Transformers for Offshore Wind Platforms: Expected Problems and Possible Approaches

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1 Transformers for Offshore Wind Platforms: Expected Problems and Possible Approaches Boris Valov (ISET/IWES, GERMANY) Abstract--In the near future the power of 12 out of 21 wind farms in Baltic Sea and North Sea of Germany will be from 1 up to 5 GW. For the connection to the power transmission system on the sea a transformer in this power range will be required but it is not available due to limitation of production facilities. Another problem is the transport of such big units. In this paper the future challenges with power for offshore wind farms are discussed and possible approaches for optimization of number, design and performance are suggested. A new modular design concept and its electrical operation were investigated and the feasibility has been verified by calculations. The proposed approaches showed ways for reduce of weight and volume of as well as for solution of logistic problems. Index Terms - Offshore wind farms, transformer, power transmission. I I. INTRODUCTION N North Sea of Germany 18 offshore wind farms have been approved (state August 2009) [1]. Further 46 wind farms are currently in the status of approval procedure [2]. The ISET-concept for the connection of approved and some planned wind farms is shown on fig. 1 [3]. 12 out of 21 approved wind farms have a rated power between 1000 MW and 5000 MW (table 1). It does not make sense to try to produce a transformer with this power level because there is a limit for production regarding the power and for transport by rail waggon regarding weight and volume. Both must not be exceeded. As a simple solution of this problem the installation of several power with lower power instead of one transformer with high power is theoretically possible but in this case more space on the sea platform is required. Thus this solution is technically and economically not optimal. The calculations showed that the alternative installation of transformer banks with twowinding or auto instead of usual with two or three windings shows better technical or economic characteristics. For the offshore transmission network according to fig. 1 AC- or DC-technology can be applied. Advantages or disadvantages of these technologies for offshore are still in discussion. For this paper calculations and analyses were carried out just for AC-technology. Boris Valov is with the Fraunhofer-Institut fuer Windenergie und Energiesystemtechnik IWES, Koenigstor 59, D Kassel, Germany ( bvalov@iset.uni-kassel.de). Legend: 1 - Offshore Bürger Windpark Butendiek; 2 - Dan Tysk; 3 - Sandbank 24; 4 - Nördlicher Grund; 5 - Amrumbank West; 6 - Nordsee Ost; 7 - ENOVA Offshore Northsea Windpower; 8 - Testfeld "alpha ventus" ehemals "Borkum West"; 9 - Borkum Riffgrund; 10 - Borkum Riffgrund West; 11 - Borkum West II; 12 - Gode Wind II;; 13 - Global Tech I; 14 - Hochsee Windpark Nordsee; 15 - Gode Wind; 16 - BARD Offshore I; 17 Meerwind Ost und Meerwind Süd; 18 - Hochsee Windpark He dreiht; A Uthland; B - Weiße Bank; C - Vento Tec Nord I; D - Offshore Windpark Austerngrund; E - Offshore-Windpark "Deutsche Bucht"; F - Vento Tec Nord II; M - Borkum Riffgat; O - Offshore Windpark Nordergründe; Fig. 1: ISET-concept for offshore transmission network II. CAPACITIES FOR PRODUCTION AND TRANSPORT OF POWER TRANSFORMERS The biggest power transformer so far has a rated power of 1100 MVA and a weight of 559 tons [4]. Such a power transformer exceeds the limit for the weight and the volume of the biggest rail waggon Uaai 839 which is about 454 tons and 191 m 3 [5]. The biggest sea crane Wind Luft 1 for installation of wind turbine generators is not able to carry more than 500 tons [6]. Therefore a transformer has to be designed so that the weight of its heaviest unit must not exceed this value. From these data it follows that the maximum power of a transformer for offshore sea platforms is not allowed higher than 1000 MVA. This requirement is relevant for all wind farms in table 1.

2 TABLE I GERMAN WIND FARMS WITH TARGETED POWER ABOVE 1000 MW Targeted power (MW) "Arkona-Becken Südost" 1005 "Godewind" 1100 "BARD Offshore 1" 1200 "Nordsee Ost" 1250 "ENOVA Offshore Northsea Windpower" 1406 "Global Tech I" 1440 "DanTysk" 1500 "Borkum Riffgrund West" 1800 "Nördlicher Grund" 2010 "Hochsee Windpark Nordsee" 2540 "Sandbank 24" 4905 "Arkona-Becken Südost" 1005 From the mentioned numbers basic values for the relations between power, weight and volume for power with two windings can be derived which are significant for designing the substations on sea platforms: - weight / power is about 0,5 t/mva, - weight / volume is about 2,38 t/m 3, - power / weight is about 2 MVA/t - power / volume is about 5,8 MVA/m 3. The first three basic values are constant for all types of power. The last basic value is variable and depends on the transformer s operation. An increasing of the relation power / volume should be an important target with designing power for offshore sea platforms. The mentioned limits and relations are also valid for reactive power compensation units on sea platforms which should be designed as inductive shunts. They are operated like a transformer with only one winding. The requirement for reactive power compensation is determined by the capacitive characteristics of the sea cable und the level of the operating voltage. The calculations showed that the required rated power of the inductive shunts for most wind farms from table 1 are higher than 1000 MVAr. In general the same limits for production and transport for these shunts are valid like for power. III. PROPOSED TYPE AND POWER OF TRANSFORMERS The problem with the transport of limit-rating can be solved by using the following alternative types: A. 3-phase 3-winding transformer with equal medium voltage windings, B. 3-phase auto transformer, C. Transformer bank winding, D. Transformer bank consisting of three single phase auto. For these types of the following characteristics are relevant: A. 3-phase 3-winding transformer with equal medium voltage windings is cheaper than two 3-phase 2-winding (basic option today for most of the wind farms) but it has a worse power / weight relation. Therefore it is not recommended for wind farms with a power of more than 1000 MVA. However, this type of transformer is explicit recommended for wind farms with a power of less than 1000 MVA because it can secure very good damping of opposite system perturbation between the wind farms connected at its two medium voltage sides. B. With a 3-phase auto transformer the biggest relation power / weight compared to a 3-phase transformer with two windings can be reached because one part of the winding can be designed just for the differential current and thereby a winding with higher power can be installed in the same transformer s volume. C. In a transformer bank consisting of three single phase with two windings the relation power / weight is further improved. In this case a single phase 2- winding transformer can be produced with the same weight and volume but with a power higher than 1000 MVA. The power of the transformer bank is the result of adding the powers of the three single phase and it can reach approximately MVA. D. The desired power of 5000 MVA for the wind farm "Sandbank 24" can presumably be reached just with a transformer bank consisting of three single phase auto. The further advantage of an auto transformer is the possibility of integration of another winding for internal energy supply of the sea platform. Thereby the installation of an additional transformer is not necessary any more.

3 "Global Tech I" 1440 MW IV. APPROACH FOR HIGH POWER OFFSHORE WIND FARMS The power of the wind farms in table 1 can be separated into three groups: A. from 1005 to 1800 MW, B. from 1800 to 2540 MW, C. more than 2540 MW. The designing procedure for is different according to each group. The suggested solutions are shown in tables II and III. A. Wind Farms with MW For this power range the above mentioned limits for production and transport are not exceeded if two 3-phase 2- winding or two 3-phase auto are used in a substation on a sea platform. Besides the reduction of weight and volume of a transformer the reliability and availability of a substation and the whole power transmission system rise in this case. "DanTysk" 1500 MW "Arkona-Becken Südost" 1005 MW Two with 500 MVA, ca. 250 tons and 105 m 3 each Fig. 3: Substation with two 3-phase auto B. Wind Farms with MW In this power range the use of a transformer bank with three single phase 2-winding (fig. 4) or with three single phase auto (fig. 5) is suggested. Fig. 2: Substation with two 3-phase 2-winding The suggested types and rated powers of the for this power range are shown in table II. TABLE II SUGGESTED NUMBER AND POWER OF TRANSFORMERS Fig. 4: Substation with a transformer bank with three single phase "Godewind" 1100 MW "BARD Offshore 1" 1200 MW "Nordsee Ost" 1250 MW "ENOVA Offshore Northsea Windpower" 1406 MW Number and nominal power (MVA) Two with 630 MVA, ca. 315 tons and 132 m 3 each Two with 800 MVA, ca. 400 tons and 168 m 3 each Fig. 5: Substation with a transformer bank with three single phase auto

4 The suitable rated powers of the for each option are shown in table III. TABLE III SUGGESTED NUMBER AND POWER OF TRANSFORMERS "Borkum Riffgrund West" 1800 MW "Nördlicher Grund" 2010 MW Number and nominal power (MVA) winding with 630 MVA, ca. 315 tons and 132 m 3 each. consisting of 3 single phase auto with 630 MVA, ca. 315 tons and 132 m 3 each. winding with 800 MVA, ca. 400 tons and 168 m 3 each. consisting of three single phase auto with 800 MVA, ca. 400 tons and 168 m 3 each. parallel operated sea cables. In opposite to a delta-connection of windings in onshoregrids a star-connection of windings in offshore-grids can be used at the medium voltage side of because the compensation of the unbalance of loads is not important any more. Generated electricity in generators or inverters of wind power generators is always symmetrical. With the use of a star-connection at the medium voltage side of a transformer the volume of these windings can be reduced, too. For this reason it is suggested to connect the medium voltage winding of power in a star (fig. 3, 5 and 6). The standard IEC also allows this option. "Hochsee Windpark Nordsee" 2540 MW winding with 1000 MVA, ca. 500 tons and 210 m 3 each. consisting of three single phase auto with 1000 MVA, ca. 500 tons and 210 m 3 each. C. Wind Farms with more than 2540 MW In this case it is about the wind farm "Sandbank 24" with a targeted power of 4905 MW. Because an installation in one transformer bank of such high power is not possible, the substation should consist of two transformer banks (fig. 6). For this wind farm it is suggested to install three single phase auto in each transformer bank. Each auto transformer has a power of 800 MVA, a weight of ca. 400 tons and a volume of ca. 168 m 3. V. NEUTRAL POINT TREATMENT Power for sea platforms are usually designed for a high voltage level of kv and a medium voltage level of kv. For the high voltage side of a transformer the star connection is preferred because the isolation of this winding is necessary to design only for phase-to-earth voltage and thereby the winding volume can be reduced. Low-resistive neutral earthing on the high voltage side of leads to a fast and selective disconnection in case of a fault in one of several Fig. 6: Substation with two banks from three single phase auto The solution with a star connection at the medium voltage side has further advantages, too: - it is not necessary any more to compensate the big capacitive short circuit current in medium voltage sea cables with the help of additional units which need space on sea platforms, - unit- and grid protection is simplified and the selectivity is improved. In general the economic efficiency rises with the auto transformer if the value of medium voltage tap nears to value of the high voltage tap. Thereby it is recommended for the internal cable connection in the wind farm with auto not to use 35 kv but 60 kv as the next voltage level according to the standard IEC The increasing of the voltage level causes a reduction of the current, of the power losses and of copper demand. Thereby the economic

5 efficiency rises. alternative types of power has demonstrated that a reduction of space is possible. VI. POWER LOSSES AND SHORT CIRCUIT POWER The proposed approaches showed ways for reduce of weight and volume of as well as for solution of logistic problems. The use of the suggested of different types and power levels causes increasing of power losses and short circuit currents at the medium voltage side in case of identical feed-in power from wind farms. For example, the power concentration in one tank of a transformer with a limit volume of about 190 m 3 for one single phase auto transformer is significant higher than the power concentration of one 3-phase transformer with the same volume. Thereby the maximum nominal power and total power losses of the transformer rise proportionally. The increase of short circuit currents at the medium voltage side of the transformer (without feed-in from wind farm) is determined by the decrease of its impedance. This causes a network enhancement for the points of common coupling of the wind turbine generators. The calculated changing ratios are clarified by the data of table IV. TABLE IV POWER LOSSES AND SHORT CIRCUIT POWER Type One 3-phase 2-winding transformer One 3-phase auto transformer Change of power losses (p.u.) Ratio for increase of circuit power (p.u.) 1,00 1,00 0,92 1,04-1,07 VIII. ACKNOWLEDGMENT This contribution has been developed at ISET/IWES within the project RAVE Research at alpha-ventus. Netzintegration von Offshore-Windparks funded by Forschungszentrum Juelich GmbH - PTJ. IX. REFERENCES [1] Bundesamt für Seeschifffahrt und Hydrographie (BSH): Windparks. Dokument im Internet, Meeresnutzung/Wirtschaft/Windparks/index.jsp. [2] Windparks in der Nordsee. Dokument im Internet, [3] Valov Boris. Outlook in the future of German North Sea Power System for Offshore Wind Power. Wind-Kraft Journal. German Offshore. 5. Ausgabe S [4] RP-Online: Schwertransport zum Braunkohlekraftwerk. Dokument im Internet, gladbach/nachrichten/moenchengladbach/43423?skip=0&refback=/pu blic/article/moenchengladbach/693939/fuenf-wochen-auf-dem-wegnach-sachsen.html article [5] Wikipedia die freie Enziklopedie: Sonderwagen (Güterwagen der Gattung U). PDF-Dokument im Internet, _Gattung_U). [6] Bard-Magazin. July X. BIOGRAPHIES Transformer bank consisting of three single phase 2- winding Transformer bank consisting of three single phase auto 1,88 1,60-1,94 2,13 1,62-2,00 In this table the data of a transformer with two windings were assumed to 1,0 because today this type is usually proposed for all offshore wind farms as a basic option. VII. CONCLUSIONS The above mentioned limits for weight and volume will not be exceeded if the proposed alternative types, power levels and numbers of the are used with the design of substations on sea platforms. The feasibility of the proposed options was proved due to network calculations for the operation of power transmission systems inclusive power, sea cables and reactive power compensation units on the sea platform and on the continent. The estimation of required space on the sea platform for Dr. Boris Valov was born in Tomsk in West Siberia (Russia), on July 28, He studied and gained the title of PhD at the Polytechnic University Tomsk, Russia. At this University he has been working in the field of mathematical analyses of power quality, stability in Electrical Power Systems and Networks for more than 25 years. During the time from 2000 to 2007 Boris Valov worked as a scientist at Kassel University at the Institute of Electrical Power Supply Systems. His was working with analyses and design of interconnection of Wind Power Plants to Electrical Power Supply Systems and Networks. Since March 2007 he is a scientist at the Institut fuer Solare Energieversorgungstechnik ISET e.v. (Fraunhofer-Institut fuer Windenergie und Energiesystemtechnik IWES) focussing on interconnection of Renewables into Electrical Power Supply Systems and Networks. He has more than 120 publications in the area of Power Quality and Grid Integration of Renewables.