Optimization of the coupled grid connection of offshore wind farms

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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

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