IWES-Concept 2010 for Offshore Power Transmission System 2020

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1 IWES-Concept 2010 for Offshore Power Transmission System 2020 B. Valov, P. Strauß, T. Degner, C. Jansen; Fraunhofer-Institut für Windenergie und Energiesystemtechnik (IWES) B. Valov EXTERNAL ARTICLE English Introduction The German government plans to increase the amount of renewable energies in electricity production step by step within the upcoming years, aiming at 30 percent in 2020 [1]. A great part of this production is supposed to be covered by building offshore wind farms in the North Sea. The first German offshore wind farm alpha ventus, that is in operation since the end of 2009, can be seen as an important first step. Currently, the planning and approval of offshore wind farms in Germany has a highly dynamic character. An equal dynamic characterization can be seen in other countries adjoining the North Sea. The high potential of electricity production of offshore wind farms is a precondition for realizing the political ambitions. In addition, a realization depends crucially on providing sufficient absorbing power of the main grid connection points. Studies performed for this article show that this is not currently the case in Germany. This problem may also occur in other European countries. Possible solutions should be discussed today to ensure feasibility in In this article, existing and newly developed plans to ensure the achievement of the political ambitions will be discussed. The focus of the work done at Fraunhofer IWES was to develop an offshore transmission system which is able to transmit the generated power of all offshore wind farms to the shore, where it is successfully absorbed by the grid connection points. Furthermore, integration into a possible future Supergrid is considered. Current Plans for Future Offshore Wind Farms According to the Windenergie-Agentur Bremerhaven [2] and the German Federal Maritime and Hydrographic Agency [3], the current state regarding quantity and power of offshore wind farms in the German North Sea is as follows (Tab. 1) [4, 5]: Before the power, generated by offshore wind farms, can reach any customer s household, several obstacles have to be overcome: 1. Distances from 50 to 100 km, from the wind power plants at sea to the substation on shore, 2. Additional distances up to 50 km from the substation at shore to the main grid connection points, 3. Limited absorption power of the main grid connection points, 4. Transmission through the main transmission and distribution grid to reach the final customers. 44 DEWI MAGAZIN NO. 37, AUGUST 2010

2 Werbung GWU / Willmers 1/1 s/w

3 Current status of offshore wind farms Quantity of offshore wind farms Rated power [GW] Quantity of wind turbines *) In Operation Approved Planned Sum Tab. 1: Status of offshore wind farms in the German North Sea in 2006 and 2010 *) equivalent to 5 MW wind turbines Fig. 1: The transpower offshore gmbh Concept for the extension of offshore power transmission system in the North Sea (with approval of transpower offshore gmbh) Fig. 2: ISET-Concept 2007 of the offshore power transmission system in the North Sea Legend: 1 - Offshore Bürger Windpark Butendiek; 2 - Dan Tysk; 3 - Sandbank 24; 4 - Nördlicher Grund; 5 - Amrumbank West; 6 - Nordsee Ost; 7 - Offshore North Sea Windpower; 8 - Borkum West; 9 - Borkum Riffgrund; 10 - Borkum Riffgrund West; A Uthland; B - Weiße Bank; C - Vento Tec Nord I; D - Offshore Windpark Austerngrund; E - Offshore-Windpark Deutsche Bucht ; F - Vento Tec Nord II; G - Global Tech I; H - Hochsee Windpark Nordsee; I - Hochsee Windpark Hedreiht; K - BARD Offshore I; L - Gode Wind; M - Borkum Riffgat; O - Offshore Windpark Nordergründe; P Meerwind. 46 DEWI MAGAZIN NO. 37, AUGUST 2010

4 In Germany, the last issue has been discussed in detail in 2005, based on studies of the German Energy Agency: Energiewirtschaftliche Planung für die Netzintegration von Windenergie in Deutschland an Land und Offshore bis zum Jahr 2020 [6]. However, the first three questions remain unsolved. Discussions about possible solutions have begun [7, 8, 9], but so far no final decisions have been made. Energy Transmission from Sea to Land Another big problem remains the transmission of the electricity produced offshore to the main grid connection points at land, with distances of up to 120 km. While at sea level only sea cables can be used, the part on land can be developed using land cables, overhead lines, gas insulated transmission lines or a combination of those [10]. Related to the decision of the used transmission line system, the question has to be answered which current to use: direct or alternating current [11]. Both technologies are used in the German North Sea at present: the connection of the first German offshore wind farm alpha ventus uses HVAC, while the connection of the sea substation BorWin1 was developed using HVDC. Compared to the huge amount of planned electricity production of GW, the installed capacities of 60 MW for the HVAC and 400 MW for the HVDC transmission line are marginal and thus, these connections can only be seen as test projects. Experiences made in these testing projects should initiate productive discussions. From the authors` point of view, both technologies should be used in an efficient complementary way. Nowadays, all operational wind power plants are running - and have been connected - using HVAC. As a result, the HVAC technology for offshore purpose has already proven its suitability and knowledge of use could be acquired. This is why in the following studies for designing an offshore transmission system only HVAC is considered. In the model used, unwanted capacitive loading currents which occur in the cable insulation have been neutralized using reactors. Loadflow calculations have proven the feasibility of offshore transmission systems based on HVAC only. A next step for plans towards an offshore transmission system is to set up a design under economic and technical aspects. A lot of possible designs with lots of variations and different approaches can be made. Each model will be characterized by its specific costs. These costs rise to around several billion Euros for offshore transmission systems in giga watt size. A decision about a preferred optimal design of such an offshore transmission grid could save some billion Euros of investment costs by Concepts for Energy Transmission According to the law for speeding up planning processes of infrastructure projects [12] the local grid operator has to plan, build and run connection lines from substations at sea to the most suited main grid connection point at land. This will affect all offshore wind farms whose construction will have started until the end of 2015 and therefore, the amount of offshore wind farms can hardly be foreseen. In addition, different concepts of energy transfer from the offshore sector to the German main grid are presented. transpower offshore gmbh Concept Fig. 1 shows the future concept for the grid connection of offshore wind farms in the German North Sea of the local grid operator transpower offshore gmbh 1. Part of this concept is a clustering of reasonable offshore wind farms. The electricity produced by each cluster will be transmitted with discrete cable connections. Two traces for sea cable laying have already been approved, although only one is in use with one HVAC and one HVDC cable lay. For crossing the island Norderney, ductwork has been laid and some of the over 30 existing buried empty tubes have been used. The rest of the ductwork will be used for upcoming cable installations. To absorb the electricity produced offshore three grid connections points are currently planned: Diele, Dörpen/West and Büttel. ISET 2 -Concept 2007 Umeå, Sweden Anti-icing and De-icing technologies Problem solving Grid connection Risk Assessment Research concerning the planning of future offshore-transition systems has shown that a radiant system with mainly separate connections of each offshore wind power plant with the German grid is preferable. The technical and eco 1 The transmission system operator transpower stromübertragungs gmbh has the responsibility for operation, maintenance and if necessary the reinforcement of its transmission grid. In charge for the offshore part is the subsidiary transpower offshore gmbh. 2 The part of the new Fraunhofer IWES institute in Kassel founded arose from the former Institut für Solare Energieversorgungstechnik - Verein an der Universität Kassel e.v. (ISET) DEWI MAGAZIN NO. 37, AUGUST

5 Fig. 3: IWES-Concept 2010 of offshore power transmission system in the Nord Sea with an example of an integration into a trans-european Supergrid Legend: A - Nordergründe; B - Godewind II; C - Godewind; D - OWP Delta Nordsee I; E - Borkum Riffgrund; F - alpha ventus (Borkum West); G - Borkum West II; H - Borkum Riffgrund West; I - Meerwind; J - Nordsee Ost ( Amrumbank ); K - Amrumbank West; L - Butendiek; M - Dan Tysk; N - Nördlicher Grund; O - Sandbank 24; P - Global Tech I; Q - Hochsee-Windpark Nordsee; R - He Dreiht ; S - BARD Offshore I; T - Veja Mate; U - OWP Delta Nordsee II; V - MEG Offshore I; 1 - Borkum Riffgat; 2 - Innogy Nordsee 1; 3 - Diamant; 4 - Borkum Riffgrund II; 5 - Euklas; 6 - Borkum Riffgrund West II; 7 - OWP West; 8 - Kaskasi; 9 - Hochsee Testfeld Helgoland; 10 - Uthland; 11 - Nordpassage; 12 - Sandbank 24 ext.; 13 - Weiße Bank; 14 - AreaC III; 15 - AreaC II; 16 - AreaC I; 17 - Skua; 18 - Sea Wind I; 19 - Albatros; 20 - Notos; 21 - Sea Wind II; 22 - He dreiht II; 23 - Bight Power II; 24 - Bight Power I; 25 - Aquamarin; 26 - OWP Deutsche Bucht ; 27 OWP - Austerngrund ; 28 - Bernstein; 29 - Citrin; 30 - Sea Storm; 31 - Sea Storm II; 32 - VentoTec Nord I; 33 - VentoTec Nord II; 34 - Aiolos; 35 - Sea Wind III; 36 - Kaikas; 37 - GAIA I; 38 - GAIA II; 39 - GAIA III; 40 GAIA IV; 41 - Horizont I; 42 - Horizont II; 43 - Horizont III; 44 - NSWP 4; 45 - NSWP 5; 46 - NSWP 6; 47 - NSWP 7; 48 - H2-20 nomical disadvantages of this approach are elaborately exemplified in [13]. In 2007, the ISET-Concept 2007 for the construction of a transmission system was proposed (Fig. 2) [14, 15]. This concept plans the development of a transmission system for a joint use by all offshore wind farms. The standardized transmission system establishes multiple advantages for the operation management in normal operation mode, as well as for grid failures. For example, in normal operation mode, the losses in the transmission system can be kept at a minimum level by controlling the load flow. With load flows at giga watt level, this means savings of several million Euros annually. A new aspect in this concept is the introduction of interconnections between the wind power plants Global Tech I - Weiße Bank and the wind power plants Gode Wind Meerwind. This allows adjustments of applied load of the grid connection points on land, because they have different absorption capacities. Thanks to the interconnections, variations of the feeder line of single wind power plants are damped and thus dynamic disturbances to the German grid are reduced. The concept covers optimization of capacity and placement of compensation reactors, reciprocal support at black starts, energy support for auxiliary power, reduction of sea substations, sea cables and transformers. Furthermore the impact on protected landscape is minimized and recommendations from the dena study, regarding the necessity of a coordinated construction of an offshore transmission system, are considered. Load flow calculations performed for the ISET-Concept 2007 proved the feasibility of an offshore transmission system based on alternating current. IWES-Concept 2010 The new advanced concept, which was updated to development plans of December 2009 (Fig. 3), is a combination of the two concepts of transpower offshore gmbh and ISET- Concept 2007 and the political attempt of Germany and Europe to connect offshore wind farms to a future Trans- European Supergrid. In comparison to the two other concepts the new concept features: 70 wind power plants ( Tab. 1) instead of 23 from ISET- Concept 2007, Interconnections introduced in ISET-Concept 2007 remain, In contrast to the dena study and ISET-Concept 2007, only 2 out of 4 grid connection points, Proposal of possible options for integration of a German offshore transmission system into a Trans-European Supergrid. Looking at the geographical location of existing and future offshore wind farms, it can be observed that their distribution in space of the Exclusive Economic Zone of Germany (blue box in Fig. 2 and 3) is uneven: the major part is located in the southwest while the minor part is located in the northeast of 48 DEWI MAGAZIN NO. 37, AUGUST 2010

6 Technical by nature... QUALIFIED BY EXPERIENCE the zone. This imbalance is also true for the distribution of the total offshore generated electric power in 2 pieces, GW (72.8 %) and GW (27.2 %) to be absorbed by the 2 grid connection points. The maximum and minimum capacity of absorption power of a grid connection point can basically only be determined by using grid calculations. If the fed in power exceeds the absorption capacity of the grid connection point, no convergence after lots of iterations in the grid calculations can be achieved. Studies have shown that the minimum requested absorption power, the sub transient threephase short-circuit apparent power, should be equal to 22 GVA in Diele and 16 GVA in Büttel to successfully absorb the total fed in offshore power of 32.6 GW and 13.2 GW, respectively. The value of the sub transient three-phase shortcircuit apparent power at the grid connection points Diele and Büttel is mainly determined by the total fed in power of power plants in Northern Germany using synchronous generators. Operation of the synchronous generators follows the current fluctuating demand in electricity and is thus mutable within certain limits itself. Analysis has shown that for practical use the following limits should be applied (Tab. 2). Tab. 2 shows, that the initial symmetrical short-circuit power in the grid connection points can be less than 22 GVA or 16 GVA. In this case, the power generated offshore cannot be absorbed completely and thus has to be limited. In the long term, grid connection reinforcements are required. However, increasing the sub transient three-phase short-circuit apparent power has technical limits, which are caused by protection measures of the grid equipment, insulation coordination, electromagnetic field strength and allowed current losses. Therefore, power generated offshore should not only be fed into a single grid connection point, but into several. As an alternative to the costly and time-consuming grid reinforcement measures, we propose a new concept of flexible distribution of the total offshore power between the two approved grid connection points. This can be achieved through an interconnection of the offshore transmission system between the wind power plants Aiolos - White Bank and AreaC - Sea Wind. Again, load flow calculations showed the feasibility of the balancing power of such an interconnection between the grid connection points. Additional flexibility can be achieved by connecting the offshore transmission system with a future offshore Trans-European Supergrid. Such an integration could offer the following benefits: Required grid reinforcements reduced to a minimum because excessive power can be absorbed by the Supergrid, Possibility to use the great amount of hydro storage capacity in other countries,.e. g. Norway, Participation in international electricity markets, which could result in reduced overcharges of the German electricity grid, Improvement of the reliability of the offshore transmission system, DEWI MAGAZIN NO. 37, AUGUST

7 Sub transient three-phase short-circuit apparent power Voltage level (GVA) (kv) minimal maximal , , , ,9 10 Tab. 2: Relevant values of the sub transient three-phase short-circuit apparent power of different voltage levels according to Transmission Code 2007 [16]. Flexibility in grid operation management. Some studies considering the design of a future Supergrid are already at hand [17, 18,19]. However, they mainly cover the economic value of interconnection to a future Supergrid. The feasibility of an electric realization remains uncertain until today [20]. A future Supergrid would have a certain absorption power itself, like any physical grid. While the Supergrid would be set up using HVDC transmission technology, its absorption power will be limited by the transmission capacity of this technology. This limit currently add up to 1.5 GW per unit. The required amounts of HVDC units and converter sea substations have lately been discussed. In the introduced IWES- Concept converter sea substations are proposed: West, Center and East. Their absorption power was estimated at 2 GW each, which is expected to be the future limit for HVDC units. Presence of the applied 2 grid connection points, 3 converter sea substations for a connection to a future Supergrid and 2 interconnections between the grid connection points has highly increased the flexibility and reliability of the offshore transmission system. AC / DC Connections in the Offshore Transmission System A final decision regarding the choice of transmission technology has not been made yet. There are advantages and disadvantages for each type mentioned. The main disadvantage of the AC technology is the high capacitance of the sea cables, which limits the transmission distance at 380 kv voltage level to about 50 km without compensating measures. For longer distances, reactive power compensation utilities have to be installed, which cause additional cost and space requirement. Compared to this disadvantage, the AC transmission technology provides high reliability, proven long term operation and high durability. Considering the great amount of potentially more than 40 GW of power generated offshore, the disadvantages of the HVAC transmission technology might be considered less important, since the following can be applied for HVDC transmission technology: High complexity, Great space requirements at the sea substations, Lowered reliability through chain connected use of power electronics (converter), Average life cycles of 10 to 15 years for power electronics, High maintenance cost, Overall greater amount of current losses compared to AC technology, Possibility of fed in of unwanted harmonics and spreading through the German transmission grid, Own grid protection requirements which may affect the selectivity of the existing grid protection measures. The information given shows the necessity of one or several comparative studies for AC / DC technology use to reduce costs in the amount of multiple millions of Euros. For the development of the IWES Concept 2010 only HVAC at 380 kv voltage level was applied. The capacitance of the sea cables has been offset using compensation reactors. Load flow calculations have shown the feasibility of the developed offshore transmission system according to the current grid operator requirements [21]. Summary The developed IWES-Concept 2010 of an offshore power transmission system supports the current plans for electricity transmission in the German North Sea and contributes to an optimized development of offshore wind power integration into the German Power Transmission System and a Trans- European Supergrid. The proposed solutions aim at increasing the reliability of the German Power Transmission System and the flexibility in operation management of the offshore transmission system and at reducing investment cost in the range of several million Euros. We show the feasibility of the IWES-Concept 2010 by appropriate load flow calculations. Acknowledgement Parts of this article have been developed in scope of the project Windenergieforschung am Offshore-Testfeld. The authors thank the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety and the Project Management Jülich for their support. Responsibility for this article is with the authors. Contact Dr./OAK Moskau Boris Valov, Fraunhofer Institut für Windenergie und Energiesystemtechnik IWES, R&D Division Engineering and Network Integration, Group Electricity Grids, Koenigstor 59, D Kassel, Ger many. Tel.: , Fax: Mail: boris.valov@iwes.fraunhofer.de; 50 DEWI MAGAZIN NO. 37, AUGUST 2010

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