section in [mm 2 ]. For the ±150 kv single phase HVDC land cable no given prices have been found. Due to the fact that the only difference with the 132 kv and 150 kv HVAC land cable is a several mm thinner insulation layer, it is assumed that the price trend will have a similar slope but a slightly lower base price. A linear price trend for the ±150 kv HVDC land cable can than be given as: 99

100 P ±150 kv, single phase DC land : C (46) In the formula above P ±150 kv is given in [M /km] and C as the conductor cross section in [mm 2 ]. One price has been found for a ±300 kv HVDC land cable. As was mentioned before the thicker the insulation the more complex it becomes to maintain the quality of the insulation. With respect to the price trend for 220 kv HVAC and ±300 kv HVDC land cables it is therefore assumed that the slope of the linear price approximation is the same as for 132 kv and 150 kv HVAC land cables, but the base price will be higher. 220 kv HVAC land cables have already been used, so the linear price approximation is made such that it intersects the given price for a ±300 kv DC land cable which is typically similar to the 220 kv HVAC land cable. The base price will be about 45 k /km higher which seems to be reasonable when one takes the more difficult manufacturing process into account. The linear approximation for 220 kv HVAC and ±300 kv HVDC land cables can than be given as: P 220 kv AC & ±300 kv DC, single phase land : C (47) In the formula above P 220 kv AC & ±300 kv DC is given in [M /km] and C is the conductor cross section in [mm 2 ]. In the figure below the linear price trends and the prices found for land cables are given. Calculated price aluminum single phase AC and DC land cables Price [MEURO] Conductor Cross Section [mm2] 132 kv and 150 kv AC 220 kv AC and 300 kv DC 150 kv DC Given Price Source kv AC Given Price Source kv DC Price Manufacturer kv AC Figure 6.13: Calculated price trend for AC and DC land cables Conclusions cable cost model In the previous sections a cost model for HVAC and HVDC cables has been given. Based on many sources found and prices given by manufacturers, linear approximations have been made for the prices of submarine and land HVAC and HVDC cables. For the type of cables where several prices were given, the linear 100

101 approximations seem to fit the data quite well. For the type of cables where little or no price information was found, assumptions have been made based on typical cable designs and assumptions with respect to the manufacturing process. The final linear cost model for cables is given as: With: P = αc + β (48) P: price of single or three phase cable [M /km] α, β: constants [-] C: conductor cross section [mm 2 ] The constants α and β for the different type of cables are given as: Table 6.2: Constants α and β of cost model for different type of cables. System Voltage Submarine cable Land cable α β α β 132 kv * AC 150 kv * kv * DC ±150 kv * ±300 kv * As was mentioned 220 kv three phase submarine HVAC cables have not been used in any commercial project thus far. Therefore the production of this type of cable is limited and accordingly the prices found in literature or given by manufacturers will differ from each other or be relatively high. Some assumptions are made in order to come to a cost model for this type of cable. Because the actual price for these cables can vary with the modeled price a sensitivity analysis needs to be made which takes price fluctuations into account in order to see how well the results will be with respect to the modeled cable price Cost model cable installation The costs for cable installation differ between onshore and offshore installation and also between HVAC and HVDC cables. The price for cable installation offshore and onshore depends on many factors. In the next sections the cost model for the cable installation onshore and offshore will be given for both HVAC and HVDC cables Installation costs three phase submarine HVAC cables For offshore cable installation large vessels are required with large turntables. The amount of vessels available depends on the total cable length and weight. These vessels have a high day rate and therefore the total installation time required is of high importance. The time required for offshore cable installation depends on the total cable length and also to a large extend on the weather conditions. Because of too large waves the available time for installation can be 101

102 reduced. The effect of weather down time on the costs fully depends on the type of contract an offshore wind farm developer has with the installation company. The weather risk can be spread evenly or one of the companies can take the full risk. Also the type of seabed and the morphology of the seabed have their influence on the total costs as well as shipping lane crossings or crossing with other cables or pipes. In several sources and from internal data known within Evelop the following cost figures have been found for three phase submarine HVAC cables: Table 6.3: Installation costs three phase submarine HVAC cables. Installation Approximate Cable Conductor Costs [k /km] Diameter Cross section Cable Voltage Source mm 500 mm² 132 kv [39] mm 800 mm² 132 kv [39] mm 800 mm² 132 kv [58] mm 800 mm² 150 kv [56] mm 1000 mm² 132 kv [39] mm 500 mm² 220 kv [39] mm 800 mm² 220 kv [39] [63] [62] As can be seen in the table above the installation costs for three phase submarine HVAC cables ranges from about 240 k /km to 310 k /km. Because there are too many factors which in the end determine the installation costs, the installation costs for submarine HVAC cables used in the cost model will be a fixed price per km. Based on the installation costs as given above a mean price of 275 k /km will be used for the installation of three phase submarine AC cables Installation costs single phase submarine HVDC cables As was mentioned before the installation costs for submarine cables depend on the type of cable (e.g. single phase or three phase cable), the cable weight and length and the weather. Single phase submarine HVDC cables have a smaller diameter than three phase submarine HVAC cables, but HVDC systems require in general two cables. The Cross Sounds Cable and Estlink HVDC projects used only one ship to transport and install both cables at the same time [22]. Installation costs given in literature range from 200 k /km [62] to 568 k /km [39]. Probably the latter cost is in the case the two cables are installed independently. Due to environmental restrictions with respect to the electromagnetic field, the cables in general have to be buried in the same trench which can be done by a single ship. With the high day rate of installation ships and the assumption that a few more ships are available which can install two single phase submarine DC cables than one three phase submarine AC cable with the same power rating (i.e. due to the 102

103 lower weight in general of two single phase submarine HVDC cables compared to a single three phase submarine HVAC cable), the installation costs for two single phase submarine DC cables is approximated at 250 k /km Installation costs single phase land cables For the installation of single phase land cables no advanced machines are required of which there is only a limited number of. Also the installation is not as weather dependant. Therefore the installation costs for land cables are in general lower than for submarine cables. The installation costs do depend on the cable route (e.g. can the cable be laid along roads, how many roads or rivers need to be crossed with a directional drilling, etc.). Approximately every km of land cable a cable joint is required due to the limitations in cable drum size and transportation limitations. For the HVDC system only two cables are used, so approximately every km two joints have to be made. For the HVAC system onshore three single phase cables are used which are cross bonded (e.g. the earthed screens are cross linked at every joint in order to minimize losses due to induced currents in the screens). Therefore the installation costs for three single phase HVAC land cables will be higher than for two single phase HVDC land cables. In literature installation costs for three single phase HVAC land cables of k /km is given [63]. This is in line with the installation costs known within Evelop [56]. Therefore the installation costs for three single phase HVAC land cables is approximated at 175 k /km. In literature costs for the installation of two single phase HVDC land cables are in the range of 100 k /km to 200 k /km [60], [62]. As was indicated the onshore cable installation cost depend on the number of directional drillings which are required to cross road, rivers and/or buildings. Therefore the latter might be the case when many and/or long directional drillings are required and the first indication of the installation with mainly trenches. Because the installation of two single phase HVDC land cables will be cheaper than the installation of three single phase HVAC land cables, the installation costs for HVDC land cables is approximated at 150 k /km when is assumed that some directional drillings will be required in general Cost model onshore and offshore substations Costs for the supply and installation of onshore and offshore substations take up a large part of the total costs for HVAC and HVDC electrical systems. HVDC converter stations are much larger and complex than HVAC substations and therefore more costly. For HVAC also not always a substation with transformer is required. When the transmission voltage is equal to the grid voltage at the grid connection point, only substation works are required on land. In the following paragraphs the substation and converter station costs of HVAC and HVDC systems are given. 103

104 Costs onshore HVAC substation The costs for the onshore HVAC substation depend on the transmission voltage and the grid voltage at which the wind farm will be connected. When the cable voltage is equal to the grid voltage no transformer is needed and only substation works are required. The substation works include circuit breakers, protection systems, grounding switches, cable switches, land costs, man hours for installing the system, etc. Typical costs are between 550 k and 750 k for a single three single phase cable system [56]. These costs are location specific because not all grid connection points are evenly complex and don t all have space readily available for placing the switch gear, protection, control systems, etc. Therefore the costs for substation works can go up to several M per three phase AC system [64]. The cost approximation for substation works is set at 1 M per three phase AC cable system. In case the cable voltage is not equal to the grid voltage also an onshore transformer is required. The total costs for an onshore HVAC substation including switch gear, circuit breakers, control system and installation as found in literature range from 21.5 k /MVA to 26.5 k /MVA [39], [57], [63]. These costs are without land purchasing costs taken into account or payments to the TSO for the grid connection. Therefore the cost approximation for an onshore HVAC substation including transformer is set at 25 k /MVA and additional 1 M for the connection of each three phase AC system to the onshore grid Costs offshore HVAC substation The costs for the offshore HVAC substation consist of costs for the foundation, the design of the compact platform with all electrical equipment which is protected to the harsh weather conditions at sea and the installation costs. In literature cost indications are in the range of 30 k /MVA to 35 k /MVA [39], [57], typically without foundation costs. Internal cost data known within Evelop are in the range of 125 k /MVA to 140 k /MVA [56]. The cost indications as given in literature seems to consist of the basic costs for the electrical equipment as used on land plus the installation costs without foundation and full substation design. The actual costs for an offshore substation consist of: the foundation including a transition piece with a number of J-tubes for the connection of the infield and transmission cables and a boat landing arrangement a compact designed substation which consists of: o a three phase transformer for stepping up the infield cable voltage to the transmission voltage o low voltage and high voltage switch gear o cable terminations o protection, control and instrumentation systems o cooling systems o earthing systems 104

105 o an emergency generator o staff and service facilities o a helipad (optional) o fire extinguishing equipment o cranes o man over board (MOB) boat o etc. the installation of the foundation and the substation platform testing and commissioning The offshore substation also must be able to withstand the harsh salty and moist marine environment, so careful design of the electrical equipment and the cooling system is required. The costs mentioned in literature are therefore on the very low side and the total costs for the design and installation of the offshore HVAC substation including foundation are approximated at 130 k /MVA. To get an indication of the design of an offshore HVAC substation the offshore substation of the Princess Amalia Wind Farm (e.g. former Q7 ) is shown in the figure below. Figure 6.14: The offshore substation of the Princess Amalia Wind Farm offshore wind farm [65]. The 140 MVA offshore substation of the Princess Amalia Wind Farm offshore wind farm has a total weight of more than 500 tons and a size of 10 by 20 meters and 12.5 meter high [65]. 105

106 Costs onshore HVDC converter station An onshore or offshore HVDC VSC converter station is a complex design with much more components than an HVAC substation and a much larger footprint. As was already mentioned the HVDC VSC technology is about ten years old and thus far only three large scale projects have been developed (e.g. the Cross Sound Cable project of 330 MW, the Murraylink of 220 MW and the Estlink of 350 MW [22]). As was mentioned before only ±150 kv HVDC VSC systems have been developed thus far and no ±300 kv HVDC VSC project is planned at the moment. Due to the constant development and the few projects developed thus far, only little cost information has been found in literature. The cost indications found are given in the table below and they include supply and installation costs. Table 6.4: Cost information found for onshore HVDC VSC converter stations. Price [k /MW] Publication Date System Voltage Source 110 October 2004 ±150 kv [67] 110 May 2005 ±150 kv [60] 100 March 2004 ±150 kv [68] 82.5 January 2007 ±300 kv [56] 97 August 2006 ±300 kv [61] 74 December 2007 ±300 kv [59] As can be seen in the table above the published cost estimates were initially around 110 k /MW for ±150 kv converter stations. The three latest cost estimates are in the range of 74 k /MW to 97 k /MW for ±300 kv converter stations. The reduction in the cost prices given can be explained because of the further development of the HVDC VSC technology which requires fewer components due to a more advances control and switching scheme than was the case several years ago. Also the IGBT design has been improved so higher current densities can be withstand [45]. In order to define the difference in costs between a ±150 kv and a ±300 kv converter station one has to look at the basic design components of a HVDC VSC converter station. In figure 6.15 the basic design components of a converter station are shown. Of these components the valves are the most important (e.g. they are the basic components for the conversion process) and also the most expensive components. The harmonic filter, the phase reactor and the DC capacitor will be more expensive for the ±300 kv converter station because of the higher operational voltage. On the total converter station costs the valves, consisting of advanced IGBT s and diodes, take up a the largest part of the component costs though. Therefore the general cost comparison between a ±150 kv and a ±300 kv station will be based on the costs for these valves. 106

107 Figure 6.15: Simplified line diagram of a HVDC VSC converter station with the major components of the system [45]. In order to withstand the high voltage, many IGBT s have to be connected in series. For handling the total current which can go up to several ka (e.g. depending on the total power to transmit), one can either connect more strings of IGBT s in parallel or increase the semiconductor active area in the IGBT modules, improve the cooling (e.g. heating up of the IGBT s limits the current handling due to thermal limitations) and optimize switching (e.g. the switching frequency is a tradeoff between losses and controllability: a higher switching frequency increases the controllability but also causes higher losses) [45]. For the same power handling a ±150 kv converter requires double the current handling as for the ±300 kv converter, but at the same time only half of the voltage needs to be withstood. In general the expected number of IGBT s needed will be about the same and therefore the costs for the valves are expected to be similar. In a paper written by Jacobson about the design of the ±300 kv converter station is stated: Converters are usually more cost-efficient in the high current end of the spectrum, in terms of money per MW [45]. This states that costs for the converter station are dependant on the design and the type of IGBT s used though. Because of too little detailed information about the cost set-up of the HVDC VSC converter station for the different voltages, the costs for a ±150 kv and ±300 kv converter station are approximated at about 85 k /MW and 93 k /MW correspondingly (i.e. 10% higher costs are assumed for the ±300 kv system due to the higher costs for the harmonic filter, the phase reactor and the DC capacitor because of the higher operational voltage). Due to the assumptions made and the fact that no commercial ±300 kv HVDC VSC project has been made or is planned yet, also for the HVDC VSC converter stations a sensitivity analysis is required in order to see how sensitive the results are with respect to the approximated converter station price. 107

108 Costs offshore HVDC converter station As was mentioned before up till now only two HVDC VSC converter stations have been placed offshore (e.g. two 40 MW converter stations were placed at the Troll A gas platform near the coast of Norway [70]). These are the first offshore HVDC VSC converter stations. The first offshore HVDC VSC system for offshore wind is planned in Germany: the 400 MW Nord E.ON 1 project [70] which will be commissioned in As was mentioned before a HVDC VSC converter station has a large footprint. An example of an offshore converter design made by Siemens is shown in the figure below [66]. The dimensions of the 675 MW offshore converter station design are 50 by 50 meters and 28 meter high which is enormous. Figure 6.16: Example of a design of a 675 MW offshore HVDC VSC converter station made by Siemens [66]. In the table below the few cost indications found in literature and internal cost details known within Evelop are shown. Table 6.5: Cost data found for an offshore HVDC converter station. Price [k /MW] Publication Date System Voltage Source Remark 225 November 2006 ±150 kv [39] Module design incl. platform, foundation and installation 175 January 2007 ±300 kv [56] Module design incl. platform, foundation and installation 128 December 2007 ±300 kv [59] August 2006 ±300 kv [61] Module design incl. platform, foundation and installation 108

109 As can be seen in the table above the price indication for both the ±150 kv and the ±300 kv converter station are in a wide range of 175 k /MW to 225 k /MW including all the costs for the module design, the platform, foundation and installation. Probably the cost as indicated in the paper of Airtricity and National Grid [59] doesn t cover all the costs which need to be taken into account (i.e. probably no installation costs and costs for the foundation). The same cost assumptions can be made for the offshore HVDC VSC converter station as was done for the onshore converter station. Because of too little details on the cost build up for the ±150 kv and ±300 kv converter station it is again assumed that the costs for a ±300 kv converter station are 10% higher due to higher costs for the harmonic filters, the phase reactors and DC capacitors as compared to a ±150 kv converter station. The cost prices for offshore HVDC VSC converter stations are approximated at 200 k /MW and 220 k /MW for the ±150 kv and ±300 kv systems correspondingly. Because of the broad range of price indications and the fact that no ±300 kv HVDC VSC commercial system has been developed yet, also for the price of an offshore HVDC VSC converter station a sensitivity analysis has to be performed in order to see what affect it has on the results Cost model reactive compensation for HVAC systems As was indicated before a cable generates reactive power under AC operation. The larger the cable length, the more reactive power is generated. In order for HVAC systems to comply with the grid connection demands (e.g. power fed into the grid within power factor limits) and increase the active power transmission capacity of the cable used, reactive power compensation has to be applied. The reactive power generated by a cable under AC operation is independent of the loading of the cable. The total reactive power generated also depends on the power factor of the power of the wind farm fed into the cable. In order to comply with the grid demands in general a fixed, passive shunt reactor is sufficient as was already explained in paragraph 5.5. In case of a connection to a weak grid (e.g. points in the grid where large fluctuations in power fed into the grid have a significant effect on the local voltage and frequency of the grid which in general is the case at the end of a transmission line of the grid where the power demand and power transmission capacity is in general low), the TSO can request for a controllable reactive power compensation system such as a SVC or a STATCOM. Because of the fact that in general a passive shunt reactor is sufficient for the reactive power compensation and this type of reactive power compensation is much cheaper than a SVC or STATCOM, also passive shunt reactors will assumed to be used for HVAC systems both onshore and offshore. In the table below cost indications as found in literature and from internal cost data known within Evelop are shown. 109

110 Table 6.6: Cost indications for onshore and offshore shunt reactors. Price [k /MVAr] Type Source 21 onshore [39] 27.4 onshore [56] 35 offshore [39] The cost indications as given in the table above include the design, supply and installation of the shunt reactive power compensation system. The cost indication for reactive power compensation onshore found in literature is in line with internal cost data known within Evelop. The price for onshore shunt reactive power compensation is therefore approximated at 25 k /MVAr. The offshore reactive power compensation has to be added to the offshore HVAC substation platform and needs to be added to the design of the offshore substation. Therefore the price for offshore reactive power compensation of 35 k /MVAr seems reasonable as is indicated in [39]. This does depend on the space required for the reactive power compensation though. As an indication the size of a 52 MVAr shunt reactive power compensation system is 4.4 x 2.15 x 3.8 m (L x W x H) [56]. Nevertheless the price per MVAr is set at 35 k for offshore reactive power compensation Verification cost model HVDC VSC system In the previous paragraphs the cost model for HVDC VSC converter stations and HVDC XLPE cables has been made. In order to get an indication how well the assumptions and approximations made fit the real costs of a commercial HVDC VSC project, a cost comparison is made with respect to the specified costs for commercial projects of ABB [22]: Murraylink ±150 kv HVDC VSC project: o 2 x 220 MW onshore converter station o 2 x 177 km 1200 mm 2 and 1400 mm 2 aluminum land cable (e.g. different cross section along the cable trajectory due to non isothermal thermal properties of the soil) o project price 100 million US$, approximately 108 M (contract date mid 2000, [69]) Cross Sound Cable (CSC) ±150 kv HVDC VSC project: o 2 x 330 MW onshore converter station o 2 x 40 km 1300 mm 2 copper submarine cable o project price 120 million US$, approximately 130 M (contract date mid 2000, [70]) Estlink ±150 kv HVDC VSC project: o 2 x 350 MW onshore converter station o 2 x 31 km 2000 mm 2 aluminum land cable o 2 x 74 km 1000 mm 2 copper submarine cable o project price 144 million US$, approximately 110 M (contract date beginning of 2005, [71]) 110

111 Nord E.ON 1 ±150 kv HVDC VSC project: o 1 x 400 MW onshore converter station o 1 x 400 MW offshore converter station o 2 x 75 km 2300 mm 2 aluminum land cable o 2 x 128 km 1200 mm 2 copper submarine cable o project price 400 million US$, approximately 291 M (contract date September 2007, [72]) With the characteristics of all four HVDC VSC projects known, the costs for all components as given in the cost model can be calculated and summed to see how well if reflects the actual costs specified for the four projects. Before the comparison is made, first the project prices are updated to the price level of the end of 2007, based on the inflation history in Europe as can be seen in the figure below. Figure 6.17: Inflation history in Europe [76]. As can be seen the average inflation in Europe is about 2% from 2001 onwards. This gives the updated project prices for mid 2007 as follows: 7.0 = Murraylink: 108 ( 1.02) M 7.0 = CSC: 130 ( 1.02) M 2.5 = Estlink: 110 ( 1.02) M 0.0 = Nord E.ON 1: 291 ( 1.02) M For the onshore HVDC VSC converter stations of the Murraylink and CSC project an initial cost price of 110 k /MW is assumed as this price was mentioned in oldest literature sources from 2004 and Possibly the cost price of the converter stations of these first two large scale ±150 kv commercial HVDC VSC projects are even higher. For the cable price used the inverse process is used as was the case for updating the cable costs found in literature. The material prices for aluminum, copper and lead have been looked up as well as the currency exchange rates at the period the contracts have been signed. The difference in material cost prices 111

112 for the cables used, have been subtracted from the cable price as specified in the cost model. By doing this an estimation is obtained for the cost price of the cables at the time of the development of the projects. For the land cable of the Murraylink project no details were given on the lengths of the 1200 mm 2 and 1400 mm 2 land cable. Therefore the mean conductor cross section of 1300 mm 2 has been assumed for the full cable length. With the assumptions as given above, the cost prices for the four projects as obtained from the cost model are shown in the table below. Project Table 6.7: Cost comparison of actual project costs and costs calculated with the HVDC VSC cost model. Land Cable [M ] Submarine Cable [M ] Substation [M ] Total Cost [M ] Supply Installation Supply Installation Onshore Offshore Model Project Difference [M ] Murraylink CSC Estlink Nord E.ON As can be seen in the table above the calculated costs don t fit the given project costs exactly. The cost model should fit the costs prices of the most recent projects the best (e.g. the Estlink and Nord E.ON 1 projects). The Murraylink and CSC projects are the first two large scale projects. Their calculated cost prices are respectively 21.9% and 33.2% too low. Because of the fact that these are the first two large scale ±150 kv HVDC VSC projects, the prices for the different components, especially for the converters, are most likely much higher as were given in the cost model. Therefore the cost model will only be compared with the two most recent projects, the Estlink and Nord E.ON 1 projects, for which the cost model must reflect the actual costs much closer. The calculated model prices for the Estlink and Nord E.ON 1 projects are respectively 5.1% too high and 17.7% too low. Several reasons can be given to explain the difference in price of which the most important one is that the prices of the cost model are generalized costs which don t take specific project details into account. The project details can have a significant influence on the final cost price of all the components, especially for large scale projects. In order to get more insight in where the difference in costs might come from, both projects are observed in more detail. For the Estlink project the main cost components are the submarine cable and the two onshore converter stations. The difference in model price is expected to come from either of these components. In the cost model the chosen cable price curve matches the given cable prices quite accurate. Therefore the difference in price is expected to come from the converter costs and not from the cable supply and installation costs. If the actual price for the converters was not set at about 85 k /MW but at 75 k /MW (e.g. the lowest cost price as given in literature, see table 6.4), this gives a difference of -7 M on the total project costs which brings 112

113 the calculated costs very close to the given project price of M. Therefore an onshore converter price of 85 k /MW might be too conservative. For the Nord E.ON 1 project the main cost components are also the submarine cable and the converter stations. As was explained above the difference is expected not to come from the submarine cables, but mainly from the costs for the converter stations. Besides these costs, also the costs for the onshore cable installation are observed in more detail. In the figure below an overview of the project is given. Figure 6.18: Overview of the Nord E.ON 1 HVDC VSC project of ABB [22]. As was given the offshore wind farm will be placed 128 km offshore. The water depths at the site are approximately around 30 meters. The very large distance to shore increases the installation costs significantly and the large water depth gives additional costs for the design of the foundation of the offshore converter station. The specified costs for offshore HVDC VSC converter stations are probably based on characteristics of most planned offshore wind farms nowadays (e.g. a distance to shore of 25 to 40 km and water depths around 20 to 25 meter). This converter station will also be the world s first offshore HVDC VSC converter station including foundation and full module design. With the highest specified cost of 225 k /MW in literature, a price in the order of 300 k /MW for the first complete offshore HVDC VSC converter station installed 128 km from shore and for the large water depth seems reasonable. 113

114 When one looks at the onshore trajectory for the 75 km long land cables, one can see that it crosses quite densely populated areas [73] with many roads and rivers. Because of this large onshore trajectory and the many directional drillings which will be required for the crossing of all the roads and rivers, the costs for the onshore cable installation can go up significantly. An onshore installation cost price for the DC land cables of 250 k /km seem to be reasonable with respect to the 150 k /km as specified previously in the cost model (e.g. the basic costs for digging the trench and making all the required joints and some directional drillings, without too many specialties taken into account). When one takes the expected higher costs for the offshore HVDC VSC converter station, the higher costs for the installation of the land cable and the expected lower costs for the onshore HVDC VSC converter station (e.g. as was done for the Estlink project) into account, this causes an expected price increase of 43.4 M. This would bring the total calculated project price to M and the difference in price is then approximately 2.8% too low. This observation shows that the specified cost for an offshore converter station is most probably too optimistic which results in a large difference in the total costs for large scale projects. Therefore the price for an offshore ±150 kv HVDC VSC converter station will most likely be in the range of 250 k /MW to 300 k /MW Verification cost model HVAC system Besides the verification of the HVDC VSC cost model, also the cost model for HVAC systems needs to be verified. The verification will be done on basis of two commercial submarine HVAC projects with an offshore substation used for offshore wind farms [74]: Horns Rev offshore wind farm: o 160 MVA offshore substation o 21 km 630 mm kv copper submarine cable o 34 km of 3 single phase 1200 mm kv aluminum land cables o 80 MVA onshore reactive power compensation o price interconnection to onshore grid approximately 40 M (contract date mid 2001, [74]), without onshore substation works Nysted offshore wind farm: o 180 MVA offshore substation o 11 km 630 mm kv copper submarine cable o 18 km of 3 single phase 1200 mm kv aluminum land cables o 40 MVA onshore reactive power compensation o Price interconnection to onshore grid approximately 30 M (contract date beginning 2002, [74]), without onshore substation works With the characteristics of both submarine HVAC projects known, the costs for all components as given in the cost model can be calculated and summed to see how well if reflects the actual costs specified for the two projects. Both project 114

115 prices come from 2001 and 2002, so first the project prices are updated to the price level of mid 2007 with an expected mean inflation rate of 2% (e.g. see figure 6.17): 6.0 = Horns Rev: 40 ( 1.02) 45.0 M 5.5 = Nysted: 30 ( 1.02) 33.5 M The cable cost price is adapted by taking the difference in material prices for aluminum, copper and lead into account between 2001 and now, as well as the currency exchange rates at the period the contracts have been signed. The difference in material cost prices for the cables used, have been subtracted from the cable price as specified in the cost model. By doing this an estimation is obtained for the cost price of the cables at the time of the development of the projects. With the assumptions as given above, the cost prices for both projects as obtained from the cost model are shown in the table below. Table 6.8: Cost comparison of actual project costs and costs calculated with the HVAC cost model. Project Land Cable [M ] Submarine Cable [M ] Substation Reactive Total Cost [M ] Difference Supply Installation Supply Installation Offshore [M ] compensation [M ] Model Project [M ] Horns Rev Nysted As can be seen in the table above, the calculated costs are overestimated based on the costs as given in the model, but the results are in the right order. The calculated price for the Horns Rev offshore wind farm is 12.9% too high and for the Nysted offshore wind farm 10.4% too high. As was the case for the HVDC VSC projects, also for the Horns Rev and Nysted HVAC offshore wind farm projects the details and time of development have to be observed more closely in order to explain the differences in price. When one looks at the costs of the different components for both projects, the costs for the offshore substation are by far the largest. A small change in the cost price per MVA can result in a large price difference of the total project cost estimate. Besides the large price increase for copper, aluminum and lead, also the price of other metals and steel has risen significantly as can be seen in figure

116 Figure 6.19: Historical price trend of steel in % (price at end of 2001 = 100 %) [75]. For the manufacturing of a regular offshore substation (typically with a monopile or tripod foundation) a lot of steel has to be used. With the large increase in metal prices a price estimate of 110 k /MVA for the offshore substation seems reasonable at the time the Horns Rev and Nysted offshore wind farms were developed, though they were the first offshore wind farms with an offshore substation. The Nysted offshore wind farm used a gravity based foundation for the offshore substation, but nevertheless a lot of steel for the offshore substation platform needed to be used. Therefore a price of 110 k /MVA also seems realistic for the offshore substation of the Nysted offshore wind farm. With this assumption the price for the substation would decrease by 3.2 M and 3.6 M for the Horns Rev and Nysted offshore wind farm respectively. With this assumption the calculated project prices become only 2.6 M (e.g. 5.8%) too high for the Horns Rev offshore wind farm and 0.1 M (e.g. 0.3%) too low for the Nysted offshore wind farm. The Horns Rev offshore wind farm became operation at the end of 2002 and the Nysted offshore wind farm at the end of Possibly there also was a price difference between the offshore substations of these offshore wind farms, but due to a lack of information no further insight can be given. These observations show that the HVAC cost model fits the actual costs quite well Conclusion cost model for HVAC and HVDC VSC systems In the previous paragraphs the cost models for HVAC and HVDC VSC systems have been developed. The cost components observed for both systems were: For the HVAC system: o Costs land and submarine cable supply o Costs onshore and offshore cable installation o Costs supply and installation of onshore and offshore substation o Costs of onshore and offshore reactive power compensation 116

117 For the HVDC VSC system: o Costs land and submarine cable supply o Costs onshore and offshore cable installation o Costs supply and installation of onshore and offshore converter station Most data has been found regarding the cost price of the different type of cables for both HVAC and HVDC VSC. With internal cost data known within Evelop [56] most of these cost prices could be verified or known data was in line with the data found in literature. This was also the case for the prices for the cable installation, the onshore and offshore substation and for the reactive compensation for HVAC systems. The HVAC cost model has been compared with two commercial HVAC systems used for the Horns Rev and Nysted offshore wind farm. It became clear that the HVAC cost model fits the actual costs of these commercial projects quite well. Less cost information and details have been found regarding the costs for HVDC VSC converter stations. With the comparison of the HVDC VSC cost model with two commercial projects, it became clear that project details can have a significant impact on the total costs of the different cost components in the system. Especially when large systems are developed of several hundreds of MW, the difference can become high with only a slight change in cost price per MW for the converter stations. The initial cost price of 200 k /MW for an offshore ±150 kv substation seemed to be too optimistic and a price in the range of 250 k /MW to 300 k /MW comes closer to actual prices of commercial projects nowadays. Therefore the price per MW is increased from 200 k to 250 k for an offshore ±150 kv HVDC VSC converter station and a price of 275 k for an offshore ±300 kv HVDC VSC converter station. Also the price for cable installation can be significantly higher, especially on land as was indicated for the Nord E.ON 1 project which has a very large onshore trajectory where many directional drillings will be required. Therefore project details need to be taken into account in order to come to a more accurate costs estimate. The final results of the HVDC VSC cost model fit the prices of the most recent commercial projects quite well. Concluding can be stated that both the HVAC and HVDC VSC cost model are reasonably accurate, at least for the commonly used technologies (e.g. for the 220 kv three phase submarine AC cable and the ±300 kv HVDC VSC system quite some assumptions had to be made in the model which can not be verified yet by commercial projects or many published cost indications). When a certain project has more special characteristics with respect to the cable installation (e.g. for example when there is a large onshore cable route and possibly many directional drillings are required for road and river crossings) or the offshore substation (e.g. installation at a large distance from shore and/or at larger water depths up to 30 meter or more), these special circumstances need to be taken 117

118 into account in the model. Overall the results of applying a sensitivity analysis gives better insight in the results of the costs as specified in the cost model. 6.3 Financial optimization of grid connection As was mentioned before the optimization of the grid connection will be based on the costs of losses of a grid connection system over the entire lifetime of the wind farm and the investment costs of the grid connection system. The modeling of the costs of both components is different because the investment will be done before the wind farm becomes operational and the losses occur during the 20 year operational lifetime of the offshore wind farm. Therefore the costs are modeled by the Net Present Value (NPV). The optimization of the grid connection will be based on the minimization of the NPV. The NPV is defined as the total present value of time series of cash flows. It is a standard method for using the time value of money to appraise long term projects. The formula for the NPV is given as: With: Ct NPV = C0 + (49) + t (1 r) NPV: Net Present Value [M ] C 0 : initial investment [M ] C t : cash flow at year t [M ] t: year [-] r: discount rate [-] In equation (49) the value of the discount rate r is extremely important because it is a measure for the current value of future cash flows. Most often a companies Weighted Average Cost of Capital (WACC) is used. For projects with high risks it is appropriate to use higher discount rates to adjust for higher risks in a project. With the development of offshore wind farms many risks are in the project. As will be shown later on the financing of a wind farm is in general about 70% by other investors, in many cases a bank. Banks might charge a higher rate of interest for projects with high risks. The Weighted Average Cost of Capital (WACC) is the rate that a company is expected to pay to finance its assets. WACC is the minimum return that a company must earn on existing asset base to satisfy its creditors, owners and other providers of capital. WACC is calculated taking into account the relative weights of each component of the capital structure. In order to be able to calculate the WACC, the financial structure of the financing of an offshore wind farm needs to be known. Most often offshore wind farm developers don t have enough capital for financing an offshore wind farm on their own. In general 30% of the investment is paid from 118

119 own capital and 70% comes from investors, most often a bank [34]. Based on this the WACC is calculated as follows: With: r = 0.3 y (b + t ) (50) c r: discount rate [-] y: required rate of return on equity, own invested capital [%] b: required rate of return on borrowings [%] t c : credit margin [%] General values for the different rates are given as [34]: y: 9% - 11%, typically 11% b: 4.8% - 5.2%, typically 5% t c : 1.5% - 2.0%, typically 1.8% With the values for the different rates as given above the discount rate r in general will be in the range of: r: 7.11% %, typically 8% The value used for the discount rate in the model is important, because a relatively low value increases the impact of losses with respect to the investment costs, while a high rate decreases the impact of losses with respect to the investment costs. Therefore in the sensitivity analysis performed also the discount rate r will be varied. Also with the modeling of the losses the development of multiple offshore wind farms over the years must be taken into account in case of a coupled grid connection (i.e. the yearly losses will vary based on the number of offshore wind farms which are operational each year). 6.4 Structure of developing an offshore wind farm In the previous paragraphs the economical model and the optimization process have been indicated in order to determine what the most optimal grid connection would be, based on the technical characteristics of the technologies available. The goal of this project is to study the feasibility of a coupled grid connection of multiple offshore wind farms. This feasibility is mainly based on technical and economical criteria. A coupled grid connection increases the complexity of the development of an offshore wind farm. Therefore more insight must be given in the complexity and the risks of the development of an offshore wind farm in order to get insight in the practical feasibility of a coupled grid connection. The focus of this study remains on the economical feasibility though. As was mentioned before the financing of an offshore wind farm is in general 30% by own capital and for 70% by investors which are in general banks. Investors never take high risks, so investment risks always need to be paid by 119

120 the offshore wind farm developer. This means that the 30% of own capital invested in the development of the offshore wind farm is in general paid at the start of the development when the permits need to be obtained and the design of the offshore wind farm is being made. The 70% of the capital of investors will in general be paid when the permits are in place and the contracts with the manufacturers of turbines and electrical equipment and installation companies are ready to be signed (i.e. when the financial close of a project is being reached). The supply and installation of the equipment form by far the largest part of the total investment costs of an offshore wind farm. Therefore in order to sign the contracts with the manufacturers and installation companies, the offshore wind farm developer must have bank guarantees of the investors. This means that the remaining risks of the further development of the offshore wind farm must be acceptable for the investors. In case of a coupled grid connection, offshore wind farm developers must cooperate in the case the full grid connection has to be paid by the developers themselves (i.e. as is assumed in this study). In general the government sets a deadline for the offshore wind farm developer at which the subsidy period starts. When a wind farm developer has delay in the development, this means a loss of income when no power is produced when the subsidy period has started. In case of a coupled grid connection the individual offshore wind farm developers must be insured in case the development of the offshore wind farm has a delay or possibly the development is stopped. The costs of a delay or stopped development of an offshore wind farm must be covered by all offshore wind farms involved in the coupled grid connection. Due to the financial structure of an offshore wind farm (i.e. 30% own capital and 70% of investors), this makes a coupled grid connection very complex. A possible solution is when part of the own capital of the individual offshore wind farm developers is used for the investment in the coupled grid connection. When the coupled grid connection is arranged, the further development of the individual offshore wind farms is than depending on the investors. Investors are normally companies with much equity. Due to the income an offshore wind farm can generate when producing power during its 20 year lifetime, this will decrease the risk for investors when only the wind farm itself needs to be further developed and the grid connection is already in place. In order to insure a coupled grid connection redundancy in the grid connection is of high importance (e.g. when there is a failure in part of the grid connection system it is unwanted to have a high loss of power generation and accordingly of income). Therefore it will in general be better to have multiple cables and substations instead of a single cable or a single offshore substation. The possibility for redundant grid connection systems depend on the technology used (e.g. adding redundancy by adding for instance an extra cable is in general not an economical solution). For a HVAC coupled grid connection system normally multiple cables are used (e.g. when the total power to be transmitted is more than 250 to 300 MW). Also multiple offshore substations are possible without 120

121 increasing the costs too much. For a HVDC coupled grid connection system redundancy is more an issue. Multiple cable systems are possible, but not directly required due to the high power transmission capacity per cable system. The redundancy is more a problem in an offshore converter station. A converter station is designed such that is has much redundancy inside the converter station itself. Adding redundancy by having multiple offshore converter stations is economically not feasible due to the high investment costs of an offshore converter station. In case of a major failure in the offshore converter station the full power transmission capacity will get lost. Due to the fact that the chance of a cable failure when buried is very small, it is advisable to add redundancy in the coupled grid connection system by having multiple offshore substations if possible by the technology used. As has been indicated above, the practical implementation of a coupled grid connection brings along many difficulties. Therefore the economical feasibility of a coupled grid connection compared to individual grid connections of offshore wind farms must be such that it is attractive for wind farm developers to develop a coupled grid connection. When the economical feasibility is too low, the initiative for a coupled grid connection will not be taken by wind farm developers because the risks are too high. In that case the government should take the lead in the development of offshore wind farms with a coupled grid connection. 6.5 Summary of economical model In this chapter the economical model has been made. For all the components of the grid connection system for both the HVAC and HVDC VSC technology formulas or cost values have been given for the investment costs of the grid connection system. In paragraph 6.1 the value for the power loss in the grid connection system has been given. With the model for the optimization of the grid connection as given in paragraph 6.3 the different possible grid connection solutions can be compared based on the investment costs and power loss of each solution. The optimized grid connection is the solution which has the lowest NPV value. In case of a coupled grid connection the development of the offshore wind farms over the years need to be taken into account with respect to the difference in power loss when the number of offshore wind farms connected changes over the years. As was indicated during the development of the economical model, some cost values are based on assumptions or little information found in literature or obtained by manufacturers. In order to study the impact of slight changes in the economical model, a sensitivity analysis needs to be performed. In the sensitivity analysis the parameters with the highest influence on the final result will be analyzed. These parameters are: Discount rate r Costs for submarine cables Costs onshore and offshore substation 121

122 Costs HVDC VSC converter stations onshore and offshore Price of electricity With respect to the optimization process the impact of the increase of a cable conductor is taken into account to check increased investment costs versus a decrease in the losses in the grid connection system. The sensitivity analysis will be performed in paragraph

123 7 Application of model With the technical and economical model as given in chapters 5 and 6 now the optimization of the grid connection of the Trident offshore wind farms can be performed. As was given in chapter 2 several scenarios of the development of these offshore wind farms will be observed. The results of the optimization process for all the scenarios are given in the following paragraphs. First the base case scenario is studied: the optimized individual grid connections. The scenarios with a coupled grid connection of two or three offshore wind farms will be referenced to the base case scenario with the optimized individual grid connections. The results obtained will be observed on basis of a sensitivity analysis of the variables with the highest influence on the outcome of the optimization process. 7.1 Optimization process Before the results of the optimization process of all scenarios will be given, first some additional optimization parameters will be discussed which are used in the model: the optimization of the type and number of cable systems required and the development of several offshore wind farms over the years. Also the operational conditions of the cables as used in the model are given as well as the metal prices which have their influence on the cable prices Optimization of cable system The grid connection system of both the individual and coupled grid connections will be optimized on both the conductor cross section and the number of cables required. Cables with a larger conductor cross section are more expensive, but with the decrease in losses over 20 years of operation due to the lower resistance of the cable, this might be a more economical solution. Also the number of onshore cable systems for the HVAC grid connection systems can be optimized. As will be shown later on, for a single wind farm with a power rating of around 300 MW in many cases already two submarine cables are required. When several of these offshore wind farms would be connected by a coupled grid connection system, this would hold 4 to 6 submarine cables in case of a HVAC grid connection system. When each submarine cable would be connected to its own onshore cable system, this would hold also 4 to 6 onshore HVAC cable systems. That many parallel cable systems require a wide cable corridor/trench and in many cases the available space for the onshore cable trajectory is limited. When several submarine HVAC cables are connected to a single three phase land cable system, this can reduce the number of onshore HVAC cable systems required significantly. Therefore the available space for the onshore cable trajectory can be used more efficient and this can give some benefits for obtaining the permits for the onshore cable trajectory. A disadvantage of a shared onshore cable system is the necessity of a switching station for the connection of the submarine cables with the onshore cables. The advantage of 123

124 lower investment costs when fewer onshore cable systems are used increases with an increase in the onshore cable length. Because of the fact that the limited space available for the onshore cable trajectory in the Netherlands is an issue, in this model also the number of onshore cable systems for the HVAC grid connection solutions will be optimized. For the HVDC VSC grid connection systems only the conductor cross section is optimized and each submarine cable will be connected to its own land cable Development of offshore wind farms over the years With the target of 6000 MW of offshore wind capacity by 2020 in the Netherlands [3] and the current status of 228 MW of installed offshore wind power, the development of offshore wind farms in the Netherlands has to speed up in order to reach the given target. When offshore wind farms have a coupled grid connection and they are developed over the years, this has effect on the optimization process for the grid connection system. With the development of the offshore wind farms over the years, also the losses will vary each year, depending on the number of offshore wind farms operational for the coupled grid connection system. Also not the entire grid connection system for the connection of all the offshore wind farms needs to be ready at the start of operation of the first offshore wind farm (e.g. depending on the individual connection of the offshore wind farms with the main offshore substation). In this study it will be assumed that the main grid connection system (i.e. all the grid connection cables and the main offshore substation) are installed at the same time so all the offshore wind farms can be connected to the grid connection system directly when they are being built. With the difference in losses over the years due to the development of the offshore wind farms over the years, this gives a decrease in the losses in case of a coupled grid connection system. Because of the fact that the development of multiple offshore wind farms will never be at the same stage of development, it is in this study assumed that for the coupled grid connection the offshore wind farms will be developed 1 year after each other. For the case scenarios it is assumed that the offshore wind farm which is located closest to the coupled grid connection will be developed first Cable length and operating conditions The length of the cable trajectory offshore is based on the locations of the offshore substations inside the offshore wind farms and the landing point at the coast. For the cable length an additional 10% of length is taken into account with respect to the geometrical shortest connection. This is due to crossings with shipping lanes, cables and pipes and the bathymetry which may cause a longer cable length. The characteristics for the cables onshore and offshore as were given in paragraph 5.4 will be used. In table 7.1 they are given once again. 124

125 Table 7.1: Typical conditions of cables at the landing point, offshore and onshore [34]. Typical cable: landing conditions offshore onshore Soil temperature 15 C 12 C 15 C Thermal resistivity 1.1 Km/W 0.8 Km/W 1.0 Km/W Burial depth 3 m 1.5 m 1.0 m The ratings of the submarine cables will be based on the worst case conditions which are in general at the cable landing at the coast. Due to the better thermal conditions further offshore the losses will be lower far offshore compared to the losses near the landing at the coast Metal prices and currency exchange rate As was mentioned in the economical model for the cables, the cable price depends to a large extend on the price of the metals used for the cables (e.g. copper, aluminum and lead). In paragraph the variation of the metal prices was shown. For the optimization process the current metal prices (e.g. August 2008) and currency exchange rate are used as are given in the table below [51], [52]. Also the price of energy and the base interest rate used in the model are given in the table. Table 7.2: Metal prices, currency exchange rate, price of energy and interest rate used in the optimization process. Variable Value Copper price 7.8 [$/kg] Aluminum price 2.8 [$/kg] Lead 1.9 [$/kg] Currency exchange rate 0.66 [ /$] Price of energy 100 [ /MWh] Interest rate r 8 [%] 7.2 Scenario 1: Individual grid connections For the optimization process is started with scenario 1 Base Case, the optimization of the individual grid connections of the three Trident offshore wind farms. In figure 7.1 the three offshore wind farms are indicated with their individual grid connection landing points and also the two coupled grid connection landing points as given in paragraph 3.2 are shown. The coordinates of the offshore wind farms and their individual landing points are given in Appendix A. The landing points for the offshore wind farms West Rijn and Katwijk at Hoek van Holland and IJmuiden' will also be the two possible landing locations for the coupled grid connection of the offshore wind farms. For the onshore cable trajectory a flat formation is assumed for the HVAC systems for the individual grid connections. In the following paragraphs the results of the optimization process of the individual grid connections of the Trident offshore wind farms will be given. 125

126 Figure 7.1: Overview of the 'Trident' offshore wind farms and their landing points Individual grid connection Scheveningen Buiten For the optimization of the individual grid connection of offshore wind farm Scheveningen Buiten the characteristics as given in Appendix A are used. The most important characteristics are shown in the table below once again. Table 7.3: Characteristics of offshore wind farm 'Scheveningen Buiten'. Characteristics Scheveningen Buiten Annual mean wind speed 9.4 [m/s] Burial depth submarine cable 1.5 [m] Burial depth onshore cable 1.0 [m] Length onshore cable 8.2 [km] Individual grid connection voltage 150 [kv] Yearly mean availability 92 [%] Yearly mean wake loss 10 [%] Infield loss at rated power 0.6 [%] When the parameters as given in the table above are used, the results of the optimized grid connection for the offshore wind farm Scheveningen Buiten are obtained as given in table

127 Table 7.4: Results of the optimization of the individual grid connection of 'Scheveningen Buiten'. System HVAC HVDC VSC Rating #2 #1 #3 #4 #5 Voltage [kv] ±150 ±300 Total costs over 20 years [M ] Total losses over 20 years [M ] Total investment costs [M ] Yearly power production [GWh] Yearly power fed into grid [GWh] Yearly losses [GWh] [%] Submarine cables Number of cables [-] Distance [km] Conductor cross section [mm 2 ] Land cables Number of cables (single phase) [-] Distance [km] Conductor cross section [mm 2 ] Offshore Substation Reactive power comp. [50%] [MVAr] Number of OHVS [-] Capacity per OHVS [MVA] 170/ / / Capacity converter station [MW] Onshore Substation Transformer required [-] YES NO YES - - Reactive power comp. [50%] [MVAr] Capacity transformer [MVA] Capacity converter station [MW] As can be seen in the table above, the optimized HVAC systems for the grid connection of the offshore wind farm Scheveningen Buiten have nearly the same costs over 20 years. The investment costs increase for the 150 kv and 220 kv HVAC systems, but the losses decrease. Of the HVAC systems the most optimal solution is the 150 kv system. The HVDC VSC systems are much more expensive as compared to the HVAC systems. The investment costs are approximately 25% to 35% higher and also the losses are about 2 to 2.3 times as high for the HVDC VSC systems compared to the HVAC systems. This is mainly because of the high costs and losses in the two converter stations. In figure 7.2 and figure 7.3 an overview is given of the losses and costs of the individual grid connection of the offshore wind farm Scheveningen Buiten. The main differences between the losses of the HVAC systems are the submarine cable losses which decrease for a higher transmission voltage. The HVDC VSC systems have the main losses in the offshore and onshore converter stations. The cable losses for the HVDC VSC systems are much smaller compared to the HVAC solutions. In figure 7.3 can be seen that the main cost components are the OHVS (e.g. mainly due to fact 2 OHVS stations are used), the submarine cables, 127

128 the submarine cable installation and the losses. The overall differences between the HVAC systems are small. The cable costs for the HVDC VSC solutions are much smaller, but the costs for the converter stations and the losses are significant and make the HVDC VSC systems much more expensive as the HVAC systems. 70 Losses Grid Connection 'Scheveningen Buiten' 60 Losses [GWh] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC VSC ±150 kv HVDC VSC ±300 kv Type of Grid Connection Infield Cables OHVS Offshore Reactive Compensation Offshore HVDC Converter Submarine Cables Onshore Cables Onshore Substation Onshore Reactive Compensation Onshore HVDC Converter Figure 7.2: Overview of losses optimized individual grid connection systems offshore wind farm 'Scheveningen Buiten'. 250 Costs Grid Connection 'Scheveningen Buiten' 200 Costs [MEURO] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC VSC ±150 kv HVDC VSC ±300 kv Type of Grid Connection OHVS Offshore Reactive Compensation Offshore HVDC Converter Submarine Cables Submarine Cable Installation Onshore Cables Onshore Cable Installation Onshore Substation Onshore Reactive Compensation Onshore HVDC Converter Losses Figure 7.3: Overview of costs optimized individual grid connection systems offshore wind farm 'Scheveningen Buiten'. 128

129 7.2.2 Individual grid connection West Rijn For the optimization of the individual grid connection of offshore wind farm West Rijn the characteristics as given in Appendix A are used and shown in the table below. Table 7.5: Characteristics of offshore wind farm 'West Rijn. Characteristics West Rijn Annual mean wind speed 9.2 [m/s] Burial depth submarine cable 1.5 [m] Burial depth onshore cable 1.0 [m] Length onshore cable 11 [km] Individual grid connection voltage 150 [kv] Yearly mean availability 92 [%] Yearly mean wake loss 10 [%] Infield loss at rated power 0.6 [%] When the parameters as given in the table above are used, the results of the optimized grid connection for the offshore wind farm West Rijn are obtained as given in the table below. Table 7.6: Results of the optimization of the individual grid connection of West Rijn. System HVAC HVDC VSC Rating Voltage [kv] ±150 ±300 Total costs over 20 years [M ] Total losses over 20 years [M ] Total investment costs [M ] Yearly power production [GWh] Yearly power fed into grid [GWh] Yearly losses [GWh] [%] Submarine cables Number of cables [-] Distance [km] Conductor cross section [mm 2 ] Land cables Number of cables (single phase) [-] Distance [km] Conductor cross section [mm 2 ] Offshore Substation Reactive power comp. [50%] [MVAr] Number of OHVS [-] Capacity per OHVS [MVA] Capacity converter station [MW] Onshore Substation Transformer required [-] YES NO YES - - Reactive power comp. [50%] [MVAr] Capacity transformer [MVA] Capacity converter station [MW]

130 As can be seen in table 7.6, the optimized HVAC systems for the grid connection of the offshore wind farm West Rijn have nearly the same costs over 20 years as was the case for offshore wind farm Scheveningen Buiten. Due to the similar size of the wind farm and nearly the same distance to shore, the comparison between the HVAC and HVDC VSC systems are similar as for offshore wind farm Scheveningen Buiten. The 220 kv HVAC solution is the most optimal due to the fact that the total rated power of the wind farm can be transmitted by a single submarine cable while for the 132 kv and 150 kv systems two cables are required. In figures 7.4 and 7.5 an overview is given of the losses and costs of the individual grid connection of offshore wind farm West Rijn. Again a decrease in the losses for the HVAC systems can be seen mainly due to a decrease in cable losses. Due to the similarity with offshore wind farm Scheveningen Buiten also in this case the converter losses dominate for the HVDC VSC systems. The losses for the HVDC VSC systems is again a factor 2 to 2.4 higher compared to the HVAC systems. In figure 7.5 the cost distribution is given for the optimized grid connection of offshore wind farm West Rijn. As was the case for Scheveningen Buiten also in this case the main cost components for the HVAC systems are the costs for the OHVS, the submarine cables, the submarine cable installation and the losses. For the HVDC VSC systems the main costs come again from the converter stations onshore and offshore and the losses in these converters. The HVDC VSC systems are 30% to 35% more expensive compared to the HVAC systems. 70 Losses Grid Connection 'West Rijn' 60 Losses [GWh] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC VSC ±150 kv HVDC VSC ±300 kv Type of Grid Connection Infield Cables OHVS Offshore Reactive Compensation Offshore HVDC Converter Submarine Cables Onshore Cables Onshore Substation Onshore Reactive Compensation Onshore HVDC Converter Figure 7.4: Overview of losses optimized individual grid connection systems offshore wind farm 'West Rijn'. 130

131 250 Costs Grid Connection 'West Rijn' 200 Costs [MEURO] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC VSC ±150 kv HVDC VSC ±300 kv Type of Grid Connection OHVS Offshore Reactive Compensation Offshore HVDC Converter Submarine Cables Submarine Cable Installation Onshore Cables Onshore Cable Installation Onshore Substation Onshore Reactive Compensation Onshore HVDC Converter Losses Figure 7.5: Overview of costs optimized individual grid connection systems offshore wind farm West Rijn Individual grid connection Katwijk For the optimization of the individual grid connection of offshore wind farm Katwijk the characteristics as given in Appendix A are used and shown in the table below. Table 7.7: Characteristics of offshore wind farm Katwijk. Characteristics Katwijk Annual mean wind speed 9.1 [m/s] Burial depth submarine cable 1.5 [m] Burial depth onshore cable 1.0 [m] Length onshore cable 10 [km] Individual grid connection voltage 150 [kv] Yearly mean availability 92 [%] Yearly mean wake loss 10 [%] Infield loss at rated power 1.1 [%] When the parameters as given in the table above are used, the results of the optimized grid connection for the offshore wind farm Katwijk are obtained as given in table

132 Table 7.8: Results of the optimization of the individual grid connection of Katwijk. System HVAC HVDC VSC Rating Voltage [kv] ±150 ±300 Total costs over 20 years [M ] Total losses over 20 years [M ] Total investment costs [M ] Yearly power production [GWh] Yearly power fed into grid [GWh] Yearly losses [GWh] [%] Submarine cables Number of cables [-] Distance [km] Conductor cross section [mm 2 ] Land cables Number of cables (single phase) [-] Distance [km] Conductor cross section [mm 2 ] Offshore Substation Reactive power comp. [50%] [MVAr] Number of OHVS [-] Capacity per OHVS [MVA] Capacity converter station [MW] Onshore Substation Transformer required [-] YES NO YES - - Reactive power comp. [50%] [MVAr] Capacity transformer [MVA] Capacity converter station [MW] As can be seen in table 7.8, the optimized HVAC systems for the grid connection of the offshore wind farm Katwijk have nearly the same costs over 20 years as was the case for offshore wind farms Scheveningen Buiten and West Rijn. Due to the similar size of the wind farm and nearly the same distance to shore, the comparison between the HVAC and HVDC VSC systems are again similar as for the other offshore wind farms. The 150 kv HVAC solution is the most optimal mainly due to the fact that no onshore transformer station is required and a single onshore cable system can be used. In figure 7.6 and 7.7 an overview is given of the losses and costs of the individual grid connection of offshore wind farm Katwijk. The main difference in the losses for the HVAC systems is again because of the decreasing cable losses for higher voltage systems. For the HVDC VSC systems again the losses in the converter station dominate. The losses for the HVDC VSC systems are 1.6 to 2 times higher compared to the HVAC systems. In figure 7.7 the costs distribution is given for the optimized grid connection of offshore wind farm Katwijk. As was the case for Scheveningen Buiten and West Rijn, also in this case the main cost components for the HVAC systems 132

133 are the costs for the OHVS, the submarine cables, the submarine cable installation and the losses. For the HVDC VSC systems the main costs come again from the converter stations onshore and offshore and the losses in these converters. The HVDC VSC systems are 30% to 35% more expensive compared to the HVAC systems. 80 Losses Grid Connection 'Katwijk' 70 Losses [GWh] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC VSC ±150 kv HVDC VSC ±300 kv Type of Grid Connection Infield Cables OHVS Offshore Reactive Compensation Offshore HVDC Converter Submarine Cables Onshore Cables Onshore Substation Onshore Reactive Compensation Onshore HVDC Converter Figure 7.6: Overview of losses optimized individual grid connection systems offshore wind farm 'Katwijk'. 250 Costs Grid Connection 'Katwijk' 200 Costs [MEURO] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC VSC ±150 kv HVDC VSC ±300 kv Type of Grid Connection OHVS Offshore Reactive Compensation Offshore HVDC Converter Submarine Cables Submarine Cable Installation Onshore Cables Onshore Cable Installation Onshore Substation Onshore Reactive Compensation Onshore HVDC Converter Losses Figure 7.7: Overview of costs optimized individual grid connection systems offshore wind farm 'Katwijk'. 133

134 7.2.4 Summary of individual grid connections In the previous paragraphs the optimized individual grid connection of all three Trident offshore wind farms are given. Due to the similar wind farm power rating and distance to shore, the results of the optimization process of the individual grid connections are similar. As could be seen for all three offshore wind farms, the losses and investment costs for the HVDC VSC systems are much higher as compared to the HVAC system. The losses are typically 30% to 35% higher and the costs 1.5 to 2.4 times higher. This is mainly due to the high investment costs for the converter stations and the losses in these converters. Therefore the HVDC VSC systems are not suited for the grid connection of individual offshore wind farms with a power rating of several hundred MW and a distance to shore of around 50 km. With an increase in distance and/or wind farm rating the differences between the HVAC and HVDC VSC systems will decrease due to the lower cable losses and costs for the HVDC VSC systems. Now the optimization of the individual grid connections of the Trident offshore wind farms are known, the scenarios with a coupled grid connection can be observed as will be done in the following paragraphs. In the sensitivity analysis later on some cost variables will be changed, but most likely the outcome of the results obtained for the individual grid connection will not change much, due to the relatively high difference between the HVAC and HVDC VSC systems. 7.3 Scenario 2: Coupled grid connections In the following paragraphs the optimization process will be done for the coupled grid connection of the Trident offshore wind farms. As was given in paragraph 2.3 there are four scenarios with a coupled grid connection which will be observed: Development of West Rijn and Scheveningen Buiten Development of West Rijn and Katwijk Development of Scheveningen Buiten and Katwijk Development of West Rijn, Scheveningen Buiten and Katwijk As was given in paragraph 3.4 there are two possible landing locations for the coupled grid connection of offshore wind farms: IJmuiden and Maasvlakte / Hoek van Holland, which are also indicated in figure 7.1. The landing point for the different scenarios depends on the total cable length required and the possibility of a grid connection. For the scenarios with offshore wind farm Scheveningen Buiten the closest landing and grid connection point is Hoek van Holland / Maasvlakte : 34.5 km offshore and approximately 20 km onshore trajectory. For the scenarios with offshore wind farm Katwijk the closes landing and grid connection point is also Hoek van Holland / Maasvlakte : 38 km offshore and approximately 20 km onshore trajectory. The onshore cable trajectory and the landing of the submarine cable are more difficult for the landing 134

135 at Hoek van Holland compared to the landing at IJmuiden'. For this study al coupled grid connections will be connected to substation Maasvlakte due to the shorter cable length required, though it might turn out in real life to be more feasible to connect to substation IJmuiden. Due to the limited space available for onshore cable trajectory trefoil formation for onshore cables for the HVAC systems are assumed. For the coupled grid connection it is of importance where the main offshore substation will be located. At this point in time it is uncertain weather it is possible to place a substation in between the offshore wind farms (e.g. depending on other offshore activities). With the location of the main offshore substation in one of the offshore wind farms it is certain that it is allowed and therefore in this study the main offshore substation will be located in one of the offshore wind farms. Up till now all offshore substation are located in the middle of the offshore wind farms. This is mainly due to the reason that the infield cable losses are then minimized and the chance of a vessel hitting the offshore substation is minimized when it is located inside the offshore wind farm (i.e. the danger of an accident happening with an OHVS is much larger as for a vessel hitting a single wind turbine). On the other hand one starts to investigate whether it is better or not to place the offshore substations at the edge of the offshore wind farms. The reasons for this is in case of an emergency access to the platform by means of a helicopter is easier and the chance of a ship hitting the tower is very small due to the safety margins of the locations of the offshore wind farms with respect to the shipping lanes of large vessels. With these observations taken into account the main offshore substation will be located in the middle of the offshore wind farms where the offshore substation of the individual offshore wind farms are also planned. The main offshore substation will be located in the offshore wind farm which is the closest to shore. The other offshore wind farms will be connected with high voltage cables to the main offshore wind farm. In case of HVDC VSC systems the interconnections between the offshore wind farms will be assumed at 150 kv HVAC. This is due to the fact that the distances between the offshore wind farms are thus far that it is most optimal to have the interconnections at high voltage. The 150 kv HVAC connection is in general the most suited HVAC system as could be seen in the previous paragraphs with the optimization of the individual grid connections. As was mentioned before the development of offshore wind farms over the years has effect on the optimization process in case of a coupled grid connection. When offshore wind farms are connected to the main grid connection system over the years, the losses per year differ and are lower compared to the case that all offshore wind farms will be developed at the same time which is normally not the case. With the target of 6000 MW of offshore wind power in the Netherlands taken into account, in this study it will be assumed that in case of a coupled grid connection the offshore wind farms will be developed one year after another. Because of the fact that it is uncertain which wind farm will be developed first, it is assumed that the offshore wind farm which is the closest to 135

136 shore will be developed first. In order to be able to give the total costs over all the years for the offshore wind farms which will be connected by a coupled grid connection over the years, the total costs and power production are given for the entire lifetime of all the offshore wind farms in the model. Therefore the power production and the losses of the individual grid connection systems should be multiplied by 20 years (i.e. the life time of the offshore wind farms, now the power production and losses are given per year for the optimized individual grid connections) in order to compare them to the power production and losses of a coupled grid connection. With the observations made in this paragraph the optimization process for the coupled grid connection of scenarios 2a till 2d can be performed as will be done in the following paragraphs Scenario 2a: West Rijn and Scheveningen Buiten For the coupled grid connection of offshore wind farms West Rijn and Scheveningen Buiten the characteristics as given in the table below will be used. Scheveningen Buiten will be developed first and one year later West Rijn will be developed. Table 7.9: Characteristics for the grid connection of offshore wind farms West Rijn and Scheveningen Buiten. Characteristics Coupled Grid Connection Annual mean wind speed West Rijn 9.2 [m/s] Annual mean wind speed Scheveningen Buiten 9.4 [m/s] Burial depth submarine cable 1.5 [m] Burial depth onshore cable 1.0 [m] Length onshore cable 20 [km] Grid connection voltage 380 [kv] Yearly mean availability 92 [%] Yearly mean wake loss 10 [%] Infield loss at rated power West Rijn 0.6 [%] Infield loss at rated power Scheveningen Buiten 0.6 [%] In tables 7.10 and 7.11 the results are given for the optimization of the coupled grid connection of offshore wind farms West Rijn and Scheveningen Buiten for both the HVAC and HVDC VSC systems. Scheveningen Buiten is located closest to the landing point at Hoek van Holland and will therefore in general have the main offshore substation. The distance between West Rijn and Scheveningen Buiten is about 14.5 km and the distance from Scheveningen Buiten to the grid connection point about 54.5 km of which 34.5 km offshore and 20 km onshore. As can be seen in table submarine cables are required for the interconnection between the wind farms for the 132 kv and 150 kv HVAC solutions and only 1 cable for the 220 kv HVAC system. For the grid connection 4 cables are required for the 132 kv HVAC system and only 3 cables for the

137 kv and 220 kv HVAC systems which is equal to the number of cables which are required for the optimized individual grid connections of these wind farms. Onshore 3 cable systems are required for the 132 kv and 220 kv HVAC systems and only 2 cable systems for the 150 kv HVAC system. The reason why the 220 kv HVAC system requires 3 onshore cable systems is because of the much larger amount of reactive power generated in the cables compared to the 150 kv HVAC system. Table 7.10: Results of the optimization of the coupled grid connection of HVAC systems for West Rijn and Scheveningen Buiten. System HVAC Rating Voltage [kv] From [-] WR SB WR SB WR SB To [-] SB GRID SB GRID SB GRID Distance [km] Costs Total costs over 20 years [M ] Total losses over 20 years [M ] Total investment costs [M ] Power 20 Year Input Connection [TWh] Year Output Connection [TWh] Year losses connection [GWh] [%] Submarine cables Number of cables [-] Distance [km] Conductor cross section [mm 2 ] Land cables Number of cables (single phase) [-] Distance [km] Conductor cross section [mm 2 ] Offshore Substation Reactive power comp. [50%] [MVAr] / / /250 Number of OHVS [-] Capacity per OHVS [MVA] Onshore Substation Reactive power comp. [50%] [MVAr] Capacity transformer [MVA] In table 7.11 the results are given for the optimized coupled grid connection with HVDC VSC as main grid connection system. As can be seen for the ±300 kv HVDC VSC system is it most optimal to have the offshore converter station located inside West Rijn though this offshore wind farm is at a larger distance from the grid connection point. The reason for this is that due to the lower cable losses for the HVDC VSC system it is in this case more optimal to have a longer HVDC VSC cable trajectory and decrease the HVAC interconnection cable length between the two offshore wind farms. In this case the OHVS inside 137

138 Scheveningen Buiten which is located closest to offshore wind farm West Rijn will be the main OHVS inside offshore wind farm Scheveningen Buiten and therefore the interconnection distance is 11.1 km. For the interconnection kv HVAC submarine cables are required for both HVDC VSC systems. For the grid connection 2 bipolar cable systems are required for the ±150 kv HVDC VSC system and only 1 bipolar cable system for the ±300 kv HVDC VSC system. For both HVDC VSC systems a 600 MW converter station is suited. The total rated power of both offshore wind farms together is MW, but due to the losses in the infield cables, interconnection cables and the wake losses a 600 MW converter station will be suited for both systems. Compared to the losses and costs of the HVAC grid connection system, the losses are about 1.35 to 1.9 times as high and the costs about 35% to 50% higher for the HVDC VSC systems. Table 7.11: Results of the optimization of the coupled grid connection of HVDC VSC systems for West Rijn and Scheveningen Buiten. System HVDC VSC Rating 5 4 Voltage [kv] 150 AC ±150 DC 150 AC ±300 DC From [-] WR SB SB WR To [-] SB GRID WR GRID Distance [km] Costs Total costs over 20 years [M ] Total losses over 20 years [M ] Total investment costs [M ] Power 20 Year Input Connection [TWh] Year Output Connection [TWh] Year losses connection [GWh] [%] Submarine cables Number of cables [-] Distance [km] Conductor cross section [mm 2 ] Land cables Number of cables (single phase) [-] Distance [km] Conductor cross section [mm 2 ] Offshore Substation Reactive power comp. [50%] [MVAr] Number of OHVS [-] Capacity per OHVS [MVA] /170 - Capacity converter station [MW] Onshore Substation Capacity converter station [MW]

139 In order to be able to compare the total solution for the HVAC and HVDC VSC grid connection systems, the total power production, power losses and costs need to be compared. In table 7.12 the overview is given for the coupled grid connection of the offshore wind farms West Rijn and Scheveningen Buiten. As can be seen in table 7.12 the most optimal coupled grid connection is the 220 kv HVAC system. The losses are only 3.1% in total including the infield losses of the offshore wind farms. The costs and losses for the HVDC VSC systems are much higher as was already indicated. Table 7.12: Overview of optimization of coupled grid connection of 'West Rijn' and 'Scheveningen Buiten'. Overview Optimization Coupled Grid Connection of West Rijn and Scheveningen Buiten Main System Total 20 Year Production Total 20 Year Losses Total Power Fed into Grid Total Costs Ranking HVAC 132 [kv] [TWh] [GWh] 4.0 [%] [TWh] [M ] 2 HVAC 150 [kv] [TWh] [GWh] 3.4 [%] [TWh] [M ] 3 HVAC 220 [kv] [TWh] [GWh] 3.1 [%] [TWh] [M ] 1 HVDC ±150 [kv] [TWh] [GWh] 5.9 [%] [TWh] [M ] 5 HVDC ±300 [kv] [TWh] [GWh] 5.6 [%] [TWh] [M ] 4 For a better insight in the main loss and cost components between the different grid connection systems a comparison is given in figures 7.8 and 7.9. In figure 7.8 can be seen that for the HVAC grid connection systems the main loss components are the offshore and onshore cables. For the HVDC VSC grid connection systems the main loss components are the offshore and onshore converter stations. The cable losses are much lower for the HVDC VSC systems compared to the HVAC systems. The losses are a factor 1.35 to 1.9 higher for the HVDC VSC systems compared to the HVAC system. These results are similar to the base case with the individual grid connection systems as could be expected Losses Coupled Grid Connection 'West Rijn' and 'Scheveningen Buiten' Losses [GWh] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC VSC ±150 kv HVDC VSC ±300 kv Type of Grid Connection Infield Cables OHVS Offshore Reactive Compensation Offshore Converter Submarine Cables Onshore Cables Onshore Substation Onshore Reactive Compensation Onshore Converter Figure 7.8: Overview of losses optimized coupled grid connection systems of 'West Rijn' and 'Scheveningen Buiten'. 139

140 In figure 7.9 an overview is given of the costs for all the grid connection systems. As was the case for the individual grid connections, the main cost components for the HVAC systems are the OHVS, the submarine cables, the submarine cable installation and the losses. For the HVDC VSC systems the main costs components are the offshore converter station and the losses. As can be seen the 220 kv HVAC system is the most optimal coupled grid connection for the offshore wind farms West Rijn and Scheveningen Buiten. The HVDC VSC systems are about 35% to 50% higher. 500 Costs Coupled Grid Connection 'West Rijn' and 'Scheveningen Buiten' Costs [MEURO] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC VSC ±150 kv HVDC VSC ±300 kv Type of Grid Connection OHVS Offshore Reactive Compensation Offshore HVDC Converter Submarine Cables Submarine Cable Installation Onshore Cables Onshore Cable Installation Onshore Substation Onshore Reactive Compensation Onshore HVDC Converter Losses Figure 7.9: Overview of costs optimized coupled grid connection systems of 'West Rijn' and 'Scheveningen Buiten'. In the table below the total costs and losses over 20 years of the optimized individual and coupled grid connection systems are shown. The coupled grid connection has slightly lower total costs. The losses are about 100 GWh more over 20 years of operation though. This indicates that the investment costs for the coupled grid connection are lower compared to the individual grid connection systems. Though the coupled grid connection system has lower total costs, 1.43% lower, the costs savings with the conditions used for this optimization process probably don t compensate for the additional complexity a coupled grid connection causes. Table 7.13: Comparison of losses and costs over 20 years for the individual and coupled grid connections of 'West Rijn' and 'Scheveningen Buiten'. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] West Rijn 220 kv HVAC kv HVAC - - Scheveningen Buiten 150 kv HVAC kv HVAC - - Total

141 7.3.2 Scenario 2b: West Rijn and Katwijk For the coupled grid connection of offshore wind farms West Rijn and Katwijk the characteristics as given in the table below will be used. Katwijk will be developed first and after one year West Rijn will be developed. Table 7.14: Characteristics for the grid connection of offshore wind farms West Rijn and Katwijk. Characteristics Coupled Grid Connection Annual mean wind speed West Rijn 9.2 [m/s] Annual mean wind speed Katwijk 9.1 [m/s] Burial depth submarine cable 1.5 [m] Burial depth onshore cable 1.0 [m] Length onshore cable 20 [km] Grid connection voltage 380 [kv] Yearly mean availability 92 [%] Yearly mean wake loss 10 [%] Infield loss at rated power West Rijn 0.6 [%] Infield loss at rated power Katwijk 1.1 [%] In tables 7.15 and 7.16 the results are given for the optimization of the coupled grid connection of offshore wind farms West Rijn and Katwijk for both the HVAC and HVDC VSC systems. Katwijk is located closest to the landing point at Hoek van Holland and will therefore in general have the main offshore substation. The distance between West Rijn and Katwijk is about 26.2 km and the distance from Katwijk to the grid connection point about 58.0 km of which 38.0 km offshore and 20 km onshore. As can be seen in table submarine cables are required for the interconnection between the wind farms for the 132 kv and 150 kv HVAC solutions and only 1 cable for the 220 kv HVAC system. For the grid connection 4 cables are required for the 132 kv HVAC system and only 3 cables for the 150 kv and 220 kv HVAC systems which is equal to the optimized individual grid connections. Onshore 3 cable systems are required for the 132 kv HVAC system and only 2 cable systems for the 150 kv and 220 kv HVAC systems. As can be seen the investment costs for the 132 kv and 220 kv HVAC systems are about equal, but due to the lower losses in the 220 kv HVAC system this is the most optimal grid connection system. 141

142 Table 7.15: Results of the optimization of the coupled grid connection of HVAC systems for West Rijn and Katwijk. System HVAC Rating Voltage [kv] From [-] WR K WR K WR K To [-] K GRID K GRID K GRID Distance [km] Costs Total costs over 20 years [M ] Total losses over 20 years [M ] Total investment costs [M ] Power 20 Year Input Connection [TWh] Year Output Connection [TWh] Year losses connection [GWh] [%] Submarine cables Number of cables [-] Distance [km] Conductor cross section [mm 2 ] Land cables Number of cables (single phase) [-] Distance [km] Conductor cross section [mm 2 ] Offshore Substation Reactive power comp. [50%] [MVAr] Number of OHVS [-] Capacity per OHVS [MVA] Onshore Substation Reactive power comp. [50%] [MVAr] Capacity transformer [MVA] In table 7.16 the results are given for the optimized coupled grid connection with HVDC VSC as main grid connection system. As can be seen the results for the ±150 kv and ±300 kv HVDC VSC systems are similar. The same type of cables are used, but for the ±150 kv system 2 bipolar cable systems are required, while for the ±300 kv system only 1 bipolar system is needed. Due to the lower investment costs for the cables and the lower cable losses due to the higher voltage, the ±300 kv system is more optimal than the ±150 kv system. The total costs for the HVDC VSC systems are much higher compared to the HVAC systems though, as was also the case for the coupled grid connection of West Rijn and Scheveningen Buiten. Due to the losses in the infield cables, interconnection cables and the wake losses a 620 MW converter station will be suited for both systems, though the total rated power of the two offshore wind farms is MW. Compared to the losses and costs of the HVAC grid connection system, the losses are about 1.35 to 1.85 times as high and the costs about 25% to 35% 142

143 higher for the HVDC VSC systems. These are in a similar range as for the coupled grid connection of West Rijn and Scheveningen Buiten. Table 7.16: Results of the optimization of the coupled grid connection of HVDC VSC systems for West Rijn and Katwijk. System HVDC VSC Rating 5 4 Voltage [kv] 150 AC ±150 DC 150 AC ±300 DC From [-] WR K WR K To [-] K GRID K GRID Distance [km] Costs Total costs over 20 years [M ] Total losses over 20 years [M ] Total investment costs [M ] Power 20 Year Input Connection [TWh] Year Output Connection [TWh] Year losses connection [GWh] [%] Submarine cables Number of cables [-] Distance [km] Conductor cross section [mm 2 ] Land cables Number of cables (single phase) [-] Distance [km] Conductor cross section [mm 2 ] Offshore Substation Reactive power comp. [50%] [MVAr] Number of OHVS [-] Capacity per OHVS [MVA] Capacity converter station [MW] Onshore Substation Capacity converter station [MW] In order to give a better overview of the comparison of the HVAC and HVDC VSC systems, the total power production, power losses and costs of all types of system are shown in table As can be seen the most optimal coupled grid connection is the 220 kv HVAC system as was also the case for the coupled grid connection of West Rijn and Scheveningen Buiten. The losses are only 3.4% in total including the infield losses of the offshore wind farms. The costs and losses for the HVDC VSC systems are much higher as was already indicated. 143

144 Table 7.17: Overview of optimization of coupled grid connection of 'West Rijn' and Katwijk'. Overview Optimization Coupled Grid Connection of West Rijn and Katwijk Main System Total 20 Year Production Total 20 Year Losses Total Power Fed into Grid Total Costs Ranking HVAC 132 [kv] [TWh] [GWh] 4.4 [%] [TWh] [M ] 2 HVAC 150 [kv] [TWh] [GWh] 4.0 [%] [TWh] [M ] 3 HVAC 220 [kv] [TWh] [GWh] 3.4 [%] [TWh] [M ] 1 HVDC ±150 [kv] [TWh] [GWh] 6.2 [%] [TWh] [M ] 5 HVDC ±300 [kv] [TWh] [GWh] 5.9 [%] [TWh] [M ] 4 For a better insight in the main loss and cost components between the different grid connection systems a comparison is given in figures 7.10 and The main loss and cost components for both the HVAC and HVDC VSC systems are similar as was the case for the coupled grid connection of West Rijn and Scheveningen Buiten. Losses [GWh] Losses Coupled Grid Connection 'West Rijn' and 'Katwijk' HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC VSC ±150 kv HVDC VSC ±300 kv Type of Grid Connection Infield Cables OHVS Offshore Reactive Compensation Offshore Converter Submarine Cables Onshore Cables Onshore Substation Onshore Reactive Compensation Onshore Converter Figure 7.10: Overview of losses optimized coupled grid connection systems of 'West Rijn' and 'Katwijk'. As was done for the coupled grid connection of West Rijn and Scheveningen Buiten, again the coupled grid connection system is compared with the optimized individual grid connection systems. In table 7.18 the total costs and losses over 20 years of the optimized individual and coupled grid connection systems are shown. As can be seen the total costs over 20 years of operation are much lower (e.g. nearly 6%) for the optimized individual grid connection systems. This is mainly due to the long cable length for the interconnection of the two offshore wind farms (i.e km) and the grid connection (i.e. 58 km) which make the coupled grid connection more expensive. Due to the higher cable voltage of 220 kv the total losses over 20 years are slightly lower though. With this comparison based on the economical variables as used in this optimization 144

145 process, the coupled grid connection will not be more feasible than the individual grid connections of offshore wind farms West Rijn and Katwijk. 500 Costs Coupled Grid Connection 'West Rijn' and 'Katwijk' Costs [MEURO] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC VSC ±150 kv HVDC VSC ±300 kv Type of Grid Connection OHVS Offshore Reactive Compensation Offshore HVDC Converter Submarine Cables Submarine Cable Installation Onshore Cables Onshore Cable Installation Onshore Substation Onshore Reactive Compensation Onshore HVDC Converter Losses Figure 7.11: Overview of costs optimized coupled grid connection systems of 'West Rijn' and 'Katwijk'. Table 7.18: Comparison of losses and costs over 20 years for the individual and coupled grid connections of 'West Rijn' and Katwijk'. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] West Rijn 220 kv HVAC kv HVAC - - Katwijk 150 kv HVAC kv HVAC - - Total Scenario 2c: Scheveningen Buiten and Katwijk For the coupled grid connection of offshore wind farms Scheveningen Buiten and Katwijk the characteristics as given in the table below will be used. Scheveningen Buiten is located closest to the grid connection point at substation Maasvlakte and will therefore be developed first. One year later offshore wind farm Katwijk will be developed. In tables 7.20 and 7.21 the results are given for the optimization of the coupled grid connection of offshore wind farms Scheveningen Buiten and Katwijk for both the HVAC and HVDC VSC systems. Scheveningen Buiten is located closest to the landing point at Hoek van Holland and will therefore in general have the main offshore substation. The distance between offshore wind farms Katwijk and Scheveningen Buiten is about 18.2 km and the distance from Scheveningen Buiten to the grid connection point about 54.5 km of which 34.5 km offshore and 20 km onshore. 145

146 Table 7.19: Characteristics for the grid connection of offshore wind farms Scheveningen Buiten and Katwijk. Characteristics Coupled Grid Connection Annual mean wind speed Scheveningen Buiten 9.4 [m/s] Annual mean wind speed Katwijk 9.1 [m/s] Burial depth submarine cable 1.5 [m] Burial depth onshore cable 1.0 [m] Length onshore cable 20 [km] Grid connection voltage 380 [kv] Yearly mean availability 92 [%] Yearly mean wake loss 10 [%] Infield loss at rated power Scheveningen Buiten 0.6 [%] Infield loss at rated power Katwijk 1.1 [%] Table 7.20: Results of the optimization of the coupled grid connection of HVAC systems for Scheveningen Buiten and Katwijk. System HVAC Rating Voltage [kv] From [-] K SB K SB K SB To [-] SB GRID SB GRID SB GRID Distance [km] Costs Total costs over 20 years [M ] Total losses over 20 years [M ] Total investment costs [M ] Power 20 Year Input Connection [TWh] Year Output Connection [TWh] Year losses connection [GWh] [%] Submarine cables Number of cables [-] Distance [km] Conductor cross section [mm 2 ] Land cables Number of cables (single phase) [-] Distance [km] Conductor cross section [mm 2 ] Offshore Substation Reactive power comp. [50%] [MVAr] Number of OHVS [-] Capacity per OHVS [MVA] / / /290 Onshore Substation Reactive power comp. [50%] [MVAr] Capacity transformer [MVA] As can be seen in table 7.20 the results for the optimized coupled grid connection of Scheveningen Buiten and Katwijk are similar as for the coupled grid connection of West Rijn and Scheveningen Buiten and the coupled grid connection of West Rijn and Katwijk. The total rated power of the two offshore 146

147 wind farms is similar as is the case for the interconnection distance and distance to the onshore grid connection point. Again the 220 kv HVAC system is the most optimal coupled grid connection. In table 7.21 the results are given for the optimized coupled grid connection with HVDC VSC as main grid connection system. The results of the optimization are also similar to the optimized coupled grid connection of West Rijn and Scheveningen Buiten and to the optimized coupled grid connection of West Rijn and Katwijk. Compared to the losses and costs of the HVAC grid connection system, the losses are about 1.4 to 1.9 times as high and the costs about 30% to 45% higher for the HVDC VSC systems. These values are in a similar range as for the other optimized coupled grid connections. Table 7.21: Results of the optimization of the coupled grid connection of HVDC VSC systems for Scheveningen Buiten and Katwijk. System HVDC VSC Rating 5 4 Voltage [kv] 150 AC ±150 DC 150 AC ±300 DC From [-] SB K SB K To [-] K GRID K GRID Distance [km] Costs Total costs over 20 years [M ] Total losses over 20 years [M ] Total investment costs [M ] Power 20 Year Input Connection [TWh] Year Output Connection [TWh] Year losses connection [GWh] [%] Submarine cables Number of cables [-] Distance [km] Conductor cross section [mm 2 ] Land cables Number of cables (single phase) [-] Distance [km] Conductor cross section [mm 2 ] Offshore Substation Reactive power comp. [50%] [MVAr] Number of OHVS [-] Capacity per OHVS [MVA] 170/ /170 - Capacity converter station [MW] Onshore Substation Capacity converter station [MW]

148 In the table below an overview is given of the optimized coupled grid connection of Scheveningen Buiten and Katwijk for both the HVAC and HVDC VSC systems. As was already indicated the most optimal coupled grid connection is the 220 kv HVAC system. The losses are only 3.2% in total including the infield losses of the offshore wind farms. The costs and losses for the HVDC VSC systems are much higher as was already indicated. Table 7.22: Overview of optimization of coupled grid connection of 'Scheveningen Buiten and Katwijk. Overview Optimization Coupled Grid Connection of Scheveningen Buiten and Katwijk Main System Total 20 Year Production Total 20 Year Losses Total Power Fed into Grid Total Costs Ranking HVAC 132 [kv] [TWh] [GWh] 4.2 [%] [TWh] [M ] 2 HVAC 150 [kv] [TWh] [GWh] 3.9 [%] [TWh] [M ] 3 HVAC 220 [kv] [TWh] [GWh] 3.2 [%] [TWh] [M ] 1 HVDC ±150 [kv] [TWh] [GWh] 6.2 [%] [TWh] [M ] 5 HVDC ±300 [kv] [TWh] [GWh] 5.8 [%] [TWh] [M ] 4 In figures 7.12 and 7.13 an overview is given of the loss and cost components of both the HVAC and HVDC VSC grid connection systems. The main loss and cost components are again similar as was the case for the coupled grid connection of West Rijn and Scheveningen Buiten and the coupled grid connection of West Rijn and Katwijk Losses Coupled Grid Connection 'Scheveningen Buiten' and 'Katwijk' Losses [GWh] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC VSC ±150 kv HVDC VSC ±300 kv Type of Grid Connection Infield Cables OHVS Offshore Reactive Compensation Offshore Converter Submarine Cables Onshore Cables Onshore Substation Onshore Reactive Compensation Onshore Converter Figure 7.12: Overview of losses optimized coupled grid connection systems of 'Scheveningen Buiten and Katwijk. 148

149 500 Costs Coupled Grid Connection 'Scheveningen Buiten' and 'Katwijk Costs [MEURO] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC VSC ±150 kv HVDC VSC ±300 kv Type of Grid Connection OHVS Offshore Reactive Compensation Offshore HVDC Converter Submarine Cables Submarine Cable Installation Onshore Cables Onshore Cable Installation Onshore Substation Onshore Reactive Compensation Onshore HVDC Converter Losses Figure 7.13: Overview of costs optimized coupled grid connection systems of 'Scheveningen Buiten and Katwijk. As was done for the coupled grid connection of West Rijn and Scheveningen Buiten, again the coupled grid connection system is compared with the optimized individual grid connection systems. In table 7.23 the total costs and losses over 20 years of the optimized individual and coupled grid connection systems are shown. As can be seen the total costs over 20 years of operation are about the same for the optimized individual grid connection systems. The losses for the optimized coupled grid connection are higher which is mainly caused due to the fact that for the individual grid connections no onshore transformer is required. With this comparison based on the economical variables as used in this optimization process, the coupled grid connection will not be more feasible than the individual grid connections of offshore wind farms Scheveningen Buiten and Katwijk. Table 7.23: Comparison of losses and costs over 20 years for the individual and coupled grid connections of 'Scheveningen Buiten and Katwijk. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] Scheveningen Buiten 150 kv HVAC kv HVAC - - Katwijk 150 kv HVAC kv HVAC - - Total Scenario 2d: West Rijn, Scheveningen Buiten and Katwijk For the coupled grid connection of all three offshore wind farms West Rijn, Scheveningen Buiten and Katwijk the characteristics as given in table 7.24 will be used. Scheveningen Buiten is located closest to the onshore grid connection point and will therefore be developed first. One year later Katwijk will be developed and two years later West Rijn will be developed. 149

150 Table 7.24: Characteristics for the grid connection of offshore wind farms West Rijn, Scheveningen Buiten and Katwijk. Characteristics Coupled Grid Connection Annual mean wind speed West Rijn 9.2 [m/s] Annual mean wind speed Scheveningen Buiten 9.4 [m/s] Annual mean wind speed Katwijk 9.1 [m/s] Burial depth submarine cable 1.5 [m] Burial depth onshore cable 1.0 [m] Length onshore cable 20 [km] Grid connection voltage 380 [kv] Yearly mean availability 92 [%] Yearly mean wake loss 10 [%] Infield loss at rated power West Rijn 0.6 [%] Infield loss at rated power Scheveningen Buiten 0.6 [%] Infield loss at rated power Katwijk 1.1 [%] In the table below the results are shown for the optimization process of the HVAC systems for the coupled grid connection of all three Trident offshore wind farms. Table 7.25: Results of the optimization of the coupled grid connection of HVAC systems for all three offshore wind farms West Rijn, Scheveningen Buiten and Katwijk. System HVAC Rating Voltage [kv] From [-] WR K SB WR K SB WR K SB To [-] SB SB GRID SB SB GRID SB SB GRID Distance [km] Costs Total costs over 20 years [M ] Total losses over 20 years [M ] Total investment costs [M ] Power 20 Year Input Connection [TWh] Year Output Connection [TWh] Year losses connection [GWh] [%] Submarine cables Number of cables [-] Distance [km] Conductor cross section [mm 2 ] Land cables Number of cables (single phase) [-] Distance [km] Conductor cross section [mm 2 ] Offshore Substation Reactive power comp. [50%] [MVAr] Number of OHVS [-] Capacity per OHVS [MVA] / / /370 Onshore Substation Reactive power comp. [50%] [MVAr] Capacity transformer [MVA]

151 In tables 7.25 and 7.26 the results are given for the optimization of the coupled grid connection of all offshore wind farms West Rijn, Scheveningen Buiten and Katwijk for both the HVAC and HVDC VSC systems. Scheveningen Buiten is located closest to the landing point at Hoek van Holland and will therefore in general have the main offshore substation. The distance between West Rijn and Scheveningen Buiten is about 14.5 km, the distance between Katwijk and Scheveningen Buiten is about 18.2 km and the distance from Scheveningen Buiten to the onshore grid connection point about 54.5 km of which 34.5 km offshore and 20 km onshore. As can be seen in table 7.25 the results for the optimized coupled grid connection of all three offshore wind farms West Rijn, Scheveningen Buiten and Katwijk are similar as for the coupled grid connection of two of these offshore wind farms. Again the 220 kv HVAC system is the most optimal coupled grid connection system. For the grid connection with the 220 kv system only a single interconnection cable is required for both West Rijn and Katwijk and 4 submarine cables are required for the connection to shore which is 1 less as compared to the optimized individual grid connections. Table 7.26: Results of the optimization of the coupled grid connection of HVDC VSC systems for all three offshore wind farms West Rijn, Scheveningen Buiten and Katwijk. System HVDC VSC Rating 5 4 Voltage [kv] 150 AC 150 AC ±150 DC 150 AC 150 AC ±300 DC From [-] WR K SB WR K SB To [-] SB SB GRID SB SB GRID Distance [km] Costs Total costs over 20 years [M ] Total losses over 20 years [M ] Total investment costs [M ] Power 20 Year Input Connection [TWh] Year Output Connection [TWh] Year losses connection [GWh] [%] Submarine cables Number of cables [-] Distance [km] Conductor cross section [mm 2 ] Land cables Number of cables (single phase) [-] Distance [km] Conductor cross section [mm 2 ] Offshore Substation Reactive power comp. [50%] [MVAr] Number of OHVS [-] Capacity per OHVS [MVA] Capacity converter station [MW] Onshore Substation Capacity converter station [MW]

152 In table 7.26 the results are given for the optimized coupled grid connection with HVDC VSC as main grid connection system. The results of the optimization are also similar to the optimized coupled grid connection of two of the offshore wind farms. Of the HVDC VSC systems again the ±300 kv system would be more optimal, but is still much more expensive as compared to the HVAC coupled grid connection systems. Compared to the losses and costs of the HVAC grid connection system, the losses are about 1.35 to 1.9 times as high and the costs about 35% to 50% higher for the HVDC VSC systems. These values are in a similar range as for the optimized coupled grid connections of two of the offshore wind farms. In table 7.27 an overview is given of the optimized coupled grid connection of all three offshore wind farms for both the HVAC and HVDC VSC systems. As was already indicated the most optimal coupled grid connection is the 220 kv HVAC system. The losses are only 3.2% in total including the infield losses of all the offshore wind farms. The costs and losses for the HVDC VSC systems are much higher as was already indicated. Table 7.27: Overview of optimization of coupled grid connection of all three offshore wind farms 'West Rijn', 'Scheveningen Buiten' and Katwijk. Overview Optimization Coupled Grid Connection of West Rijn, Scheveningen Buiten and Katwijk Main System Total 20 Year Production Total 20 Year Losses Total Power Fed into Grid Total Costs Ranking HVAC 132 [kv] [TWh] [GWh] 4.2 [%] [TWh] [M ] 2 HVAC 150 [kv] [TWh] [GWh] 3.9 [%] [TWh] [M ] 3 HVAC 220 [kv] [TWh] [GWh] 3.2 [%] [TWh] [M ] 1 HVDC ±150 [kv] [TWh] [GWh] 6.0 [%] [TWh] [M ] 5 HVDC ±300 [kv] [TWh] [GWh] 5.7 [%] [TWh] [M ] Losses Coupled Grid Connection 'West Rijn', 'Scheveningen Buiten' and 'Katwijk' Losses [GWh] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC VSC ±150 kv HVDC VSC ±300 kv Type of Grid Connection Infield Cables OHVS Offshore Reactive Compensation Offshore Converter Submarine Cables Onshore Cables Onshore Substation Onshore Reactive Compensation Onshore Converter Figure 7.14: Overview of losses optimized coupled grid connection systems of all three offshore wind farms 'West Rijn', 'Scheveningen Buiten' and Katwijk. 152

153 In figures 7.14 and 7.15 an overview is given of the loss and cost components of both the HVAC and HVDC VSC coupled grid connection systems. The main loss and cost components are again similar as was the case for the coupled grid connection of only two offshore wind farms. 800 Costs Coupled Grid Connection 'West Rijn', 'Scheveningen Buiten' and 'Katwijk Costs [MEURO] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC VSC ±150 kv HVDC VSC ±300 kv Type of Grid Connection OHVS Offshore Reactive Compensation Offshore HVDC Converter Submarine Cables Submarine Cable Installation Onshore Cables Onshore Cable Installation Onshore Substation Onshore Reactive Compensation Onshore HVDC Converter Losses Figure 7.15: Overview of costs optimized coupled grid connection systems of all three offshore wind farms 'West Rijn', 'Scheveningen Buiten' and Katwijk. As was done for the coupled grid connection of two of the offshore wind farms, again the coupled grid connection system is compared with the optimized individual grid connection systems. In table 7.28 the total costs and losses over 20 years of the optimized individual and coupled grid connection systems are shown. As can be seen the total costs over 20 years of operation are about the same for the optimized individual grid connection systems as for the optimized coupled grid connection system. The losses for the optimized coupled grid connection are higher which is mainly caused due to the fact that for the individual grid connections no onshore transformer is required. The cable losses of the coupled grid connection are lower as for the individual grid connections though. With this comparison based on the economical variables as used in this optimization process, the coupled grid connection of all three offshore wind farms will not be more feasible than the individual grid connections. Table 7.28: Comparison of losses and costs over 20 years for the individual and coupled grid connections of all three wind farms 'West Rijn', 'Scheveningen Buiten' and Katwijk. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] West Rijn 220 kv HVAC kv HVAC - - Scheveningen Buiten 150 kv HVAC kv HVAC - - Katwijk 150 kv HVAC kv HVAC - - Total

154 7.3.5 Summary of coupled grid connections In the previous paragraphs the optimized coupled grid connections where given for the coupled grid connection of two or three of the Trident offshore wind farms. For the coupled grid connection Hoek van Holland was chosen as landing point in all cases with correspondingly substation Maasvlakte as grid connection point. Though the onshore cable length is longer at landing location Hoek van Holland as compared to landing location IJmuiden, the submarine cable length is quite much shorter in most cases which makes the landing at Hoek van Holland a more economical solution. In real life the landing location IJmuiden might be more feasible due to the difficulties encountered both offshore and onshore when landing at Hoek van Holland, but that is out of the scope of this study. With the coupled grid connection of all scenarios 2a up to 2d the same conclusions can be drawn. In all cases the most optimal coupled grid connection system is a 220 kv HVAC system, followed relatively close by the 132 kv and 150 kv HVAC systems correspondingly. The distance to shore is too small to make the HVDC VSC systems a feasible and suitable alternative for the coupled grid connection due to the high costs and high base losses of the offshore and onshore converter stations. For the HVAC systems the main loss components are the submarine and onshore cables. The main cost components for the HVAC systems are the OHVS stations, the submarine cables, the submarine cable installation and the losses. When the costs and losses of the optimized coupled grid connections are compared with the optimized individual grid connections, the total costs are typically similar but the losses are in most cases slightly higher. The losses are in most cases higher due to the fact that for the individual grid connections in most situations no onshore transformer is required. In most cases 1 cable is required less for the grid connection in case of a coupled grid connection compared to the optimized individual grid connections. Overall can be concluded that in case the optimized coupled grid connection is less costly as individual grid connections the difference in costs are so small that a coupled grid connection will most likely not be a feasible solution due to the complexity of a coupled grid connection. The conclusions as given above are based on economical variables as given in the beginning of this chapter. As was stated in the economical model, not all modeled costs have many references on which the model is based. Also the future development of the metal prices is not certain at the moment. Therefore it is important to check the influence of the results when changing some of the cost components in the economical model. In the next paragraphs the sensitivity analysis will be performed. 154

155 7.4 Sensitivity analysis of cost components In order to check the sensitivity of the results obtained in the previous paragraphs, a sensitivity analysis will be performed in order to check how sensitive the results are to changes of the most important economical parameters. With the sensitivity analysis the results are compared between the individual and coupled grid connections in order to see if some economical parameters change whether the individual or coupled grid connections are economically the most feasible. Also the HVAC and HVDC VSC systems will be compared to see what the effect of a change in some economical parameters has for the results of a certain scenario. In the economical model was already mentioned that for the cost modeling of some cable types and for the offshore HVDC VSC converter station a little number of references have been found. The cost models for some cable types and for the offshore HVDC VSC converter station were therefore based on some assumptions. Actual prices can vary with respect to the costs given by the model. Also the metal prices have their influence on the cost prices of components and prices vary for the different manufacturers. Therefore the cable cost price and the offshore HVDC VSC converter price will be taken into account in the sensitivity analysis. As was indicated also the price of energy lost is of high importance because it indicates the relative importance of energy loss over 20 years compared to the investment costs. The price of energy changes per country and over time depending on the subsidy scheme a country has. Therefore also a change in the price of power loss should be taken into account in the sensitivity analysis. Another variable which is closely related to the costs of the losses over the operational lifetime of the wind farm is the interest rate r. The interest rate indicates the net present value of costs in the future and in this case the costs of future power loss. The higher the interest rate the lower the impact of future power loss and vice versa. A decrease of the interest rate is beneficial for the HVDC VSC systems because they have higher losses compared to the HVAC systems. The final sensitivity analysis which will be performed is about the length of the offshore cable which will be varied for the individual and coupled grid connections in order to see when the coupled grid connections become economically more feasible and to check when the HVDC VSC systems become economically more feasible than the HVAC systems. A sensitivity analysis will be performed concerning the five given parameters as follows: Cable price: base case 100%, variable from 100% to 130% Offshore HVDC VSC converter station: base case 225 k /MW, varied to 175 k /MW 155

156 Price of electricity: base case 100 /MWh, variable from 80 /MWh to 140 /MWh Interest rate r : base case 8%, variable from 6% to 10% Offshore cable length: varied from 40 km to 100 km In the following paragraphs the results of the sensitivity analysis is given Sensitivity analysis: cable price For the sensitivity analysis the price of cables has the largest influence for the HVAC systems due to the fact that more cables and more expensive cable are used compared to the HVDC VSC systems. Due to the rise in the metal prices the last years and the expectancy that the prices of metals will keep rising, only an increase in the cable price will be taken into account in the sensitivity analysis. For the sensitivity analysis the cable price will be varied from 100% to 130% with steps of 10%. First the results will be given for the optimized individual grid connections and after that the optimized coupled grid connections. Table 7.29: Results of sensitivity analysis of price of cables for optimized individual grid connection of 'West Rijn'. Individual Grid Connection West Rijn Price of Cables [%] System Total 20 Year Losses Total Costs HVAC 132 [kv] 682 [GWh] 3.50 [%] [M ] HVAC 150 [kv] 576 [GWh] 2.95 [%] [M ] 100 HVAC 220 [kv] 526 [GWh] 2.70 [%] [M ] HVDC ±150 [kv] 1202 [GWh] 6.17 [%] [M ] HVDC ±300 [kv] 1162 [GWh] 5.97 [%] [M ] HVAC 132 [kv] 686 [GWh] 3.52 [%] [M ] HVAC 150 [kv] 576 [GWh] 2.95 [%] [M ] 110 HVAC 220 [kv] 526 [GWh] 2.70 [%] [M ] HVDC ±150 [kv] 1202 [GWh] 6.17 [%] [M ] HVDC ±300 [kv] 1162 [GWh] 5.97 [%] [M ] HVAC 132 [kv] 686 [GWh] 3.52 [%] [M ] HVAC 150 [kv] 580 [GWh] 2.97 [%] [M ] 120 HVAC 220 [kv] 526 [GWh] 2.70 [%] [M ] HVDC ±150 [kv] 1202 [GWh] 6.17 [%] [M ] HVDC ±300 [kv] 1162 [GWh] 5.97 [%] [M ] HVAC 132 [kv] 686 [GWh] 3.52 [%] [M ] HVAC 150 [kv] 580 [GWh] 2.97 [%] [M ] 130 HVAC 220 [kv] 532 [GWh] 2.73 [%] [M ] HVDC ±150 [kv] 1204 [GWh] 6.18 [%] [M ] HVDC ±300 [kv] 1162 [GWh] 5.97 [%] [M ] 156

157 Table 7.30: Results of sensitivity analysis of price of cables for optimized individual grid connection of 'Scheveningen Buiten'. Individual Grid Connection Scheveningen Buiten Price of Cables [%] System Total 20 Year Losses Total Costs HVAC 132 [kv] 784 [GWh] 3.47 [%] [M ] HVAC 150 [kv] 670 [GWh] 2.97 [%] [M ] 100 HVAC 220 [kv] 608 [GWh] 2.69 [%] [M ] HVDC ±150 [kv] 1374 [GWh] 6.09 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.97 [%] [M ] HVAC 132 [kv] 784 [GWh] 3.47 [%] [M ] HVAC 150 [kv] 672 [GWh] 2.98 [%] [M ] 110 HVAC 220 [kv] 608 [GWh] 2.69 [%] [M ] HVDC ±150 [kv] 1380 [GWh] 6.11 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.97 [%] [M ] HVAC 132 [kv] 784 [GWh] 3.47 [%] [M ] HVAC 150 [kv] 672 [GWh] 2.98 [%] [M ] 120 HVAC 220 [kv] 610 [GWh] 2.71 [%] [M ] HVDC ±150 [kv] 1382 [GWh] 6.12 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.97 [%] [M ] HVAC 132 [kv] 784 [GWh] 3.47 [%] [M ] HVAC 150 [kv] 672 [GWh] 2.98 [%] [M ] 130 HVAC 220 [kv] 610 [GWh] 2.71 [%] [M ] HVDC ±150 [kv] 1382 [GWh] 6.12 [%] [M ] HVDC ±300 [kv] 1350 [GWh] 5.98 [%] [M ] Table 7.31: Results of sensitivity analysis of price of cables for optimized individual grid connection of 'Katwijk'. Individual Grid Connection Katwijk Price of Cables [%] System Total 20 Year Losses Total Costs HVAC 132 [kv] 888 [GWh] 3.95 [%] [M ] HVAC 150 [kv] 744 [GWh] 3.30 [%] [M ] 100 HVAC 220 [kv] 698 [GWh] 3.10 [%] [M ] HVDC ±150 [kv] 1396 [GWh] 6.20 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.98 [%] [M ] HVAC 132 [kv] 888 [GWh] 3.95 [%] [M ] HVAC 150 [kv] 744 [GWh] 3.30 [%] [M ] 110 HVAC 220 [kv] 702 [GWh] 3.12 [%] [M ] HVDC ±150 [kv] 1398 [GWh] 6.21 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.98 [%] [M ] HVAC 132 [kv] 888 [GWh] 3.95 [%] [M ] HVAC 150 [kv] 744 [GWh] 3.30 [%] [M ] 120 HVAC 220 [kv] 702 [GWh] 3.12 [%] [M ] HVDC ±150 [kv] 1404 [GWh] 6.23 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.98 [%] [M ] HVAC 132 [kv] 888 [GWh] 3.95 [%] [M ] HVAC 150 [kv] 744 [GWh] 3.30 [%] [M ] 130 HVAC 220 [kv] 702 [GWh] 3.12 [%] [M ] HVDC ±150 [kv] 1406 [GWh] 6.24 [%] [M ] HVDC ±300 [kv] 1349 [GWh] 5.99 [%] [M ] 157

158 With an increase in the cable price from 100% to 130% the only difference for all three offshore wind farms for their optimized individual grid connection is that the conductor cross section of the onshore cables decreases. The number of cables required and the type of submarine cables remain the same. With a decrease in the conductor cross section of the onshore cables the losses become slightly higher, but the investment costs decrease slightly. Due to the fact that the number of cables in the optimized grid connection for the different systems does not change with an increase in the cable price, the final results for the optimized individual grid connections hardly change. The price increase for the HVAC systems is higher due to the fact that more cables are used which are also more expensive. The price increase is higher with an increase in the system voltage. The overall changes in the results remain small though. In the four following tables the results are given for the optimized coupled grid connections of two or three of the Trident offshore wind farms. The results are given for all type of systems to see how the results change with an increase in the cable price with respect to the difference between the HVAC and HVDC VSC systems. Table 7.32: Results of sensitivity analysis of price of cables for optimized coupled grid connection of West Rijn and 'Scheveningen Buiten'. Coupled Grid Connection West Rijn and Scheveningen Buiten Price of Cables [%] System Total 20 Year Losses Total Costs HVAC 132 [kv] [GWh] 4.01 [%] [M ] HVAC 150 [kv] [GWh] 3.40 [%] [M ] 100 HVAC 220 [kv] [GWh] 3.10 [%] [M ] HVDC ±150 [kv] [GWh] 5.88 [%] [M ] HVDC ±300 [kv] [GWh] 5.57 [%] [M ] HVAC 132 [kv] [GWh] 4.01 [%] [M ] HVAC 150 [kv] [GWh] 3.40 [%] [M ] 110 HVAC 220 [kv] [GWh] 3.10 [%] [M ] HVDC ±150 [kv] [GWh] 5.88 [%] [M ] HVDC ±300 [kv] [GWh] 5.57 [%] [M ] HVAC 132 [kv] [GWh] 4.01 [%] [M ] HVAC 150 [kv] [GWh] 3.40 [%] [M ] 120 HVAC 220 [kv] [GWh] 3.10 [%] [M ] HVDC ±150 [kv] [GWh] 5.88 [%] [M ] HVDC ±300 [kv] [GWh] 5.57 [%] [M ] HVAC 132 [kv] [GWh] 4.01 [%] [M ] HVAC 150 [kv] [GWh] 3.40 [%] [M ] 130 HVAC 220 [kv] [GWh] 3.10 [%] [M ] HVDC ±150 [kv] [GWh] 5.88 [%] [M ] HVDC ±300 [kv] [GWh] 5.57 [%] [M ] 158

159 Table 7.33: Results of sensitivity analysis of price of cables for optimized coupled grid connection of 'West Rijn and Katwijk'. Coupled Grid Connection West Rijn and Katwijk Price of Cables [%] System Total 20 Year Losses Total Costs HVAC 132 [kv] [GWh] 4.36 [%] [M ] HVAC 150 [kv] [GWh] 4.03 [%] [M ] 100 HVAC 220 [kv] [GWh] 3.34 [%] [M ] HVDC ±150 [kv] [GWh] 6.24 [%] [M ] HVDC ±300 [kv] [GWh] 5.87 [%] [M ] HVAC 132 [kv] [GWh] 4.41 [%] [M ] HVAC 150 [kv] [GWh] 4.03 [%] [M ] 110 HVAC 220 [kv] [GWh] 3.34 [%] [M ] HVDC ±150 [kv] [GWh] 6.24 [%] [M ] HVDC ±300 [kv] [GWh] 5.87 [%] [M ] HVAC 132 [kv] [GWh] 4.41 [%] [M ] HVAC 150 [kv] [GWh] 4.03 [%] [M ] 120 HVAC 220 [kv] [GWh] 3.34 [%] [M ] HVDC ±150 [kv] [GWh] 6.24 [%] [M ] HVDC ±300 [kv] [GWh] 5.87 [%] [M ] HVAC 132 [kv] [GWh] 4.47 [%] [M ] HVAC 150 [kv] [GWh] 4.03 [%] [M ] 130 HVAC 220 [kv] [GWh] 3.34 [%] [M ] HVDC ±150 [kv] [GWh] 6.24 [%] [M ] HVDC ±300 [kv] [GWh] 5.87 [%] [M ] Table 7.34: Results of sensitivity analysis of price of cables for optimized coupled grid connection of 'Scheveningen Buiten and Katwijk'. Coupled Grid Connection Scheveningen Buiten and Katwijk Price of Cables [%] System Total 20 Year Losses Total Costs HVAC 132 [kv] [GWh] 4.14 [%] [M ] HVAC 150 [kv] [GWh] 3.91 [%] [M ] 100 HVAC 220 [kv] [GWh] 3.20 [%] [M ] HVDC ±150 [kv] [GWh] 6.16 [%] [M ] HVDC ±300 [kv] [GWh] 5.81 [%] [M ] HVAC 132 [kv] [GWh] 4.14 [%] [M ] HVAC 150 [kv] [GWh] 3.91 [%] [M ] 110 HVAC 220 [kv] [GWh] 3.20 [%] [M ] HVDC ±150 [kv] [GWh] 6.20 [%] [M ] HVDC ±300 [kv] [GWh] 5.83 [%] [M ] HVAC 132 [kv] [GWh] 4.14 [%] [M ] HVAC 150 [kv] [GWh] 3.91 [%] [M ] 120 HVAC 220 [kv] [GWh] 3.24 [%] [M ] HVDC ±150 [kv] [GWh] 6.20 [%] [M ] HVDC ±300 [kv] [GWh] 5.83 [%] [M ] HVAC 132 [kv] [GWh] 4.14 [%] [M ] HVAC 150 [kv] [GWh] 3.97 [%] [M ] 130 HVAC 220 [kv] [GWh] 3.24 [%] [M ] HVDC ±150 [kv] [GWh] 6.20 [%] [M ] HVDC ±300 [kv] [GWh] 5.83 [%] [M ] 159

160 Table 7.35: Results of sensitivity analysis of price of cables for optimized coupled grid connection of West Rijn, Scheveningen Buiten and 'Katwijk'. Coupled Grid Connection West Rijn, Scheveningen Buiten and Katwijk Price of Cables [%] System Total 20 Year Losses Total Costs HVAC 132 [kv] [GWh] 4.18 [%] [M ] HVAC 150 [kv] [GWh] 3.88 [%] [M ] 100 HVAC 220 [kv] [GWh] 3.15 [%] [M ] HVDC ±150 [kv] [GWh] 5.98 [%] [M ] HVDC ±300 [kv] [GWh] 5.71 [%] [M ] HVAC 132 [kv] [GWh] 4.24 [%] [M ] HVAC 150 [kv] [GWh] 3.88 [%] [M ] 110 HVAC 220 [kv] [GWh] 3.21 [%] [M ] HVDC ±150 [kv] [GWh] 6.01 [%] [M ] HVDC ±300 [kv] [GWh] 5.73 [%] [M ] HVAC 132 [kv] [GWh] 4.24 [%] [M ] HVAC 150 [kv] [GWh] 3.88 [%] [M ] 120 HVAC 220 [kv] [GWh] 3.21 [%] [M ] HVDC ±150 [kv] [GWh] 6.01 [%] [M ] HVDC ±300 [kv] [GWh] 5.73 [%] [M ] HVAC 132 [kv] [GWh] 4.24 [%] [M ] HVAC 150 [kv] [GWh] 3.88 [%] [M ] 130 HVAC 220 [kv] [GWh] 3.21 [%] [M ] HVDC ±150 [kv] [GWh] 6.01 [%] [M ] HVDC ±300 [kv] [GWh] 5.73 [%] [M ] In tables 7.32, 7.33, 7.34 and 7.35 the results are given for the sensitivity analysis of the cable price for the optimized coupled grid connections. As can be seen no large changes in the results can be seen as was the case for the optimized individual grid connections. The only difference with respect to the cables is the conductor cross section of the onshore cables which reduces for some systems with an increase in cable price. The number of cables and the type of submarine cables used in the optimized coupled grid connections don t change with an increase in the cable price. With an increase in the cable price the differences between the HVAC and HVDC VSC systems become smaller but remain significant. The final results of the most optimal system do not change for any of the coupled grid connections. Now is known that a change in the cable price does not much influence the results of the optimized results for the individual and coupled grid connections, the results between the individual and coupled grid connections have to be observed. In the following four tables the results of the sensitivity analysis of the cable price for the individual and coupled grid connection are compared. For the comparison the cable price increase to 130% is assumed in order to observe the results in the extreme case. 160

161 Table 7.36: Comparison of losses and costs over 20 years for the individual and coupled grid connections of 'West Rijn' and 'Scheveningen Buiten' for a cable price of 130%. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] West Rijn 220 kv HVAC kv HVAC - - Scheveningen Buiten 150 kv HVAC kv HVAC - - Total Table 7.37: Comparison of losses and costs over 20 years for the individual and coupled grid connections of 'West Rijn' and Katwijk for a cable price of 130%. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] West Rijn 220 kv HVAC kv HVAC - - Katwijk 150 kv HVAC kv HVAC - - Total Table 7.38: Comparison of losses and costs over 20 years for the individual and coupled grid connections of 'Scheveningen Buiten' and Katwijk for a cable price of 130%. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] Scheveningen Buiten 150 kv HVAC kv HVAC - - Katwijk 150 kv HVAC kv HVAC - - Total Table 7.39: Comparison of losses and costs over 20 years for the individual and coupled grid connections of all three wind farms 'West Rijn', 'Scheveningen Buiten' and Katwijk for a cable price of 130%. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] West Rijn 220 kv HVAC kv HVAC - - Scheveningen Buiten 150 kv HVAC kv HVAC - - Katwijk 150 kv HVAC kv HVAC - - Total In the four tables above the results are shown for the comparison of the individual and coupled grid connections of the Trident offshore wind farms for an increase in the cable price to 130% of the base price. In this extreme case it can be seen that for the coupled grid connection of West Rijn and Scheveningen Buiten, the coupled grid connection of Scheveningen Buiten and Katwijk and for the coupled grid connection of all three Trident offshore wind farms the coupled grid connection becomes economically more feasible than the individual grid connection of the offshore wind farms. This difference is caused mainly 161

162 because of the fact that for these coupled grid connection based on the 220 kv HVAC technology 1 cable is required less as compared to the optimized individual grid connections. The difference in price remains below 4% though which is only very small. As was mentioned an increase in the cable price to 130% of the prices as given in the economical model is quite extreme. Combined with the added complexity of a coupled grid connection, the slight advantage of some coupled grid connection configurations will most likely not be enough for offshore wind farm developers to develop a coupled grid connection for their offshore wind farms Sensitivity analysis: price offshore converter For the sensitivity analysis of the price of an offshore HVDC VSC converter station the influence of a decrease in the cost price is studied. In the cost model for the offshore HVDC VSC converter several prices are mentioned which have been found in literature. With a reference to the investments costs in an offshore HVDC VSC system for a commercial project in Germany, the price for an offshore ±150 kv HVDC VSC converter was set at 225 k /MW. The lowest cost price mentioned in literature was 175 k /MW. This is a significant difference in price which can have influence on the outcome of the optimization process due to the high investment costs for the offshore HVDC VSC converter station. Therefore in this sensitivity analysis the price for an offshore ±150 kv HVDC VSC converter is set at 175 k /MW which is compared to the base case scenario with a price of 225 k /MW. As was indicated in the economical model the price for the offshore ±300 kv HVDC VSC converter is assumed to be 10% higher and will thus be set at k /MW. Due to the fact that the HVDC VSC systems have an advantage for the transmission of a large amount of power over a long distance, the comparison will only be done for the coupled grid connection of all three Trident offshore wind farms Results Sensitivity Analysis of Price Offshore Converter of 175 keuro/mw Costs [MEURO] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC ±150 kv HVDC ±300 kv Type of System OHVS Offshore Reactive Compensation Offshore HVDC Converter Submarine Cables Submarine Cable Installation Onshore Cables Onshore Cable Installation Onshore Substation Onshore Reactive Compensation Onshore HVDC Converter Losses Figure 7.16: Overview of cost components of sensitivity analysis of the prices of offshore ±150 kv and ±300 kv HVDC VSC converter stations of 175 k /MW and k /MW respectively for the coupled grid connection of all three Trident offshore wind farms. 162

163 Results Sensitivity Analysis of Price Offshore Converter of 225 keuro/mw Costs [MEURO] HVAC 132 kv HVAC 150 kv HVAC 220 kv HVDC ±150 kv HVDC ±300 kv Type of System OHVS Offshore Reactive Compensation Offshore HVDC Converter Submarine Cables Submarine Cable Installation Onshore Cables Onshore Cable Installation Onshore Substation Onshore Reactive Compensation Onshore HVDC Converter Losses Figure 7.17: Overview of cost components of sensitivity analysis of the prices of offshore ±150 kv and ±300 kv HVDC VSC converter stations of 225 k /MW and k /MW correspondingly for the coupled grid connection of all three Trident offshore wind farms. As can be seen in figures 7.16 and 7.17 the total costs of the offshore converter station has decreased significantly when the prices of an offshore converter station is decreased to 175 k /MW and k /MW for the ±150 kv and ±300kV systems correspondingly. For the ±150 kv and ±300 kv HVDC VSC systems the prices for the offshore converter stations decrease correspondingly by 47 M and 52 M. This corresponds to a decrease in price of about 22%. Even when the price of an offshore ±150 kv HVDC VSC converter station is set at the price of 175 k /MW, the HVAC systems are still economically more feasible as compared to the HVDC VSC systems. The reason for this is that the benefits of the HVDC VSC systems (i.e. low cable losses and costs) become of more importance when the distance to shore increases. The losses and the investment costs of the converters are still that high that the decrease in cable losses and costs of the HVDC VSC systems don t compensate for this. Also for the coupled grid connection still OHVS stations are required in each offshore wind farm in order to connect it to the main offshore substation. The costs for these OHVS and the interconnection cables are also required for the HVDC VSC systems. Therefore the HVDC VSC systems are not suited for a coupled grid connection of a total power of around 1000 MW at a distance of approximately 55 km from the onshore grid connection point (i.e. 35 km offshore and 20 km onshore length) as is the case for the three Trident offshore wind farms. When the distance to the onshore grid connection point increases the HVDC VSC systems will become economically suitable. In paragraph the sensitivity analysis with respect to the distance to shore is shown Sensitivity analysis: price of electricity For the sensitivity analysis of the price of electricity the influence of the losses is studied. For the sensitivity analysis the price of electricity will be varied from 80 /MWh to 140 /MWh with steps of 20 /MWh. With a decrease in the price of 163

164 electricity the importance of losses decreases and vice versa. In the following three tables the results are given for the optimized individual grid connections with a change in the price of electricity. Table 7.40: Results of sensitivity analysis of price of electricity for optimized individual grid connection of 'West Rijn'. Individual Grid Connection West Rijn Price of Electricity [ /MWh] System Total 20 Year Losses Total Costs HVAC 132 [kv] 686 [GWh] 3.52 [%] [M ] HVAC 150 [kv] 580 [GWh] 2.97 [%] [M ] 80 HVAC 220 [kv] 532 [GWh] 2.73 [%] [M ] HVDC ±150 [kv] 1204 [GWh] 6.18 [%] [M ] HVDC ±300 [kv] 1162 [GWh] 5.97 [%] [M ] HVAC 132 [kv] 682 [GWh] 3.50 [%] [M ] HVAC 150 [kv] 564 [GWh] 2.95 [%] [M ] 100 HVAC 220 [kv] 526 [GWh] 2.70 [%] [M ] HVDC ±150 [kv] 1202 [GWh] 6.17 [%] [M ] HVDC ±300 [kv] 1162 [GWh] 5.97 [%] [M ] HVAC 132 [kv] 676 [GWh] 3.47 [%] [M ] HVAC 150 [kv] 572 [GWh] 2.94 [%] [M ] 120 HVAC 220 [kv] 522 [GWh] 2.68 [%] [M ] HVDC ±150 [kv] 1202 [GWh] 6.17 [%] [M ] HVDC ±300 [kv] 1158 [GWh] 5.95 [%] [M ] HVAC 132 [kv] 672 [GWh] 3.45 [%] [M ] HVAC 150 [kv] 566 [GWh] 2.91 [%] [M ] 140 HVAC 220 [kv] 520 [GWh] 2.67 [%] [M ] HVDC ±150 [kv] 1196 [GWh] 6.14 [%] [M ] HVDC ±300 [kv] 1158 [GWh] 5.95 [%] [M ] Table 7.41: Results of sensitivity analysis of price of electricity for optimized individual grid connection of 'Scheveningen Buiten'. Individual Grid Connection Scheveningen Buiten Price of Electricity [ /MWh] System Total 20 Year Losses Total Costs HVAC 132 [kv] 784 [GWh] 3.47 [%] [M ] HVAC 150 [kv] 672 [GWh] 2.98 [%] [M ] 80 HVAC 220 [kv] 610 [GWh] 2.71 [%] [M ] HVDC ±150 [kv] 1382 [GWh] 6.12 [%] [M ] HVDC ±300 [kv] 1350 [GWh] 5.98 [%] [M ] HVAC 132 [kv] 784 [GWh] 3.47 [%] [M ] HVAC 150 [kv] 670 [GWh] 2.97 [%] [M ] 100 HVAC 220 [kv] 608 [GWh] 2.69 [%] [M ] HVDC ±150 [kv] 1374 [GWh] 6.09 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.97 [%] [M ] HVAC 132 [kv] 784 [GWh] 3.47 [%] [M ] HVAC 150 [kv] 668 [GWh] 2.95 [%] [M ] 120 HVAC 220 [kv] 604 [GWh] 2.68 [%] [M ] HVDC ±150 [kv] 1374 [GWh] 6.09 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.97 [%] [M ] 164

165 140 HVAC 132 [kv] 782 [GWh] 3.46 [%] [M ] HVAC 150 [kv] 664 [GWh] 2.94 [%] [M ] HVAC 220 [kv] 602 [GWh] 2.67 [%] [M ] HVDC ±150 [kv] 1374 [GWh] 6.09 [%] [M ] HVDC ±300 [kv] 1346 [GWh] 5.96 [%] [M ] Table 7.42: Results of sensitivity analysis of price of electricity for optimized individual grid connection of 'Katwijk'. Individual Grid Connection Katwijk Price of Electricity [ /MWh] System Total 20 Year Losses Total Costs HVAC 132 [kv] 888 [GWh] 3.95 [%] [M ] HVAC 150 [kv] 744 [GWh] 3.30 [%] [M ] 80 HVAC 220 [kv] 702 [GWh] 3.12 [%] [M ] HVDC ±150 [kv] 1406 [GWh] 6.24 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.98 [%] [M ] HVAC 132 [kv] 44.4 [GWh] 3.95 [%] [M ] HVAC 150 [kv] 744 [GWh] 3.30 [%] [M ] 100 HVAC 220 [kv] 698 [GWh] 3.10 [%] [M ] HVDC ±150 [kv] 1396 [GWh] 6.20 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.98 [%] [M ] HVAC 132 [kv] 888 [GWh] 3.95 [%] [M ] HVAC 150 [kv] 744 [GWh] 3.30 [%] [M ] 120 HVAC 220 [kv] 696 [GWh] 3.09 [%] [M ] HVDC ±150 [kv] 1396 [GWh] 6.20 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.98 [%] [M ] HVAC 132 [kv] 888 [GWh] 3.95 [%] [M ] HVAC 150 [kv] 740 [GWh] 3.28 [%] [M ] 140 HVAC 220 [kv] 696 [GWh] 3.09 [%] [M ] HVDC ±150 [kv] 1396 [GWh] 6.20 [%] [M ] HVDC ±300 [kv] 1344 [GWh] 5.97 [%] [M ] In tables 7.40, 7.41 and 7.42 the results are given for a variation in the electricity price for the optimized individual grid connections of the Trident offshore wind farms. Because of the increase in importance of the losses with an increase in the price of electricity, the optimized results have decreasing losses. The decrease in losses is in all cases only small though, because of the fact that the only improvements are an increase in the conductor cross section for the onshore cables. The number of cables and the submarine cables don t change in any case because the cost savings of losses by adding a cable or increasing the conductor cross section of the submarine cables does not compensate for the added investment costs (e.g. or a decrease in conductor cross section for the submarine cables is not even possible due to the power transmission requirements). As can be seen also the difference between the HVAC and HVDC VSC systems becomes larger with an increase in the price of electricity due to the much larger losses in the HVDC VSC systems. In the four following tables the results are given for the optimized coupled grid connections of two or three of the Trident offshore wind farms. The results are 165

166 given for all type of systems to see how the results change with a decrease or increase in the price of electricity. Table 7.43: Results of sensitivity analysis of price of electricity for optimized coupled grid connection of West Rijn and 'Scheveningen Buiten'. Coupled Grid Connection West Rijn and Scheveningen Buiten Price of Electricity [ /MWh] System Total 20 Year Losses Total Costs HVAC 132 [kv] [GWh] 4.01 [%] [M ] HVAC 150 [kv] [GWh] 3.40 [%] [M ] 80 HVAC 220 [kv] [GWh] 3.10 [%] [M ] HVDC ±150 [kv] [GWh] 5.88 [%] [M ] HVDC ±300 [kv] [GWh] 5.57 [%] [M ] HVAC 132 [kv] [GWh] 4.01 [%] [M ] HVAC 150 [kv] [GWh] 3.40 [%] [M ] 100 HVAC 220 [kv] [GWh] 3.10 [%] [M ] HVDC ±150 [kv] [GWh] 5.88 [%] [M ] HVDC ±300 [kv] [GWh] 5.57 [%] [M ] HVAC 132 [kv] [GWh] 3.68 [%] [M ] HVAC 150 [kv] [GWh] 3.40 [%] [M ] 120 HVAC 220 [kv] [GWh] 3.10 [%] [M ] HVDC ±150 [kv] [GWh] 5.85 [%] [M ] HVDC ±300 [kv] [GWh] 5.50 [%] [M ] HVAC 132 [kv] [GWh] 3.68 [%] [M ] HVAC 150 [kv] [GWh] 3.35 [%] [M ] 140 HVAC 220 [kv] [GWh] 3.10 [%] [M ] HVDC ±150 [kv] [GWh] 5.82 [%] [M ] HVDC ±300 [kv] [GWh] 5.48 [%] [M ] Table 7.44: Results of sensitivity analysis of price of electricity for optimized coupled grid connection of 'West Rijn and Katwijk'. Coupled Grid Connection West Rijn and Katwijk Price of Electricity [ /MWh] System Total 20 Year Losses Total Costs HVAC 132 [kv] [GWh] 4.47 [%] [M ] HVAC 150 [kv] [GWh] 4.03 [%] [M ] 80 HVAC 220 [kv] [GWh] 3.39 [%] [M ] HVDC ±150 [kv] [GWh] 6.24 [%] [M ] HVDC ±300 [kv] [GWh] 5.87 [%] [M ] HVAC 132 [kv] [GWh] 4.36 [%] [M ] HVAC 150 [kv] [GWh] 4.03 [%] [M ] 100 HVAC 220 [kv] [GWh] 3.34 [%] [M ] HVDC ±150 [kv] [GWh] 6.24 [%] [M ] HVDC ±300 [kv] [GWh] 5.87 [%] [M ] HVAC 132 [kv] [GWh] 4.26 [%] [M ] HVAC 150 [kv] [GWh] 4.03 [%] [M ] 120 HVAC 220 [kv] [GWh] 3.34 [%] [M ] HVDC ±150 [kv] [GWh] 6.19 [%] [M ] HVDC ±300 [kv] [GWh] 5.84 [%] [M ] 166

167 140 HVAC 132 [kv] [GWh] 4.18 [%] [M ] HVAC 150 [kv] [GWh] 4.03 [%] [M ] HVAC 220 [kv] [GWh] 3.34 [%] [M ] HVDC ±150 [kv] [GWh] 6.17 [%] [M ] HVDC ±300 [kv] [GWh] 5.84 [%] [M ] Table 7.45: Results of sensitivity analysis of price of electricity for optimized coupled grid connection of 'Scheveningen Buiten and Katwijk'. Coupled Grid Connection Scheveningen Buiten and Katwijk Price of Electricity [ /MWh] System Total 20 Year Losses Total Costs HVAC 132 [kv] [GWh] 4.14 [%] [M ] HVAC 150 [kv] [GWh] 3.97 [%] [M ] 80 HVAC 220 [kv] [GWh] 3.24 [%] [M ] HVDC ±150 [kv] [GWh] 6.20 [%] [M ] HVDC ±300 [kv] [GWh] 5.83 [%] [M ] HVAC 132 [kv] [GWh] 4.14 [%] [M ] HVAC 150 [kv] [GWh] 3.91 [%] [M ] 100 HVAC 220 [kv] [GWh] 3.20 [%] [M ] HVDC ±150 [kv] [GWh] 6.16 [%] [M ] HVDC ±300 [kv] [GWh] 5.81 [%] [M ] HVAC 132 [kv] [GWh] 4.14 [%] [M ] HVAC 150 [kv] [GWh] 3.73 [%] [M ] 120 HVAC 220 [kv] [GWh] 3.16 [%] [M ] HVDC ±150 [kv] [GWh] 6.04 [%] [M ] HVDC ±300 [kv] [GWh] 5.71 [%] [M ] HVAC 132 [kv] [GWh] 4.14 [%] [M ] HVAC 150 [kv] [GWh] 3.73 [%] [M ] 140 HVAC 220 [kv] [GWh] 3.16 [%] [M ] HVDC ±150 [kv] [GWh] 6.04 [%] [M ] HVDC ±300 [kv] [GWh] 5.71 [%] [M ] Table 7.46: Results of sensitivity analysis of price of electricity for optimized coupled grid connection of West Rijn, Scheveningen Buiten and 'Katwijk'. Coupled Grid Connection West Rijn, Scheveningen Buiten and Katwijk Price of Electricity [ /MWh] System Total 20 Year Losses Total Costs HVAC 132 [kv] [GWh] 4.24 [%] [M ] HVAC 150 [kv] [GWh] 3.88 [%] [M ] 80 HVAC 220 [kv] [GWh] 3.21 [%] [M ] HVDC ±150 [kv] [GWh] 6.01 [%] [M ] HVDC ±300 [kv] [GWh] 5.73 [%] [M ] HVAC 132 [kv] [GWh] 4.18 [%] [M ] HVAC 150 [kv] [GWh] 3.88 [%] [M ] 100 HVAC 220 [kv] [GWh] 3.15 [%] [M ] HVDC ±150 [kv] [GWh] 5.98 [%] [M ] HVDC ±300 [kv] [GWh] 5.71 [%] [M ] HVAC 132 [kv] [GWh] 4.10 [%] [M ] HVAC 150 [kv] [GWh] 3.90 [%] [M ] 120 HVAC 220 [kv] [GWh] 3.15 [%] [M ] HVDC ±150 [kv] [GWh] 5.93 [%] [M ] HVDC ±300 [kv] [GWh] 5.66 [%] [M ] 167

168 140 HVAC 132 [kv] [GWh] 4.10 [%] [M ] HVAC 150 [kv] [GWh] 3.69 [%] [M ] HVAC 220 [kv] [GWh] 3.15 [%] [M ] HVDC ±150 [kv] [GWh] 5.91 [%] [M ] HVDC ±300 [kv] [GWh] 5.65 [%] [M ] In tables 7.43, 7.44, 7.45 and 7.46 the results are shown for a change in the price of electricity for the coupled grid connection of two or three of the Trident offshore wind farms. With an increase in the price of electricity the losses become of more importance and therefore the optimized systems have a decrease in losses. The decrease in losses is in most systems only small and is caused only by an increase in the conductor cross section of the onshore and interconnection cables. In none of the cases the type of offshore grid connection cable has changed. For some cases also the number of onshore cable systems decreases. This is the case for the 132 kv HVAC system for the coupled grid connection of West Rijn and Scheveningen Buiten (i.e. from 3 to 2 onshore cables systems), for the 150 kv HVAC system for the coupled grid connection of Scheveningen Buiten and Katwijk (i.e. from 3 to 2 onshore cable systems) and for the 150 kv HVAC system for the coupled grid connection of all three Trident offshore wind farms (i.e. from 4 to 3 onshore cable systems). In none of the optimized coupled grid connections the most optimal system changes with an increase or decrease in the price of electricity. The difference between the HVAC and HVDC VSC systems also increase with an increase in the price of electricity due to the much higher losses for the HVDC VSC systems. Now is known that a change in the price of electricity does not influence the results of the optimized results for the individual and coupled grid connections, the results between the individual and coupled grid connections have to be observed. In the following four tables the results of the sensitivity analysis of the price of electricity for the individual and coupled grid connection are compared. For the comparison the price of electricity of 140 /MWh is assumed in order to observe the results in the extreme case. Table 7.47: Comparison of losses and costs over 20 years for individual and coupled grid connections of 'West Rijn' and 'Scheveningen Buiten' for an electricity price of 140 /MWh. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] West Rijn 220 kv HVAC kv HVAC - - Scheveningen Buiten 150 kv HVAC kv HVAC - - Total

169 Table 7.48: Comparison of losses and costs over 20 years for the individual and coupled grid connections of 'West Rijn' and Katwijk for an electricity price of 140 /MWh. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] West Rijn 220 kv HVAC kv HVAC - - Katwijk 150 kv HVAC kv HVAC - - Total Table 7.49: Comparison of losses and costs over 20 years for the individual and coupled grid connections of 'Scheveningen Buiten' and Katwijk for an electricity price of 140 /MWh. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] Scheveningen Buiten 150 kv HVAC kv HVAC - - Katwijk 150 kv HVAC kv HVAC - - Total Table 7.50: Comparison of losses and costs over 20 years for the individual and coupled grid connections of all three wind farms 'West Rijn', 'Scheveningen Buiten' and Katwijk for an electricity price of 140 /MWh. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] West Rijn 220 kv HVAC kv HVAC - - Scheveningen Buiten 150 kv HVAC kv HVAC - - Katwijk 150 kv HVAC kv HVAC - - Total In the four tables above the results are shown for the comparison of the individual and coupled grid connections of the Trident offshore wind farms for an increase in the price of electricity to 140 /MWh. With this high price of electricity the losses for the individual and coupled grid connection are decreases as far as possible due to the increased importance of the losses. As can be seen the differences between the optimized individual and coupled grid connections are only small and remain below 1.2%. In some cases the individual grid connection is slightly better, in other cases the coupled grid connection. For the coupled grid connection of all three Trident offshore wind farms where a relatively large advantage of the coupled grid connection should be expected, the coupled grid connection is only 0.1% cheaper which is extremely small. These results combined with the added complexity of a coupled grid connection make a coupled grid connection economically not more feasible for offshore wind farm developers as compared to individual grid connections of their offshore wind farms when the price of electricity increases significantly at the distance of 55 km towards the onshore grid connection point as is the case for the Trident offshore wind farms. 169

170 7.4.4 Sensitivity analysis: interest rate r For the sensitivity analysis of the interest rate r the influence of losses over the 20 years of operation of the offshore wind farms is studied. For the sensitivity analysis the interest rate r will be varied from 6% to 10% with steps of 2%. With a decrease of the interest rate r to 6% the importance of losses over 20 years become of more importance and vice versa. First the results will be given for the optimized individual grid connections and after that the optimized coupled grid connections. Table 7.51: Results of sensitivity analysis of price interest rate r for optimized individual grid connection of 'West Rijn'. Individual Grid Connection West Rijn Interest Rate r [%] System Total 20 Year Losses Total Costs HVAC 132 [kv] 678 [GWh] 3.48 [%] [M ] HVAC 150 [kv] 572 [GWh] 2.94 [%] [M ] 6 HVAC 220 [kv] 522 [GWh] 2.68 [%] [M ] HVDC ±150 [kv] 1202 [GWh] 6.17 [%] [M ] HVDC ±300 [kv] 1158 [GWh] 5.95 [%] [M ] HVAC 132 [kv] 682 [GWh] 3.50 [%] [M ] HVAC 150 [kv] 576 [GWh] 2.95 [%] [M ] 8 HVAC 220 [kv] 526 [GWh] 2.70 [%] [M ] HVDC ±150 [kv] 1202 [GWh] 6.17 [%] [M ] HVDC ±300 [kv] 1162 [GWh] 5.97 [%] [M ] HVAC 132 [kv] 686 [GWh] 3.52 [%] [M ] HVAC 150 [kv] 580 [GWh] 2.97 [%] [M ] 10 HVAC 220 [kv] 526 [GWh] 2.70 [%] [M ] HVDC ±150 [kv] 1202 [GWh] 6.17 [%] [M ] HVDC ±300 [kv] 1162 [GWh] 5.97 [%] [M ] Table 7.52: Results of sensitivity analysis of interest rate r for optimized individual grid connection of 'Scheveningen Buiten'. Individual Grid Connection Scheveningen Buiten Interest Rate r [%] System Total 20 Year Losses Total Costs HVAC 132 [kv] 784 [GWh] 3.47 [%] [M ] HVAC 150 [kv] 668 [GWh] 2.95 [%] [M ] 6 HVAC 220 [kv] 604 [GWh] 2.68 [%] [M ] HVDC ±150 [kv] 1374 [GWh] 6.09 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.97 [%] [M ] HVAC 132 [kv] 784 [GWh] 3.47 [%] [M ] HVAC 150 [kv] 670 [GWh] 2.97 [%] [M ] 8 HVAC 220 [kv] 608 [GWh] 2.69 [%] [M ] HVDC ±150 [kv] 1374 [GWh] 6.09 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.97 [%] [M ] HVAC 132 [kv] 784 [GWh] 3.47 [%] [M ] HVAC 150 [kv] 672 [GWh] 2.98 [%] [M ] 10 HVAC 220 [kv] 608 [GWh] 2.69 [%] [M ] HVDC ±150 [kv] 1382 [GWh] 6.12 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.97 [%] [M ] 170

171 Table 7.53: Results of sensitivity analysis of interest rate r for optimized individual grid connection of 'Katwijk'. Individual Grid Connection Katwijk Interest Rate r [%] System Total 20 Year Losses Total Costs HVAC 132 [kv] 888 [GWh] 3.95 [%] [M ] HVAC 150 [kv] 744 [GWh] 3.30 [%] [M ] 6 HVAC 220 [kv] 698 [GWh] 3.10 [%] [M ] HVDC ±150 [kv] 1396 [GWh] 6.20 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.98 [%] [M ] HVAC 132 [kv] 888 [GWh] 3.95 [%] [M ] HVAC 150 [kv] 744 [GWh] 3.30 [%] [M ] 8 HVAC 220 [kv] 698 [GWh] 3.10 [%] [M ] HVDC ±150 [kv] 1396 [GWh] 6.20 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.98 [%] [M ] HVAC 132 [kv] 888 [GWh] 3.95 [%] [M ] HVAC 150 [kv] 744 [GWh] 3.30 [%] [M ] 10 HVAC 220 [kv] 702 [GWh] 3.12 [%] [M ] HVDC ±150 [kv] 1402 [GWh] 6.22 [%] [M ] HVDC ±300 [kv] 1348 [GWh] 5.98 [%] [M ] In tables 7.51, 7.52 and 7.53 the results are given for a variation of the interest rate r for the optimized individual grid connections of the Trident offshore wind farms. With a decrease of the interest rate r to 6% the importance of losses increases and with an increase of the interest rate r to 10% the importance of the losses decrease. Accordingly the results for the optimized individual grid connections vary only slightly. With a decrease in the interest rate r the conductor cross section of the onshore cables increases in order to minimize the losses. With an increase in the interest rate r the conductor cross section of the onshore cables decreases in order to minimize the investment costs. In none of the cases the type or number of submarine cables changes, because adding a cable or increasing the conductor cross section of the submarine cables does not compensate for the added investment costs in case of a decrease in the interest rate r. As can be seen the difference between the HVAC and HVDC VSC systems decreases for an increase in the interest rate r and increases with a decrease of the interest rate r. In the four following tables the results are given for the optimized coupled grid connections of two or three of the Trident offshore wind farms. The results are given for all type of systems to see how the results change with a decrease or increase of the interest rate r. 171

172 Table 7.54: Results of sensitivity analysis of interest rate r for optimized coupled grid connection of West Rijn and 'Scheveningen Buiten'. Coupled Grid Connection West Rijn and Scheveningen Buiten Interest Rate r [%] System Total 20 Year Losses Total Costs HVAC 132 [kv] [GWh] 3.73 [%] [M ] HVAC 150 [kv] [GWh] 3.40 [%] [M ] 6 HVAC 220 [kv] [GWh] 3.10 [%] [M ] HVDC ±150 [kv] [GWh] 5.86 [%] [M ] HVDC ±300 [kv] [GWh] 5.51 [%] [M ] HVAC 132 [kv] [GWh] 4.01 [%] [M ] HVAC 150 [kv] [GWh] 3.40 [%] [M ] 8 HVAC 220 [kv] [GWh] 3.10 [%] [M ] HVDC ±150 [kv] [GWh] 5.88 [%] [M ] HVDC ±300 [kv] [GWh] 5.57 [%] [M ] HVAC 132 [kv] [GWh] 4.01 [%] [M ] HVAC 150 [kv] [GWh] 3.40 [%] [M ] 10 HVAC 220 [kv] [GWh] 3.10 [%] [M ] HVDC ±150 [kv] [GWh] 5.88 [%] [M ] HVDC ±300 [kv] [GWh] 5.57 [%] [M ] Table 7.55: Results of sensitivity analysis of interest rate r for optimized coupled grid connection of 'West Rijn and Katwijk'. Coupled Grid Connection West Rijn and Katwijk Interest Rate r [%] System Total 20 Year Losses Total Costs HVAC 132 [kv] [GWh] 4.36 [%] [M ] HVAC 150 [kv] [GWh] 4.03 [%] [M ] 6 HVAC 220 [kv] [GWh] 3.34 [%] [M ] HVDC ±150 [kv] [GWh] 6.20 [%] [M ] HVDC ±300 [kv] [GWh] 5.85 [%] [M ] HVAC 132 [kv] [GWh] 4.36 [%] [M ] HVAC 150 [kv] [GWh] 4.03 [%] [M ] 8 HVAC 220 [kv] [GWh] 3.34 [%] [M ] HVDC ±150 [kv] [GWh] 6.24 [%] [M ] HVDC ±300 [kv] [GWh] 5.87 [%] [M ] HVAC 132 [kv] [GWh] 4.41 [%] [M ] HVAC 150 [kv] [GWh] 4.03 [%] [M ] 10 HVAC 220 [kv] [GWh] 3.34 [%] [M ] HVDC ±150 [kv] [GWh] 6.24 [%] [M ] HVDC ±300 [kv] [GWh] 5.87 [%] [M ] 172

173 Table 7.56: Results of sensitivity analysis of interest rate r for optimized coupled grid connection of 'Scheveningen Buiten and Katwijk'. Coupled Grid Connection Scheveningen Buiten and Katwijk Interest Rate r [%] System Total 20 Year Losses Total Costs HVAC 132 [kv] [GWh] 4.14 [%] [M ] HVAC 150 [kv] [GWh] 3.73 [%] [M ] 6 HVAC 220 [kv] [GWh] 3.16 [%] [M ] HVDC ±150 [kv] [GWh] 6.04 [%] [M ] HVDC ±300 [kv] [GWh] 5.71 [%] [M ] HVAC 132 [kv] [GWh] 4.14 [%] [M ] HVAC 150 [kv] [GWh] 3.91 [%] [M ] 8 HVAC 220 [kv] [GWh] 3.20 [%] [M ] HVDC ±150 [kv] [GWh] 6.16 [%] [M ] HVDC ±300 [kv] [GWh] 5.81 [%] [M ] HVAC 132 [kv] [GWh] 4.14 [%] [M ] HVAC 150 [kv] [GWh] 3.91 [%] [M ] 10 HVAC 220 [kv] [GWh] 3.24 [%] [M ] HVDC ±150 [kv] [GWh] 6.20 [%] [M ] HVDC ±300 [kv] [GWh] 5.83 [%] [M ] Table 7.57: Results of sensitivity analysis of interest rate r for optimized coupled grid connection of West Rijn, Scheveningen Buiten and 'Katwijk'. Coupled Grid Connection West Rijn, Scheveningen Buiten and Katwijk Interest Rate r [%] System Total 20 Year Losses Total Costs HVAC 132 [kv] [GWh] 4.09 [%] [M ] HVAC 150 [kv] [GWh] 3.90 [%] [M ] 6 HVAC 220 [kv] [GWh] 3.15 [%] [M ] HVDC ±150 [kv] [GWh] 5.93 [%] [M ] HVDC ±300 [kv] [GWh] 5.66 [%] [M ] HVAC 132 [kv] [GWh] 4.18 [%] [M ] HVAC 150 [kv] [GWh] 3.88 [%] [M ] 8 HVAC 220 [kv] [GWh] 3.15 [%] [M ] HVDC ±150 [kv] [GWh] 5.98 [%] [M ] HVDC ±300 [kv] [GWh] 5.71 [%] [M ] HVAC 132 [kv] [GWh] 4.24 [%] [M ] HVAC 150 [kv] [GWh] 3.88 [%] [M ] 10 HVAC 220 [kv] [GWh] 3.21 [%] [M ] HVDC ±150 [kv] [GWh] 6.01 [%] [M ] HVDC ±300 [kv] [GWh] 5.73 [%] [M ] In tables 7.54, 7.55, 7.56 and 7.57 the results are shown for the change of the interest rate r for the coupled grid connection of two or three of the Trident offshore wind farms. With a decrease of interest rate r the losses become more important and therefore the optimized systems have a decrease in the losses. The decrease in losses is in most systems only small and is caused only by an increase in the conductor cross section of the onshore and interconnection cables. In none of the cases the type of offshore grid connection cables has changed. For some cases also the number of onshore cable systems decreases. 173

174 This is the case for the 132 kv HVAC system for the coupled grid connection of West Rijn and Scheveningen Buiten (i.e. from 3 to 2 onshore cable systems) and for the coupled grid connection of Scheveningen Buiten and Katwijk (i.e. from 3 to 2 onshore cable systems). With an increase of the interest rate r the importance of losses decreases and accordingly the conductor cross section of the onshore and interconnection cables decrease in most cases. In none of the cases the type of offshore grid connection cable has changed when the interest rate r is increased to 10%. The difference between the HVAC and HVDC VSC systems also increase with a decrease of the interest rate r and decreases with an increase of the interest rate r due to the much higher losses for the HVDC VSC systems. Now is known that a change of the interest rate r does not influence the results of the optimized results for the individual and coupled grid connections, the results between the individual and coupled grid connections have to be observed. In the following four tables the results of the sensitivity analysis of the interest rate r for the individual and coupled grid connection are compared. For the comparison the interest rate r of 10% is assumed, because an increase of the interest rate r is more realistic than a decrease in the interest rate r. Table 7.58: Comparison of losses and costs over 20 years for individual and coupled grid connections of 'West Rijn' and 'Scheveningen Buiten' for an interest rate of 10%. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] West Rijn 220 kv HVAC kv HVAC - - Scheveningen Buiten 150 kv HVAC kv HVAC - - Total Table 7.59: Comparison of losses and costs over 20 years for the individual and coupled grid connections of 'West Rijn' and Katwijk for an interest rate of 10%. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] West Rijn 220 kv HVAC kv HVAC - - Katwijk 150 kv HVAC kv HVAC - - Total Table 7.60: Comparison of losses and costs over 20 years for the individual and coupled grid connections of 'Scheveningen Buiten' and Katwijk for an interest rate of 10%. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] Scheveningen Buiten 150 kv HVAC kv HVAC - - Katwijk 150 kv HVAC kv HVAC - - Total

175 Table 7.61: Comparison of losses and costs over 20 years for the individual and coupled grid connections of all three wind farms 'West Rijn', 'Scheveningen Buiten' and Katwijk for an interest rate of 10%. Individual Grid Connection Coupled Grid Connection System Total Costs [M ] 20 Year Losses [GWh] System Total Costs [M ] 20 Year Losses [GWh] West Rijn 220 kv HVAC kv HVAC - - Scheveningen Buiten 150 kv HVAC kv HVAC - - Katwijk 150 kv HVAC kv HVAC - - Total In the four tables above the results are shown for the comparison of the individual and coupled grid connections of the Trident offshore wind farms for an increase of the interest rate r to 10%. With this high value for the interest rate r the losses are of less importance and therefore the total costs over 20 years of losses decreases. As can be seen the coupled grid connection is economically more feasible in three of the four cases. The differences between the coupled and individual grid connections remain below 2.5% though which is rather small. These results combined with the added complexity of a coupled grid connection make a coupled grid connection economically not more feasible for offshore wind farm developers as compared to individual grid connections of their offshore wind farms in case of an increase in the interest rate r Sensitivity analysis: grid connection length The final sensitivity analysis which will be performed is to check when the break even point is reached for the individual and coupled grid connections when the distance to the onshore grid connection station is varied. Also the break even point for the HVAC and HVDC VSC systems is determined. For the sensitivity analysis the length of the offshore cable connection is varied for the individual grid connections and for the coupled grid connections of two or three of the Trident offshore wind farms. By varying the offshore cable length the break even point can be found for the optimized individual grid connection systems and the coupled grid connections. By increasing the offshore cable length, the optimized individual and coupled grid connection systems vary. In table 7.62 the optimized systems are given for the individual and coupled grid connection systems. As can be seen in table 7.62 the most optimal coupled grid connection is the same for the coupled grid connection of either two or three of the Trident offshore wind farms. For the coupled grid connection the 220 kv HVAC system is the most optimal for offshore cable lengths up to about 70 km (i.e. excluding the 20 km onshore cable trajectory to the onshore grid connection point) and for offshore cable lengths of 80 km or more the ±300 kv HVDC VSC system is the most optimal. For the optimized individual grid connection systems the most optimal system varies for the three different Trident offshore wind farms. For 175

176 offshore wind farm Scheveningen Buiten the 150 kv HVAC systems is the most suited up to an offshore cable length of 90 km (i.e. excluding 8.2 km onshore cable trajectory to the onshore grid connection point) and above 90 km the ±300 kv HVDC VSC system is the most optimal. For offshore wind farm West Rijn which is the smallest of the three Trident offshore wind farms, the most optimal system varies three times. Up to 50 km offshore cable length (i.e. excluding 11 km onshore cable trajectory to the onshore grid connection point) the 220 kv HVAC system is the most optimal. Above 50 km 2 instead of 1 offshore cables would be required for the 220 kv HVAC system and therefore the 132 kv HVAC system becomes economically more feasible. When the offshore cable length is increased further above 80 km the ±150 kv HVDC VSC systems becomes the most feasible. For offshore wind farm Katwijk up to 70 km the most feasible system is the 150 kv HVAC system. Above 70 km the ±300 kv HVDC VSC systems becomes the most feasible system. With an increase in the offshore cable length the investment costs of the HVAC systems become higher than the investment costs for the HVDC VSC systems. Therefore the break even point depends on the losses in the systems for the different grid connection options. Table 7.62: Optimized systems for the individual and coupled grid connections for a varying offshore cable length. Optimized Individual Grid Connections 40 km 50 km 60 km 70 km 80 km 90 km 100 km WR 220 kv AC 220 kv AC 132 kv AC 132 kv AC ±150 kv DC ±150 kv DC ±150 kv DC SB 150 kv AC 150 kv AC 150 kv AC 150 kv AC 150 kv AC 150 kv AC ±300 kv DC K 150 kv AC 150 kv AC 150 kv AC 150 kv AC ±300 kv DC ±300 kv DC ±300 kv DC Optimized Coupled Grid Connections WR + SB 220 kv AC 220 kv AC 220 kv AC 220 kv AC ±300 kv DC ±300 kv DC ±300 kv DC WR + K 220 kv AC 220 kv AC 220 kv AC 220 kv AC ±300 kv DC ±300 kv DC ±300 kv DC SB + K 220 kv AC 220 kv AC 220 kv AC 220 kv AC ±300 kv DC ±300 kv DC ±300 kv DC WR + SB + K 220 kv AC 220 kv AC 220 kv AC 220 kv AC ±300 kv DC ±300 kv DC ±300 kv DC As is described above the economically most feasible system varies the most for the individual grid connections when the offshore cable length is being increased. With increasing the offshore cable length the break even point can be found for which, under the given economical variables, the coupled grid connections become economically more feasible than the individual grid connections. Also the break even point between the HVAC and HVDC VSC systems can be found. In figures 7.18, 7.19 and 7.20 the results are given for the break even point of the individual and coupled grid connection of two of the Trident offshore wind farms. As can be seen in table 7.62 the economically most feasible system for offshore wind farm Scheveningen Buiten is up to 90 km the 150 kv HVAC system. For offshore wind farm West Rijn the most feasible solution for offshore cables lengths above 50 km up to 70 km is the 132 kv HVAC system. For the coupled grid connection of these two offshore wind farms the economically most feasible solution up to 70 is the 220 kv HVAC system. The investment cost for the 220 kv HVAC system is higher as compared to the 132 kv and 150 kv HVAC systems, which causes the break even point for which the coupled grid connection 176

177 becomes economically more feasible to be around 85 km, when for the individual grid connection of both offshore wind farms the HVDC VSC system becomes the most feasible. In this comparison the onshore cable lengths are used as were given for the individual and coupled grid connections. One of the main reasons why the break even point is so far from shore is because of the interconnection cables which are also required between the offshore wind farms in case of the coupled grid connection. The distance between the two offshore wind farms is about 14.5 km which is significant. When one of the economical variables would be such that the value of the losses over 20 year of operation becomes higher, the break even point is reached at a shorter offshore cable length. 600 Results Sensitivity Analysis Length of Main Grid Connection WR and SB Total Costs [MEURO] km 50 km 60 km 70 km 80 km 90 km 100 km Offshore Cable Length Coupled Grid Connection 'West Rijn' and 'Scheveningen Buiten' Sum Individual Grid Connections 'West Rijn' and 'Scheveningen Buiten' Figure 7.18: Results of sensitivity analysis of varying the offshore cable length for the individual and coupled grid connection of 'West Rijn' and 'Scheveningen Buiten'. In figure 7.19 the results are given for the varying offshore cable length with respect to the optimized individual and coupled grid connection of offshore wind farms West Rijn and Katwijk. As can be seen the break even point between the optimized individual and coupled grid connections is above 100 km of offshore cable length. The reason why this break even point lays much further as compared to the coupled grid connection of West Rijn and Scheveningen Buiten is because of the fact that from 80 km of offshore cable length onwards for both the individual and the coupled grid connection the HVDC VSC system is the most optimal. The main cost components of the HVDC VSC system are the converter stations and the price of a converter station is given per MW which is about equal for the sum of the individual grid connections and the coupled grid connection. Therefore the main difference between the optimized individual and coupled grid connections of West Rijn and Katwijk for large offshore cable lengths is the number of cables required for the HVDC VSC systems. This cost difference is only small and therefore the break even point lies above 100 km of offshore cable length. Another important reason why the break even point is so 177

178 far from shore is because of the large distance between the two offshore wind farms of 26.2 km which is required in case of a coupled grid connection. Total Costs [MEURO] Results Sensitivity Analysis Length of Main Grid Connection WR and K km 50 km 60 km 70 km 80 km 90 km 100 km Offshore Cable Length Coupled Grid Connection 'West Rijn', 'West Rijn' and 'Katwijk' Sum Individual Grid Connections 'West Rijn', 'West Rijn' and 'Katwijk' Figure 7.19: Results of sensitivity analysis of varying the offshore cable length for the individual and coupled grid connection of 'West Rijn' and 'Katwijk'. In figure 7.20 the results are given for the break even point of the individual and coupled grid connection of Scheveningen Buiten and Katwijk. As can be seen the break even point is around 90 km of offshore cable length. At 90 km of offshore cable length the most optimal system for the individual grid connection of Scheveningen Buiten is still the 150 kv HVAC system and for Katwijk and the coupled grid connection the ±300 kv HVDC VSC system is the most optimal. Due to the lower investment costs at larger distances for the HVDC VSC systems, the total costs become lower as compared to the HVAC systems, though the losses are higher. The distance between the offshore wind farms is about 18.2 km which causes extra costs for the interconnection of the offshore wind farms in case of a coupled grid connection and therefore a break even point around 90 km offshore. 178

179 600 Results Sensitivity Analysis Length of Main Grid Connection SB and K Total Costs [MEURO] km 50 km 60 km 70 km 80 km 90 km 100 km Offshore Cable Length Coupled Grid Connection 'Scheveningen Buiten' and 'Katwijk' Sum Individual Grid Connections 'Scheveningen Buiten' and 'Katwijk' Figure 7.20: Results of sensitivity analysis of varying the offshore cable length for the individual and coupled grid connection of Scheveningen Buiten and 'Katwijk'. Total Costs [MEURO] Results Sensitivity Analysis Length of Main Grid Connection WR, SB and K km 50 km 60 km 70 km 80 km 90 km 100 km Offshore Cable Length Coupled Grid Connection 'West Rijn', 'Scheveningen Buiten' and 'Katwijk' Sum Individual Grid Connections 'West Rijn', 'Scheveningen Buiten' and 'Katwijk' Figure 7.21: Results of sensitivity analysis of varying the offshore cable length for the individual and coupled grid connection of 'West Rijn', 'Scheveningen Buiten' and Katwijk. In figure 7.21 above the results are shown for the break even point of the individual and coupled grid connection of all three Trident offshore wind farms. As can be seen the break even point is around 100 km of offshore cable length. This is mainly due to the interconnection cables required for the coupled grid connection of the three offshore wind farms. The distance between West Rijn and Scheveningen Buiten is about 14.5 km and the distance between Katwijk and Scheveningen Buiten is about 18.2 km. These interconnection distances are significant and give added investment costs for the coupled grid connection of the Trident offshore wind farms. 179

180 As was already indicated, the results as shown in figures 7.18, 7.19, 7.20 and 7.21 depend on the values used for the most important economical variables. When the effect of the losses over 20 years of operation becomes more important, the break even point of the coupled and individual grid connections will shift closer to shore due to the fact that for some optimized coupled grid connections for larger distances to shore the total losses are significantly lower compared to the losses in the optimized individual grid connections. As could already be seen in table 7.62 for larger offshore cable lengths the HVDC VSC systems become economically more feasible as compared to the HVAC systems. This is due the fact that the investment costs for larger distances from shore are higher for the HVAC systems. The losses in the HVDC VSC systems remain higher, but overall theses systems have lower costs over 20 years of operation. In order to get more insight in the break even point between the HVAC and HVDC VSC systems, the results for the different systems for a varying offshore cable length for the coupled grid connection of all three Trident offshore wind farms is shown in the figure below. The length of the offshore cable is varied from 40 to 100 km and the onshore cable length remains 20 km in all cases. Also the distance between the offshore wind farms remains the same (i.e. around 15 to 20 km). Figure 7.22: Results of sensitivity analysis of grid connection length for the grid connection of all three 'Trident' offshore wind farms. As can be seen in the figure above the increase in costs of all HVAC systems have about the same slope. For the HVDC VSC systems the slope of the costs for the ±150 kv HVDC VSC system are higher compared to the ±300 kv HVDC VSC system due to the fact that the ±150 kv HVDC VSC system requires 2 bipolar cable systems (i.e. 4 cables) while the ±300 kv HVDC VSC system requires only 1 bipolar system (i.2. 2 cables). For presently available offshore technologies (i.e. 132 and 150 kv HVAC and ±150 kv HVDC VSC) the break 180

181 even point is reached with a total grid connection length of 90 to 95 km. For all systems the number of grid connection cables remains the same for the total grid connection length of 60 km to 120 km. As was stated the ±300 kv HVDC VSC uses a single bipolar system with therefore only 2 cables which makes this system more suited with an increase in the transmission distance. As soon as the 220 kv HVAC system is available on the market the break even point with the ±300 kv HVDC VSC system is around 95 km and the break even point with the ±150 kv HVDC VSC system is even around 105 km. Therefore for the grid connection of offshore wind farms with a total power up to 1000 MW the most feasible solution up to a total grid connection distance of approximately 100 km will be the 220 kv HVAC system. For longer distances or when more power needs to be transmitted over a long distance, the HVDC VSC systems become more feasible. This will be the case when for instance a large scale offshore transmission grid is being developed with interconnection stations offshore Conclusions sensitivity analysis In the previous paragraphs the sensitivity analysis of the optimization process has been performed. For the sensitivity analysis the cable price, the interest rate r, the price of electricity, the price for offshore converter stations and the offshore cable length has been varied. These parameters have in general the most influence on the outcome of the optimization process. In paragraph 7.3 the results were given for the coupled grid connection of two or three of the Trident offshore wind farms. The results indicated that the total costs over 20 year of operation were quite similar for the optimized individual and coupled grid connections. With increasing the cable price was shown that the results for the optimized grid connection for both the individual and coupled grid connection hardly changed. Due to the fact that the costs for 220 kv HVAC cables are higher as compared to the 132 kv and 150 kv HVAC systems, the total investment costs for the coupled grid connections increased more as compared to the optimized individual grid connections. Therefore when the price of cables increases this is only beneficial for the coupled grid connections when there is a saving on the number of grid connection cables compared to the optimized individual grid connections. When the base price for the offshore HVDC VSC converter was decreased to 175 k /MW, the total investment costs for the HVDC VSC systems decreased with approximately 50 M for the case of the coupled grid connection of all three Trident offshore wind farms. The total investment costs remain still much higher as compared to the HVAC systems and therefore the HVDC VSC systems will not be suited for grid connections relatively close to shore as is the case for the Trident offshore wind farms. The interest rate r will most likely only be higher as the stated value of 8% and not lower due to the financial structure of offshore wind farms. With an increase in the interest rate r the importance of losses over 20 years of operation 181

182 decreases. This is beneficial for the coupled grid connection which typically has lower investment costs and higher operational losses as compared to the individual grid connections. As was shown for an increase of the interest rate r to 10% in some cases of a coupled grid connection the coupled grid connection is economically more feasible as compared to the optimized individual grid connections, but the difference remains below 2.5% which is most probably too small to compensate for the added complexity of a coupled grid connection. With an increase in the price of electricity the losses over 20 years of operation become more important. Due to the fact that the total losses of the coupled grid connection are higher as compared to the sum of the optimized individual grid connections, this is a disadvantage for the coupled grid connection in case of the coupled grid connection of the Trident offshore wind farms. The reason for the higher losses of the coupled grid connection is mainly due to the distance of the interconnection of the offshore wind farms which is already 15 to 18 km which is significant on a total grid connection distance of about 50 km. Finally the sensitivity of the length of the offshore cable was studied with the economical parameters as given in the economical model, in order to study the break even point for the individual and coupled grid connections. Also the break even point of the HVAC and HVDC VSC systems was studied. As could be seen the break even points for the coupled grid connections lay far offshore at a distance of 85 to over 100 km from shore. The main reason for this large distance is due to the fact that the interconnection cables between the offshore wind farms are about 15 to 18 km as was stated before, which give added investment costs for the coupled grid connection. Another reason is because of the fact that the investment costs for the 220 kv HVAC system, which is the economically the most feasible for the coupled grid connection up to 80 km, are higher per km as compared to the 150 kv system which is typically the most suitable system for the individual grid connections. Therefore from a project developer s point of view the coupled grid connection will economically not be more feasible compared to the individual grid connection. A coupled grid connection also has a much higher complexity which makes it less feasible due to the risks in the development of offshore wind farms. With varying the length of the offshore cable also the break even point between the HVAC and the HVDC VSC systems was studied. The HVDC VSC system has the advantage when much power has to be transmitted over a long distance. Therefore this study was performed for the coupled grid connection of all three Trident offshore wind farms. With the current technologies available (i.e. 132 kv and 150 kv HVAC and ±150 kv HVDC VSC) the break even point is around 90 km of total grid connection length. When the 220 kv HVAC and ±300 kv HVDC VSC system become commercially available the break even point between these systems is around 95 km of total grid connection length. This shows that for total grid connection lengths up to about 100 km the HVAC system remains 182

183 economically the most feasible for a power transmission up to 1000 MW of multiple offshore wind farms. Overall can be concluded on basis of the sensitivity analysis performed, that the results of the optimization of both the individual and coupled grid connections hardly change when the most important economical factors are changed. Even for rather extreme variations compared to the base values as assumed in the economical model, the differences in the results were minor. The only changes were mostly in the conductor cross section of the onshore or interconnection cables and in some cases also the number of onshore cable systems. These variations only cause a slight difference in the total investment costs and the overall losses. Therefore can be concluded that even with quite substantial changes in the parameters of the economical model, the overall results will hardly change. An increase or a decrease (e.g. if technically possible) in the number of grid connection cables or a change in the conductor cross section of the submarine cables of the main grid connection causes a difference in the investment price which is much higher as compared to the differences in the total costs of losses due to the difference in losses. 183

184 8 Results and conclusions In this study a model has been made for the optimization of individual and coupled grid connections of offshore wind farms. Individual grid connections have the advantage that the development of offshore wind farms is independent and the connection with the onshore grid can be closer as compared to a coupled grid connection. A coupled grid connection increases the complexity of the grid connection and makes the development of offshore wind farms not independent. The advantage of the coupled grid connection is that in general less grid connection cables are required and therefore less space for cable trajectories offshore, less cable landings and dune crossings and fewer onshore cable systems are required. Also there is synergy with reactive power compensation and voltage control in case of a HVAC grid connection system which causes investment costs which can be lower. The advantages and disadvantages of individual and coupled grid connections as given above indicate that a coupled grid connection might be a feasible solution for the connection of multiple offshore wind farms which are located relatively close to each other. In order to study the possibility of a coupled grid connection, a case scenario study of the development of three offshore wind farms is done which are located relatively close to each other: the Trident offshore wind farms which are being developed approximately 30 km off the Dutch coast. The optimization process for the grid connection of offshore wind farms consist of two parts: a model for calculating the losses in the various systems and an economical model for the investment costs and the costs of the losses over 20 years of operation. In the model for the optimization of the grid connection all commercially available grid connection technologies are taken into account (i.e. 132 kv and 150 kv HVAC systems and ±150 kv HVDC VSC system) as well as technologies which are soon commercially available (i.e. 220 kv HVAC system and ±300 kv HVDC VSC system). For the modeling of the cable losses standard calculation methods have been used which take the temperature dependence of the losses in the cable into account. For the other components in the HVAC system practical calculation were used as applied in industry. A model for the losses in HVDC VSC converter station was based on losses published of two commercial HVDC VSC projects and data obtained by ABB. For the economical model data was used as found in literature and cost data known within Evelop. For the grid connection technologies currently available (i.e. 132 kv and 150 kv HVAC and ±150 kv HVDC VSC) quite much references have been found in literature and costs data was known within Evelop. Therefore the cost models for these systems could be made quite accurate. For the grid connection technologies which are currently not yet economically available (i.e. the 220 kv HVAC and ±300 kv HVDC VSC systems) less or no data has been 184

185 found in literature or was known within Evelop. The cost model for these systems was therefore based on some assumptions and referred to the currently existing technologies. With data published about some commercial projects the economically model for the 132 kv and 15 kv HVAC and for the ±150 kv HVDC VSC could be validated and turned out to be reasonably accurate. For the optimization of the system for the individual and coupled grid connections the optimization is done on basis of the investment costs and the losses over 20 years of operation of the offshore wind farms. Availability, failure rates and mean repair times and costs also have their impact on the economical optimization of the grid connection, but because of the fact that too little information about these variables is known for offshore ±150 kv HVDC VSC systems and the 220 kv HVAC and ±300 kv HVDC VSC system which are currently not commercially available yet, this would only decrease the reliability of the outcome of the optimization process. Therefore only the losses and the investment costs are taken into account in the optimization process. For the final optimization process the total costs over 20 years of operation of the offshore wind farms is calculated by means of the net present value (NPV) which models the future losses of the grid connection systems to current values. For the NPV the price of electricity and the interest rate r are of high importance because they indicate the importance of losses. Both values were based on typical values valid in the offshore wind farm industry. With the calculation of the losses and the economical model the optimization process is studies for a case study of the grid connection of the three Trident offshore wind farms which are being developed in front of the Dutch coast. For the case study several different scenarios were studied: individual grid connection of all three offshore wind farms and the coupled grid connection of two or all three of the Trident offshore wind farms. By comparing the results of the optimization process of the individual and coupled grid connections of the offshore wind farms, conclusions could be drawn about the feasibility of a coupled grid connection of the offshore wind farms. With the economical parameters as set in the economical model it turns out that in some cases the coupled grid connection is economically more feasible as compared to the optimized individual grid connections. The differences were very small in all cases though (i.e. only 1% to 4% lower costs over 20 years of operation). With the added complexity and risks of a coupled grid connection this slight advantage will most probably be too low for offshore wind farm developers to start cooperating and develop a coupled grid connection. In order to test the sensitivity of the results obtained for the optimized individual and coupled grid connections, a sensitivity analysis was performed for the most important parameters. For the sensitivity analysis the cable price, the price of electricity, the price of an offshore HVDC VSC converter station and the interest rate r have been varied. The cable price depends to a large extend on the metal prices and with the increase of the price of the metals used over the past years 185

186 and the expectance of a further rice in the coming future, the sensitivity of the increase in the cable price was studied. Up to an increase of the cable price of 130% the results of the optimization process of the individual and coupled grid connections remained the same. This was mainly due to the fact that for the coupled grid connections the 220 kv HVAC system would be the most optimal which is more expensive than the other HVAC systems which are used in the optimized individual grid connections. Though the optimized coupled grid connections requires in most cases fewer grid connection cables, this does not compensate for the increase in the cable price. With the sensitivity analysis of the price of electricity and the interest rate r the importance of the losses over 20 years of operation was studied. With an increase in the price of electricity and a decrease in the interest rate r the importance of losses over 20 years increases and vice versa. It turns out that for both variables the increase in the importance of losses is not beneficial for the coupled grid connection because for most coupled grid connections the losses were higher as compared to the individual grid connections, due to the added losses in the interconnection of the offshore wind farms. With the sensitivity analysis of the price of an offshore HVDC VSC converter station the impact of a decrease in the price was studied for the coupled grid connection of all three Trident offshore wind farms. With a decrease in price of the offshore converter by 22% the total costs for the HVDC VSC systems of the coupled grid connection of all three Trident offshore wind farms decreased by about 50 M. Though the large decrease in price for the HVDC VSC systems, the total costs for the HVDC VSC systems remain much higher as compared to the HVAC systems for the relatively short distance to the onshore grid as is the case for the Trident offshore wind farms. This shows that for possible coupled grid connections of offshore wind farms off the Dutch coast which are typically located within 60 km from shore, the HVDC VSC system will not be an economically feasible grid connection technology. The final sensitivity analysis performed was about the variation of the offshore cable length. By this sensitivity analysis the break even points were studied for the optimized individual and coupled grid connections and also for the HVAC and HVDC VSC grid connection technologies. It turns out that the break even points for the optimized individual and coupled grid connections is with offshore cable lengths of about 85 to 100 km. The main reason for this is that for the optimized coupled grid connections the 220 kv HVAC system is the most economical. This system is more expensive per km as the 132 kv and 150 kv HVAC systems which are the most optimal systems for the individual grid connections up to about 70 to 90 km of offshore cable length. From an offshore cable length of approximately 80 km onwards the HVDC VSC systems become the most economical for most of the individual and coupled grid connections. Due to the fact that for the coupled grid connection also the interconnection cables are required and these distances are 15 to 18 km in case of the Trident offshore 186

187 wind farms, the break even points are reached from 85 km onwards for the different connection possibilities. When the importance of losses increases due to a change in the economical parameters as used in the economical model, the break even points for the optimized individual and coupled grid connections will come closer to shore due to the fact that for larger distances to shore the losses of the coupled grid connection become in general below the losses of the individual grid connections. For the break even point of the HVAC and HVDC VSC systems it turns out that the break even point of the currently existing technologies (i.e. 132 kv and 150 kv HVAC and ±150 kv HVDC VSC systems) is around 95 km of total grid connection length and for the available technologies in the near future (i.e. 220 kv HVAC and ±300 kv HVDC VSC) the break even point is around 90 km of total grid connection length. This analysis was done for the coupled grid connection of all three Trident offshore wind farms with a total rated power of approximately 1000 MW. This shows that the HVDC VSC technology is only suited for very long transmission distances, for example in case of an offshore transmission grid. Based on the study done for the Trident offshore wind farms and the sensitivity analysis performed, one can conclude that from a project developer s point of view a coupled grid connection is not an economically feasible solution when all the grid connection costs have to be paid by the offshore wind farm developer. Besides the economical side of the coupled grid connection, a coupled grid connection is also much more complex as compared to an individual grid connection when multiple developers are involved and it makes the development of several offshore wind farms dependent on each other. This increases the risks in the development of offshore wind farms which are already high. Therefore it is advisable for governments, in this case the Dutch government, to allocate a transmission system operator which is also responsible for the development of the electrical grid offshore, as is currently the case for instance in Germany, when it wants to regulate the number of grid connection cables with the growth of offshore wind power in the coming future. With the target of the Dutch government of 6000 MW of offshore wind power in 2020, many new offshore wind farms need to be build and accordingly many grid connection cables are required. When it is not beneficial for offshore wind farm developers to cooperate and start developing coupled grid connections, as has been indicated in this study, a significant amount of offshore and onshore cable connection will be required for all individual offshore wind farms. When there is a TSO which is responsible for the offshore grid connection up to the point where the offshore wind farms are located (e.g. the Socket at Sea principle) and the investment costs for the grid connection are socialized, the total costs for the development of an offshore wind farm and accordingly the price per kwh will go down. In this case the electrical infrastructure for the grid connection is also depreciated over the lifetime of the cables, which is typically 30 to 40 years, and not just over the lifetime of the offshore wind farm which is typically 20 years. Such a controlled growth of offshore wind energy will cause a structured offshore transmission grid. 187

188 9 Recommendations In this study both for the modeling of the losses and for the economical model some assumptions have been made. Some recommendations for the improvement of the optimization model will be given below. In practice the cable rating used for the grid connection cables of an offshore wind farm is based on worst case scenarios as if the cable is used under continuous operation of close to or at rated power. This conservative approach ensures a safe operation of the grid connection cables under any scenario. This approach has also been used in this study and causes the cables to be over dimensioned. Due to the typical Weibull distribution of the wind speed distribution over a year, the cable is most of the time loaded at 30% to 80% of its rated power. The maximal rating of a cable is limited by the maximum operational temperature of the insulation in the cable, which is typically 90 C for XLPE cables. The actual operational temperature of the cable will vary over the cable length depending on the history of the power transmission and the local soil conditions. The thermal capacitance of the cable and the soil surrounding the cable determine the temperature increase or decrease of a cable, depending on the power transmission through the cable over time. Grid connection cables are typically designed such that it contains fibers for data transmission for the operation of the offshore wind farm. Nowadays also systems are available which use these fibers to measure the temperature distribution of the cable over its length. With these types of systems hot spots in the cable trajectory can be localized, but also a continuous monitoring of the cable temperature over time can be done. Accordingly also studies on the thermal capacitance of the surrounding soil of the cables needs to be performed to check the impact of the varying loading of the cable. When these studies are performed and this is done for several offshore wind farms, more insight is obtained in how well the grid connection is designed and whether the rating of the grid connection cables could be decreased. For grid connection cables with lengths of over 50 km this can save quite some investment costs. For such a study also the availability of the offshore wind farm should be taken into account. For the economical model data has been used as has been found in literature or which was known within Evelop. For the optimization model also the 220 kv HVAC and ±300 kv HVDC VSC systems are taken into account, while they are currently not commercially available yet. Due to the fact that there exist no commercial projects which used either of these technologies, there was very little or no information found at all for some of the components for the grid connection of these technologies. With some assumptions and references to the other HVAC and HVDC VSC technologies also economical models for these technologies were developed. These models could not be validated by any commercial project as was the case for the other HVAC and HVDC VSC technologies. The results of the optimization process indicate that due to the high transmission capability of 188

189 these technologies, the 220 kv HVAC and ±300 kv HVDC VSC are the most economical solutions for large scale power transmission as is the case for the coupled grid connection of offshore wind farms. Because of the fact that the economical model for these technologies is mainly based on assumptions, it is recommended to verify the cost model of these technologies as soon as commercial projects with these technologies are being developed. 189

190 Appendices Appendix A: Trident offshore wind farms In the following sections the characteristics for all three Trident offshore wind farms will be given in detail, as obtained from their individual EIA s [1]. A.1 Offshore wind farm Katwijk of WEOM The offshore wind farm Katwijk is planned at about 24 km offshore from the coast of the city Katwijk and will have a surface area of approximately 42 km 2. From the EIA of offshore wind farm Katwijk [1] the following characteristics have been obtained: o Wind farm capacity: 342 MW o Number of turbines: 114 o Type of turbine used: Vestas V90, 3MW, approximately 70 m hub height o Wind farm coordinates: Coordinate system ED50-UTM31 Coordinate X Y o Mean wind speed at hub height: o Coastal landing point: V mean, 70m : 9.1 m/s IJmuiden Coordinate system ED50-UTM31 Coordinate Landing point X Y o Onshore grid connection station: o Number of OHVS: Velsen/Beverwijk (i.e. 150 kv station) 1 (i.e. 34/150 kv, 450 MVA) Coordinate system ED50-UTM31 Coordinate OHVS X Y o Transmission voltage: 150 kv AC o Offshore transmission cable length: 48.7 km o Onshore transmission cable length: approximately 10 km o Wind farm availability: approximately 92% o Wind farm wake loss: approximately 10% o Infield electrical losses: approximately 1% 190

191 A.2 Offshore wind farm Scheveningen Buiten of Evelop The offshore wind farm Scheveningen Buiten is planned at about 28 km offshore from the coast of the city Scheveningen and will have a surface area of approximately 31 km 2. From the EIA of offshore wind farm Scheveningen Buiten [1] the following characteristics have been obtained: o Wind farm capacity: MW o Number of turbines: 89 o Type of turbine used: GE 3.6, 3.6MW, approximately 77 m hub height o Wind farm coordinates: Coordinate system ED50-UTM31 Coordinate X Y o Mean wind speed at hub height: o Coastal landing point: V mean, 77m : 9.4 m/s Noordwijk aan Zee Coordinate system ED50-UTM31 Coordinate Landing point X Y o Onshore grid connection station: o Number of OHVS: Sassenheim (i.e. 150 kv station) 2 (i.e. 33/150 kv) Coordinate system ED50-UTM31 Coordinate OHVS: #1 OHVS: #2 X Y o Transmission voltage: o Offshore transmission cable length: o Onshore transmission cable length: 150 kv AC 49 km 8.2 km o Wind farm availability: approximately 92% o Wind farm wake loss: 9,8% o Infield electrical losses: approximately up to 1% 191

192 A.3 Offshore wind farm West Rijn of Airtricity The offshore wind farm West Rijn is planned at about 40 km offshore from the coast of the city Scheveningen and will have a surface area of approximately 47 km 2. From the EIA of offshore wind farm West Rijn [1] the following characteristics have been obtained: o Wind farm capacity: MW o Number of turbines: 79 o Type of turbine used: GE 3.6, 3.6MW, approximately 75 m hub height o Wind farm coordinates: Coordinate system ED50-UTM31 Coordinate X Y o Mean wind speed at hub height: o Coastal grid connection point: V mean, 75m : 9.2 m/s Hoek van Holland Coordinate system ED50-UTM31 Coordinate Landing point X Y o Onshore grid connection station: Westerlee (i.e. 150 kv station) o Number of OHVS: 1 Coordinate system ED50-UTM31 Coordinate OHVS X Y o Transmission voltage: o Offshore transmission cable length: o Onshore transmission cable length: 150 kv 48.7 km 11 km o Wind farm availability: 92% o Wind farm wake loss: approximately 10% o Infield electrical losses: approximately up to 1% 192

193 Appendix B: Cable construction and cable systems In this appendix the cable construction of submarine and land cables is given and the possible configuration of cable systems for both HVAC and HVDC VSC cable systems. B.1 Submarine and land cable design For offshore HVAC power connections always three phase cables are used. This is mainly because they are cheaper to install than three separate single phase cables and offshore the bending radius of a cable can be higher for installation due to the availability of large vessels. Installation vessels with a capacity for cable lengths of 50 km are available (i.e. the maximal length possible on a vessel is determined by the total weight and size of the cable). Due to these long cable lengths possible offshore cable joints are in many cases not necessary for the offshore wind industry. Most often an offshore cable has a copper conductor though copper is more expensive than aluminum. Copper has a lower resistance than aluminum, so for the same power rating a smaller conductor can be used which makes the cable lighter, easier to install and it gives lower losses. Onshore mainly single core cables are used, most often with an aluminum conductor. The main reason for this is that single core cables are cheaper to produce, easier to install (i.e. smaller bending radius) and the earthing arrangements of the sheaths can be such that the circulating currents can be limited to acceptable levels. Mainly aluminum conductors are used because they are cheaper. The increase in cable diameter because of the lower power rating is for single core cables in general not much of a problem for installation. Though an aluminum cable has higher losses the overall costs for an aluminum cable are most often still lower than for single core cables with a copper conductor. In figure B.1 a typical three core AC submarine cable is shown. The XLPE cable consists typically of 13 layers [25]: o Conductor (copper or aluminum) o Conductor screen: ensures that the conductor presents a smooth surface to the insulation. Particularly for the higher voltage cables which operate at the highest electrical stresses (i.e. electrical field strength [V/m]), the screen must be as smooth as possible to minimize any local high stresses which can cause small and local discharges which damage the cable and make the lifetime of the cable go down (i.e. degradation of the insulation material). Specially formulated super smooth semiconducting compounds are used for the conductor screen. o Insulation material o Insulation screen: to ensure the electrical field is confined to the polymeric insulation an extruded semiconducting polymer screen is applied overall. The material used is similar to that used for the conductor screen. 193

194 o Swelling tape: protects the cable for moisture in case of damage to the cable. When damage occurs and water penetrates, the swelling tape will swell causing the cable parts to be pressed more firmly against each other and closing up lengthwise cavities and fissures underneath the swollen swelling tape. This causes a lengthwise water tightness which is such that the water can penetrate along the cable approximately only about 1 m of cable length within 24 hours. This gives sufficient time for repair and accordingly only a short length of cable needs to be replaced. The swelling tape also provides mechanical protection for the cable core by absorbing the radial thermal expansion of the insulation and by cushioning side wall loading at bends. o Sheath (Lead alloy, copper or aluminum laminated sheath): protects the insulation layer due to its strength, prevents the cable against moisture and/or water penetration and works as the neutral earth return path along the cable for earth short circuit currents. It also acts as return conductor for the capacitive charging current and acts as a shield for the electromagnetic field. o Polyethylene sheath: prevents moisture penetration and acts as anti corrosion covering of the sheath. o Fiber optic cable: used for data transmission to monitor and control the turbines and their electrical equipment. The fiber can also be used for temperature measurement of the cable to detect hot spots along the cable trajectory. o Filler: filling up the space between the circular binder and the three separate conductors by a good thermal conducting material which also protects against moisture. o Binder: layer binds the filling material and the conductors. o Bedding: usually made of polyethylene as a layer between the binder and the armor as an insulation layer with good thermal conductivity. o Armor: protects the cable for the high tensile loading of the cable encountered when laying the cable and also protects the cable against damaging by trailing ships anchors or fishing gear. The armouring is twisted to give extra strength to the cable due to the residual tension in the cable after laying, caused by a twist in the cable itself. Nonmagnetic materials are used in order to reduce the induced currents in the armor and accordingly the armor losses. o Outer serving: is a moisture impermeable barrier to protect the cable against the surroundings. 194

195 Figure B.1: Overview of single phase submarine HVDC cable and three phase submarine HVAC cable [ref. ABB]. Single phase submarine cables typically have the same design both for AC and DC cables. The main difference between land and submarine cables is the armor being absent. The armor is added to submarine cables to give extra strength to the cable which is required for the installation, the severe conditions in which the cable operates and the possibility of getting hit by a ships anchor. On land this extra protection layer is not required and unwanted because of the reduced flexibility and increased weight of an armored cable. The typical design life of a high voltage cable is 30 to 40 years. B.2 HVAC land cable systems As was mentioned before single phase land cables have a similar design as a three phase submarine cable, except of the armor layer. Due to the alternating current and alternating voltage a voltage will be induced along the metallic sheath for cables under AC operation. The sheaths need to be earthed in order to conduct the capacitive charging current and possible short circuit currents. Earthing of the sheaths can be done in several ways: o Single end bonded o Both end bonded o Cross bonded 195

196 The single and both ends bonding is shown in the figure below. Figure B.2: Configuration of single or both end bonded cable and induced voltage distribution [26]. Single end bonded One end of the cable sheath is earthed so that the other end is open and a standing voltage will appear which is induced linearly along the cable length. In order to ensure the relevant safety requirements the open end of the cable sheath has to be protected with a surge arrester (protection system which protects for overvoltage) to protect the cable for too high voltages which damage the insulation. This type of bonding is usually only applied for circuits shorter than 1 km. Both ends bonded Both ends of the cable sheath are earthed. With this method no standing voltages occur at the cable ends which gives the cable safe operation. A disadvantage of both ends bonding is that now circulating currents can flow in the sheath which are proportional to the conductor currents and therefore reduce the cable ampacity significantly. This is uneconomic for longer lengths or high voltage cables and is therefore only applied to medium or low voltage cables for short connections. Cross bonding The cross bonding configuration of three single core cables is shown in figure B.3. This earthing method is applied for longer route lengths where joints are required (e.g. several km s of cable length). A cross bonding system consists of three equal sections with cyclic sheath crossing after each section. The termination points are both bonded to earth. Along each section a standing voltage is induced. For an ideal cross bonding system the three sections are exactly equal so that no residual voltage occurs and thus no sheath current flows. The sheath losses can be kept very low with this method. In addition to cross linking the sheaths the conductor phases can also be transposed cyclically. This is done for equal loading of the cables (e.g. three single core cables in flat formation will have the middle cable being the hottest and accordingly the losses 196

197 in the three cables will differ, transposing the cables can balance this). Transposition is normally only done for very long cable lengths (e.g. several tens of km s) or when there are several parallel circuits. Due to the typical onshore cable length for the grid connection of offshore wind farms of several tens of km s, cross bonding of the onshore cables for the HVAC system will be assumed. Figure B.3: Cross bonded cable configuration and induced voltage distribution [26]. B.3 HVDC Cable systems DC cables are always single core cables. The design of DC cables is similar to AC cables. Submarine DC cables are strengthened with an armor layer. Due to the fact that for direct current the full cross sectional area of the conductor can be used (e.g. no need for segmented conductor to avoid the skin effect), smaller conductors can be used compared to their AC equivalent cables. A smaller conductor makes the cable lighter, have a smaller radius and accordingly a smaller bending radius which makes installation easier. Due to the smaller radius and smaller bending radius much more ships are available which can be used for the installation of submarine DC cables than there are for three core AC cables. This makes the installation of DC cables cheaper on average. DC cable systems can be designed in several ways: o Monopolar o Bipolar o Duplex systems Monopolar In figure B.4 a monopolar system is shown. 197

198 Figure B.4: Monopolar HVDC system configuration [27]. A monopolar system is the simplest configuration with a single converter at each end and a single DC cable. The system operates with a conductor voltage at +80 kv, +150 kv or +300 kv and the earth is used as return conductor. Therefore large earth electrodes are required at each end of the connection. This system has reduced installation costs due to the single cable used. The main disadvantage is the high static electromagnetic field which is proportional to the current magnitude. Because of possible environmental risks (e.g. impact on animal life or possible influence on the human body) and the disturbance of some electrical equipment due to the large electromagnetic field, this type of systems is in general not designed any more. Bipolar In the figure below a bipolar system is shown. Figure B.5: Bipolar HVDC system configuration [27]. A bipolar HVDC system has one converter station at each end but uses two cables which operate in opposite polarity (e.g. ±80 kv, ±150 kv or ±300 kv). Because of the opposite polarity the electromagnetic field can be reduced significantly when then the cables are laid close to each other, preferable in the 198

199 same trench. This is the main type of installation for HVDC systems. In case of a failure of one of the converters or of one of the cables, this will result in the loss of the entire power transmission capacity of the system. For more reliability and redundancy duplex systems can be used, but these are most costly. Therefore this type of HVDC system is in general used and will be assumed in this study. Duplex bipolar In the figure below an overview is given of a duplex bipolar system. Figure B.6: Duplex bipolar HVDC system configuration [27]. A duplex bipolar HVDC system has two converters installed at each end, both of half the power transmission capacity, and two cables as is the case for the bipolar system. Now one of the converters operates at a positive polarity and the other on the negative polarity. When the mid point of the converter pairs on both sides are earthed the reliability of the system is improved. In case of a failure of one converter or cable only half of the power transmission capacity gets lost, but the system will operate as a monopolar system. Because of an increase in costs and environmental concerns as mentioned before this is unwanted and therefore this type of system will generally not be used. Duplex monopolar In figure B.7 a duplex monopolar system is given. 199

200 Figure B.7: Duplex monopolar HVDC system configuration [27]. A duplex monopolar system has two independent converters at each end and uses two cables with opposite polarity. This system solves the problem of the high electromagnetic field of a monopolar system and has a higher reliability because of the independent systems, but increases the operating costs substantially because of the two independent systems. It has therefore only been used once so far at the Moyle submarine interconnector between Scotland and Northern Ireland [22]. 200

201 Appendix C: Cable characteristics In this appendix the cable characteristics are given of the cables used in this study. The characteristics of the three phase submarine HVAC cables are based on published data of the FXBTV cable design [39]. The characteristics of the single phase HVAC and HVDC land and submarine cables are based on published data by ABB [26], [29], [42]. The characteristics are valid for the conditions as given in the table below. Table C.1: Typical conditions of cables offshore (i.e. more than 3 km from shore) and onshore. Offshore Onshore Thermal resistance soil/seabed: 0.8 [Km/W] 1.0 [Km/W] Temperature at burial depth: 12 [ C] 15 [ C] Burial depth: 1.5 [m] 1.0 [m] Parallel heat sources: No No Soil moisture migration: No No C1: Submarine three phase HVAC cables In the three tables below the characteristics of the three phase submarine HVAC cables of 132 kv, 150 kv and 220 kv are given. Table C.2: Characteristics of 132 kv submarine three phase HVAC cable. Nominal Voltage [kv] Cross Section of Conductor [mm²] Max Continuous Load [MVA] Rated Current [ka] Conductor AC resistance at 20 C Total Phase AC resistance at 20 C (incl. sheath and armor) [Ω/km] [Ω/km] Capacitance per phase [µf/km] Capacitive charging current, per phase, at nominal voltage [A/km] Table C.3: Characteristics of 150 kv submarine three phase HVAC cable. Nominal Voltage [kv] Cross Section of Conductor [mm²] Max Continuous Load [MVA] Rated Current [ka] Conductor AC resistance at 20 C Total Phase AC resistance at 20 C (incl. sheath and armor) [Ω/km] [Ω/km] Capacitance per phase [µf/km] Capacitive charging current, per phase, at nominal voltage [A/km]

202 Table C.4: Characteristics of 220 kv submarine three phase HVAC cable. Nominal Voltage [kv] Cross Section of Conductor [mm²] Max Continuous Load [MVA] Rated Current [ka] Conductor AC resistance at 20 C Total Phase AC resistance at 20 C (incl. sheath and armor) [Ω/km] [Ω/km] Capacitance per phase [µf/km] Capacitive charging current, per phase, at nominal voltage [A/km] C2: Onshore single phase HVAC cables In the three tables below the characteristics of the single phase HVAC land cables of 132 kv, 150 kv and 220 kv are given [26], [42]. Table C.5: Characteristics of single phase HVAC 132 kv land cables. 202

203 Table C.6: Characteristics of single phase HVAC 150 kv land cables. Table C.7: Characteristics of single phase HVAC 220 kv land cables. 203

204 C3: Submarine single phase HVDC cables In the two tables below the characteristics of the single phase submarine HVDC cables of ±150 kv and ±300 kv are given [26], [29]. Table C.8: Characteristics of single phase submarine HVDC ±150 kv cables. Table C.9: Characteristics of single phase submarine HVDC ±300 kv cables. C4: Onshore single phase HVDC cables In the two tables C.10 and C.11 the characteristics of the single phase HVDC land cables of ±150 kv and ±300 kv are given [26], [29]. 204

205 Table C.10: Characteristics of single phase HVDC ±150 kv land cables. Table C.11: Characteristics of single phase HVDC ±300 kv land cables. C5: Correction factors The maximal current through a cable specified by a cable manufacturer is always given for specified conditions (i.e. a certain soil temperature, thermal resistivity of the soil and burial depth). For other soil temperatures, thermal resistivity and burial depths correction factors can be calculated by which the given maximal current has to be multiplied in order to get the actual rated current allowed for the cable under certain conditions. In the following sections the correction factors are given for both the submarine and land cables for both the HVAC and HVDC cables. 205

206 C5.1 Correction factors: submarine HVAC cables In the following three tables the correction factors are given for the three phase submarine HVAC cables for a variation in the soil thermal resistance, the burial depth and the soil temperature. Table C.12: Correction factor for soil thermal resistance of submarine HVAC cables. Heat Correction Resistance Factor 0.5 [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] Table C.13: Correction factor for burial depth of submarine HVAC cables. Burial Depth Correction Factor 1.0 [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m]

207 Table C.14: Correction factor for soil temperature of submarine HVAC cables. Soil Correction Temperature Factor 10 [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] C5.3 Correction factors: submarine HVDC cables In the following three tables the correction factors are given for the submarine HVAC cables for a variation in the soil thermal resistance, the burial depth and the soil temperature. Table C.15: Correction factor for soil thermal resistance of submarine HVDC cables. Heat Correction Resistance Factor 0.5 [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] Table C.16: Correction factor for soil temperature of submarine HVDC cables. Soil Correction Temperature Factor 10 [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C]

208 Table C.17: Correction factor for burial depth of submarine HVDC cables. Burial Depth Correction Factor 1.0 [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] C5.4 Correction factors: HVDC land cables In the following three tables the correction factors are given for the single phase HVAC and HVDC land cables for a variation in the soil thermal resistance, the burial depth and the soil temperature. Table C.18: Correction factor for soil thermal resistance of single phase HVAC and HVDC land cables. Heat Correction Resistance Factor 0.6 [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W] [Km/W]

209 Table C.19: Correction factor for burial depth of single phase HVAC and HVDC land cables. Burial Correction Depth Factor 0.5 [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] [m] Table C.20: Correction factor for soil temperature of single phase HVAC and HVDC land cables. Soil Correction Temperature Factor 5 [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C] [ C]

210 Appendix D: Power curves In order to calculate the power production of offshore wind farms, the power curves of offshore wind turbines are important. Nowadays six offshore wind turbines are available and several are under development and will become available within a few years. In the table below the power curves are given of the currently available and future offshore wind turbines. The data for the power curves has either been found at the websites of the turbine manufacturer or is estimated on basis of given graphical power curves [28]. Table D.1: Overview of power curves of available or future offshore wind turbines. Type of turbine: [-] Vestas Vestas GE Siemens Vestas Multibrid REpower Bard V80 V V120 M5000 5M VM Cut-in wind speed: [m/s] Cut-out wind speed: [m/s] Rated wind speed: [m/s] Rated power: [MW] Rotor diameter: [m] Wind Speed Power Power Power Power Power Power Power Power [MW] [MW] [MW] [MW] [MW] [MW] [MW] [MW] 0 [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s] [m/s]

211 Appendix E: Cost figures of HVAC and HVDC cables The cost model for the HVAC and HVDC submarine and land cables is based on cost information found in literature and from tender bid prices known within Evelop [56]. The tender bid prices known within Evelop come from various companies which are not specified in this study. In several sources the total price for a cable of a certain length was given and in some cases also including the installation costs. Not always were the cable dimensions given. In those cases the cable dimensions have been estimated based on the given total power which needed to be transported and the cable characteristics as given in Appendix C. All the cost prices have been converted to and the cable installation costs per km cable as given in paragraph have been subtracted. The total copper, aluminum and lead prices have been calculated and accordingly the total price ratio of the copper, aluminum and lead used in the cable, so one gets an indication what percentage of the cable price basically comes from the raw material price. In the following paragraphs the given or calculated cost price are given as found in literature or from the tender bid prices [56] and also the updated material costs and accordingly the updated cable prices. E.1 Submarine AC cables The three phase submarine HVAC cables under consideration are 132 kv, 150 kv and 220 kv cables. The characteristics of the original cost prices for three phase submarine HVAC cables are given in the table below. Costs [M /km] Voltage Table E.1: Original cost prices three phase submarine AC cables. Conductor Copper Lead Total Price Total Price Price Ratio Cross Section Price Price Copper Lead Copper + [mm²] Date [USD/kg] [USD/kg] [k /km] [k /km] Lead Jan [57] Jan [56] Nov [39] Jan [58] Nov [39] Nov [39] Jan [56] Nov [56] Dec [56] Nov [39] Nov [39] Dec [59] Nov [56] In the table above one can see that the total price of the copper and lead needed for the construction of the cables takes up to about 37% of the total cable price. For larger conductor cross sections and for higher voltages (e.g. accordingly a Source 211

212 Costs [M /km] thicker insulation layer) the manufacturing process becomes more expensive and accordingly the cable prices go up. When the material prices for copper and lead are updated to the material price of December 2007 [51], the cable prices become as given in the table below. Voltage Table E.2: Updated cost prices three phase submarine AC cables. Conductor Copper Lead Total Price Total Price Price Ratio Cross Section Price Price Copper Lead Copper + [mm²] Date [USD/kg] [USD/kg] [k /km] [k /km] Lead Jan [57] Jan [56] Nov [39] Jan [58] Nov [39] Nov [39] Jan [56] Nov [56] Dec [56] Nov [39] Nov [39] Dec [59] Nov [56] Costs [M /km] In the table above one can see that the cable prices with the updated material prices change in price up to about k 25 per km cable. E.2 Submarine DC cables The submarine DC cables under consideration are ±150 kv and ±300 kv cables. The characteristics of the original cost prices for single phase submarine DC cables are given in the table below. Voltage Table E.3: Original cost prices single phase submarine DC cables. Conductor Copper Lead Total Price Total Price Price Ratio Cross Section Price Price Copper Lead Copper + [mm²] Date [USD/kg] [USD/kg] [k /km] [k /km] Lead ± May [60] ± May [60] ± Oct [56] ± Nov [39] ± Aug [61] ± Mar [62] ± Dec [59] ± Jan [56] In the table above one can see that the price ratio of the copper and lead is between 24% and 38% since the copper price has reached values above 6 US$/kg, while several years ago the price ratio was only about 12% to 18%. Source Source 212

213 Costs [M /km] When the material prices for copper and lead are updated to the material price of December 2007 [51], the cable prices become as given in the table below. Voltage Table E.4: Updated cost prices single phase submarine DC cables. Conductor Copper Lead Total Price Total Price Ratio Cross Section Price Price Copper Price Lead Copper + [mm²] Date [USD/kg] [USD/kg] [k /km] [k /km] Lead ± May [60] ± May [60] ± Oct [56] ± Nov [39] ± Aug [61] ± Mar [62] ± Dec [59] ± Jan [56] Costs [M /km] With the updated cable prices as given in the table above it becomes clear that price ratio of the copper and lead used in the cables ranges from about 22% to 38% for cables given. E.3 AC and DC land cables AC and DC land cables are similar in design. The only main difference is the insulation thickness. Typical insulation thicknesses are [42] [53]: 132 kv AC land cable: 15 mm 150 kv AC land cable: 17 mm 220 kv AC land cable: 23 mm ±150 kv DC land cable:12 mm ±300 kv DC land cable:24 mm The 132 kv AC, 150 kv AC and ±150 kv DC land cables have a copper screen of 95 mm 2 and the 220 kv AC and ±300 kv DC land cables have a copper screen of 185 mm 2. From the given insulation thicknesses and size of the copper wire screen one can conclude that the 132 kv and 150 kv AC land cables will be similar in price as well as the 220 kv AC and ±300 kv DC land cables. Fewer sources have been found where cost prices are given for land cables than for submarine cables. In the table below the characteristics of the original cost prices for single phase AC and DC land cables are given. Voltage Source Table E.5: Original cost prices single phase AC and DC land cables. Conductor Aluminum Copper Total Price Total Price Price Ratio Cross Section Price Price Aluminum Copper Aluminum [mm²] Date [USD/kg] [USD/kg] [k /km] [k /km] + Copper Source AC 800 jan [56] AC 1200 apr [56] AC 1600 nov [56] ±300 DC 2500 Mar [62] 213

214 Costs [M /km] In the table above it can be seen that the price ratio of the aluminum conductor and copper wire screen are significantly lower than for copper submarine cables with a lead sheath which is obvious because of the difference in price between copper and aluminum. When the material prices for aluminum and copper are updated to the material price of December 2007 [51], the cable prices become as given in the table below. As can be seen the price ratio of the aluminum and copper used ranges from 12% to 19%. Voltage Table E.6: Updated cost prices single phase AC and DC land cables. Conductor Aluminum Copper Total Price Total Price Price Ratio Cross Section Price Price Aluminum Copper Aluminum [mm²] Date [USD/kg] [USD/kg] [k /km] [k /km] + Copper Source AC 800 jan [56] AC 1200 apr [56] AC 1600 nov [56] ±300 DC 2500 Mar [62] 214

215 References [1]: Data obtained from the individual EIA s of the three wind farms, either at or at the visitor centre of Commissie MER in Utrecht, The Netherlands. [2]: Nieuwe energie voor het klimaat Werkprogramma Schoon en zuinig, report about the energie strategy made by the Dutch Ministry of Economical Affairs, available at: [3]: Dutch Ministry of Economical Affairs, Connect II: Eindrapport, report of the Dutch Ministry of Economical Affairs on the planned growth of offshore wind in the Netherlands, Den Haag, November 2005, available at [4]: Information found at [5]: Windenergie op zee: Een transitiepad naar een duurzame elektriciteitsvoorziening, report of the Dutch strategy group Transition Group Offshore Wind about the planned growth of offshore wind in the Netherlands, November 2007, available at: [6]: A letter from the Dutch State Secretary of Transport, Public Works and Water Management to the Dutch Government about the future of offshore wind energy on the North Sea and accordingly a governmental decision, April 4 th 2008, Den Haag, the Netherlands, available at: [7]: Information about the new Dutch subsidy system for renewable energy, the SDE, available at the website of the Dutch Ministry of Economical Affairs, [8]: Information obtained at: [9]: TenneT, Visie 2030, a report about the vision of the Dutch TSO TenneT on the Dutch transmission system in 2030, February 2008, available at: [10]: Information obtained from the Poseidon website: [11]: Information obtained from Drs. David de Jager, consultant on Energy & Environment, Ecofys Netherlands B.V., one of the initiators of the Poseidon concept. [12]: Jager, D. de, Wijk, A. van, POSEIDON: Sustainable energy supply sets out to sea, Econcern, June [13]: TenneT, Kwaliteits- en Capaciteitsplan , a report about the quality and capacity plan of the Dutch transmission system for the period 2008 to 2014 made by the Dutch TSO TenneT, February 2008, available at: [14]: KEMA, Connect 6000 MW-II: Elektrische infrastructuur op zee, a study for the integration of 6000 MW offshore wind power into the Dutch onshore transmission grid done by KEMA, September 2005, Arnhem, the Netherlands, available at: [15]: Engel, J., Elektriciteitsinfrastructuur op zee voor 6000 MW windvermogen, advisory report to the Dutch government about the policy 215

216 towards the development of an electrical infrastructure for offshore wind, June 2004, Delft, the Netherlands, available at [16]: Ummels, B.C., Hendriks, R.L., Kling, W.L., Inpassing van grootschalig windvermogen op zee in het Nederlandse elektriciteitsvoorzieningssysteem, report about the large scale integration of offshore wind power in the Dutch transmission system performed by the Technical University of Delft, February 2007, Delft, the Netherlands, available at: [17]: Royal Haskoning, Connect 6000 MW Eindrapport, study for the connection of 6000 MW offshore wind power into the Dutch transmission grid performed by Royal Haskoning for the Ministry of Economical Affairs, Den Haag, July 2004, available at: [18]: Information obtained about the Randstad 380kV project of the Dutch TSO TenneT: [19]: Information obtained about the Randstad 380kV project of the Dutch TSO TenneT: [20]: Information about offshore wind farms found at: (last visit August 2008). [21]: Information obtained from M. Nichols, submarine cable specialist working at ETA (trainer of Cable Laying Course at 27 th and 28 th of May 2008 at Evelop). [22]: Information obtained about the HVDC VSC technology: [23]: Information on the first application of HVDC VSC technology for the offshore wind industry: 0.aspx. [24]: ABB, It s time to connect, report on the technical description of HVDC VSC technology by ABB, October 2007, available at: [25]: Moore, G.F., Electric cables handbook, 1997, London, England. [26]: Cable information obtained from ABB: [27]: Cowdroy, S., Hill, J., Halliday, M., Study on the development of the offshore grid connection of round two wind farms, study done by Econnect for the DTI, January 2005, London, England, available at: [28]: Information obtained about the power curves of offshore wind turbines: [29]: ABB, Success of Troll-A HVDC Light Project is Paving the way Offshore, mid 2006, available at: [30]: Gerdes, G., Tiedemann, A., Zeelenberg, S., Case study: European offshore wind farms A survey for the analysis of the experiences and leasons learnt by developers of offshore wind farms Final Report, study performed by the Deutsche Windguard, Deutsche Energie-Agentur GmbH (DENA) and University of Groningen, January 2006, available at: 216

217 [31]: DTI, Capital grant scheme for the North Hoyle offshore wind farm annual report July 2005 to June 2006, July 2006, available at: [32]: Information obtained about the North Hoyle offshore wind farm in Wales from a press release: [33]: Larsson, A., Practical experiences gained at Lillgrund offshore wind farm, paper presented at 7 th international workshop on large scale integration of wind power and on transmission networks for offshore wind farms, May, 2008, Madrid, Spain. [34]: Data known within Evelop. [35]: Schachner, J., Power connections for offshore wind farms, Technical University of Delft, January 2004, available at: [36]: Anders, G.J., Rating of electric power cables, IEEE Press power engineering series, 1997, New York, United States. [37]: Moore, G.F., Electric cables handbook, 1997, London, England. [38]: Op den Velde, W., Thermal resistance of soil, study performed by ECN and NEGMicon, May 2002, available at: [39]: DTI & Ofgem, A security standard for offshore transmission networks, report of a consultation held by DTI and Ofgem, January 2007, London, England, available at: [40]: ICES/CIEM and EuroGOOS, Update report on North Sea conditions 2 nd quarter 2006, Institute of marine research, Bergen, Norway. [41]: Brakelmann, H. and Anders, G.J., Improvement in Cable Rating Calculations by Consideration of Dependence of Losses on Temperature, published in IEEE Transactions on Power Delivery, [42]: ABB, XLPE Cable Systems user guide, version 2, available at: [43]: Brakelmann, H., Thermal analysis of cable groups for wind power transmission, published paper by CIGRE, [44]: Brakelmann, H., Netzverstärkungs-Trassen zur Übertragung von Windenergie: Freileitung oder Kabel?, Rheinberg, October 2004, available at: [45]: Jacobson, B., Jiang-Häfner, Y., Rey, P., Asplund, G., HVDC with voltage source converters and extruded cables for up to ±300 kv and 1000 MW, paper presented at CIGRE B4 conference, 2006, available at: [46]: Barberis Negra, N., Evaluation of losses of HVDC Solutions for Large Offshore Wind Farms, master thesis for the Department of Electric Power Engineering of the University of Stockholm, Sweden, May [47]: Normark, B., Nielsen, E.K., Advanced power electronics for cable connection of offshore wind, paper presented at Copenhagen Wind 2005, Copenhagen, Denmark, 2005, available at: [48]: Railing, B.D., Moreau, G., Ronström, L., Cross Sound Cable Project Second generation VSC technology for HVDC, paper presented at CIGRE B4 session in 2004 about the large HVDC VSC project Cross Sound Cable, available at: 217

218 [49]: Mattsson, I., Railing, B.D., Williams, B., Moreau, G., Clarke, C.D., Murraylink, the longest underground HVDC cable in the world, paper presented at CIGRE B4 session in 2004 about the large HVDC VSC project Murraylink, available at: [50]: Mohan, N., Undeland, T.M., Robbins, W.P., Power electronics Converters, Applications and Design, book about the principles of power electronics, 2003, Hoboken, USA. [51]: Historical price data for copper, aluminum and lead obtained from: [52]: Historical currency conversion ratios obtained from: [53]: ABB, HVDC Light Cables Submarine and Land Power Cables, April 2006, Sweden, available at: [54]: Department of Trade and Industry (DTI), Meeting the energy challenge A White Paper on Energy, May 2007, available at: [55]: Information obtained about the prices of ROC s in the UK: [56]: Internal cost data Evelop obtained from several companies. [57]: Econnect, Study on the development of the Offshore Grid for Connection of the Round Two Wind farms, January 2005, London, England, available at: [58]: National Grid, TNUoS Charges Onshore Methodology Potential Application Offshore, presentation given at January 2007, available at: [59]: Veal, C., Byrne, C., Kelly, S., The cost-benefit of integrating offshore wind farm connections and subsea interconnectors in the North Sea, Airtricity and National Grid, paper presented at European Offshore Wind Conference 4-6 December 2007, Berlin, Germany. [60]: Lazaridis, L.P., Economic Comparison of HVAC and HVDC Solutions for Large Offshore Wind Farms under Special Consideration of Reliability, May 2005, Royal Institute of Technology, Stockholm, Sweden, available at: EES-0505.pdf. [61]: Econnect, Dutch offshore costing Offshore wind farm grid connection costing for AC and HVDC light configurations, October 2006, London, England. [62]: Brakelmann, H., Bipolare HVAC- und HVDC-Hochleistungs- Übertragungssysteme met VPE-isolierten See- und Landkabeln, 2007, University of Duisburg-Essen, Germany. [63]: DTI, Study of the costs of offshore wind generation, 2007, UK, available at: [64]: Cost information known within Evelop, obtained from the Dutch TSO TenneT for several grid connection points for planned offshore wind farms. [65]: Information obtained about the Q7 offshore wind farm: 218

219 [66]: Possible design of future offshore HVDC VSC substation by Siemens: OWEE_Technology.pdf. [67]: ABB, Middletown Norwalk Transmission Project; Technical description of VSC HVDC Converter and cable technology, October 2007, Ludvika, Sweden, available at: [68]: Lako, P., Kenmerken van gelijkstroom of wisselstroom hoogspanningsnetwerken, March 2004, ECN, the Netherlands, available at: [69]: Cost information obtained on the Murraylink HVDC VSC project at: [70]: Cost information obtained on the Cross Sound Cable HVDC VSC project at: [71]: Cost information obtained on the Estlink HVDC VSC project at: [72]: Cost information obtained on the Nord E.ON 1 HVDC VSC project at: 3.aspx. [73]: Observations made with Google Earth. [74]: Gerdes, G., Tiedemann, A., Zeelenberg, S., Case study: European offshore wind farms A survey for the analysis of the experiences and lessons learnt by developers of offshore wind farms final report, Deutsche WindGuard GmbH, Deutsche Energie-Agentur GmbH (DENA), University of Groningen, 2006, available at: 0POWER.pdf. [75]: Information price history steel: [76]: Statistical information about inflation obtained from: [77]: Presentation held by Günter Stark of ABB at the Hannover Messe 2008: Realisierung einer Offshore Windparkanbindung mit selbstgeführter HGU, 22 April

220 Glossary of Terms AC: Alternating current Black Start Capability: Capability of a system to start up the grid after a severe fault on the grid/complete shut down by itself. DC: Direct current DTI: Department of Trade and Industry in the UK EEZ: Exclusive Economical Zone: part of the sea outside the 12 (nautical) mile zone where the laws differ from the laws valid for the Territorial waters and mainland. EIA: Environmental Impact Assessment EWEA: European Wind Energy Association HVAC: High Voltage Alternating Current HVDC: High Voltage Direct Current HVDC Light: HVDC VSC technology developed by ABB HVDC Plus HVDC VSC technology developed by Siemens HVDC VSC: HVDC technology based on Voltage Source Converters (VSC) with IGBT s as switching component IGBT: Insulated Gate Bipolar Transistor, switching element used in HVDC VSC. Line voltage: voltage difference between two phases in a three phase AC system Modular design: Technology (HVAC or HVDC) can be connected in multiple sections (over time) by means of interconnected offshore substations and cables in between. MW: Mega Watt; 10 6 Watt NPV: Net Present Value NWEA: Dutch Wind Energy Association Ofgem: Office of Gas and Electricity Markets in the UK OHVS: Offshore High Voltage Station (for HVAC system) PE: polyethylene, an insulation material Phase voltage: PWM: voltage difference between a phase and the neutral earth Pulse Width Modulation, modulation technology for switching off/on switching elements with the modulation in a varying pulse width. RMS: 1 2 root mean square value (i.e. x for period N for N N variable x) STATCOM: STATic COMpensator, advanced reactive power compensation technology. SVC: Static Var Compensator, advanced reactive power compensation technology. TSO: Transmission System Operator WACC: Weighted Average Cost of Capital XLPE: Cross Linked PolyEthylene, type of dielectric isolation 220