Transmission technologies for collective offshore wind farm connections

Transcription

1 Transmission technologies for collective offshore wind farm connections H. Brakelmann, K. Burges Abstract: Offshore wind farms under development have capacities of 4 MW and higher and are likely to be located - km off the cost or the nearest grid connection point. Because of technology constraints, conventional HAC cable designs and configurations have serious disadvantages as for example high transmission losses and extensive space requirements. The proposed transmission concept overcomes major advantages and, hence, offers a technically and economically viable option for collective connection of offshore wind projects to onshore networks. The comparison with alternative DC technologies in the same capacity range shows promising characteristics. Index Terms HAC, XLPE, cables, offshore, wind farms, transmission systems I. BACKGROUND AND STATE-OF-THE-ART Currently, in the German waters of the North and the Baltic Sea offshore wind parks are under development with typical capacities of 4 MW in the initial pilot phase. In the final stage capacities of up to 5 MW per wind farm are intended. The distance to the onshore transmission network is up to 5 km. The situation in other European countries as the UK or Belgium is similar. The costs and losses associated with the high voltage (H) transmission link between the offshore wind farms and the point of common coupling will play a decisive role for the economics of those projects. Concerning the transmission link, different approaches are being discussed. Three-phase AC cables and the required auxiliary components (compensation) are proven and available in commercial markets. However, transfer capacities of three phase XLPE AC cables are limited, most of all as a consequence of restrictions of their physical dimensions. Figure shows the cross section of a typical 5 / 7 k cable. Key data indicating the technically feasible maximum are a copper cross section of about mm, a cable diameter of more than 3 mm and a specific weight of about kg/m. The resulting capacity of such a cable is limited to about 5 MA, depending on the distance. For typical wind farms currently under development in Europe, this results in connections using cables which are laid one by one in two separate trenches. The copper cross section of cables for 45 k, currently H. Brakelmann is with the Universität Duisburg-Essen, Department ETS, Bismarckstraße 8, Gebäude BE,, D-4757 Duisburg, Germany. ( [email protected]). K. Burges is with the Ecofys GmbH, Stralauer Platz 34, 43 Berlin, Germany. ( [email protected]). being introduced to the market, will be restricted to about 8mm, resulting in a maximum capacity of about 35 MA. Nexans [] proposed to use single core 4-k XLPE cables. The cross section of the conductor would be about mm and the copper armouring should have a similar cross section. The maximum capacity for a 5 km line according to [] is about 9 MA. The concept requires three parallel cable trenches to be implemented one by one. One problem of such a configuration is the high current induced in the armouring. If the cables are buried with technically relevant distance in between (5 m or more) these shield currents easily are of the same magnitude as the currents in the inner conductor. The disadvantages of such a configuration are evident: heavy and expensive cables, and compared to three core cables much higher losses. Summarising, with state-of-the-art technology the technically feasible capacity limit for AC offshore transmission is about 37 MA for 3 core cables and about MA in case of 3 single core cables []. Higher capacities inevitably require more parallel cable routes. Alternatively, DC concepts have been discussed already for several years. Conventional, current source converters (CSC) based on thyristor switches have already been applied for long distance, bulk electricity transport up to high voltage and power levels (e.g. bipolar + 45 k, MA, Figure ). Concepts adapted to offshore wind energy have been proposed (e.g. by AREA [3]) but for a number of technical and economical reasons prospects are questionable. Figure : typical three core XLPE submarine cable for 5/7 k, 3** mm; left ABB (FXBT), right: Nexans (TKFA)

2 Figure : +/- 45 k/ mm - HDC-submarine cable (oil paper) of ABB; maximum capacity bipolar: MW The capacity of commercial IGBT based voltage source (SC) converters has been increased significantly during the last years. Converter capacities up to 5 MW emerge in the market [], though from a limited number of suppliers (HDC light, HDC plus). Using state-of-the-art single core XLPE DC cables (see Figure 3) with a rated voltage of +5 k and a cross section of mm, the maximum capacity per circuit would be about 4 MW. This would be sufficient for most offshore wind projects currently under development in the North Sea. Recently, ABB announced introduction of ±3 k DC cables with a transmission capacity up to MW and availability of DC converter modules of the same size. The two DC cables would be laid simultaneously in one trench. Limitations of this technology are the substantial costs for the power converters, respective space requirements, in particular offshore, and the conversion losses amounting for about 3% per converter, depending on loading. Figure 3: +/-5-k XLPE-DC cable (source: ABB) The number of potential projects under development in the North Sea results in substantial space requirements for cable routes. This is critical as the coastal areas are sensitive, to a large extent declared as natural parks and as such more or less protected. Consequently, planning and licensing of offshore routes is a complex and time consuming process with an uncertain result. For that reason, high capacity offshore collectors have been proposed. These should transmit the capacity of a number of offshore wind farms to shore via one single route. However, the required capacity of such a collective transmission line had to be MA and because of the technology limitations explained above, only DC concepts with their inherent disadvantages may be considered. Otherwise the transmission link will consist of numerous, parallel AC cables with little advantage compared to individual connections [9]. II. SIX PHASE, BIPOLAR AC SYSTEM The concept discussed here uses two conductors for each phase of the supplying AC system [7]. The adjacent conductors may be laid simultaneously e.g. using one vessel with two turntables. In case of limited cross sections these conductors can be integrated in one cable (similar to the Mǿllheroj-HDC-configuration). Each pair of conductors related to one phase are supplied with AC current of opposite polarity, e.g. with a phase angle of 8 (see Figure 4). The resulting six phase system may be considered as a bipolar conventional 3 phase AC system. Figure 4: configuration of the six single core submarine cables (source ABB) The voltages in the individual cables in Figure 4 can be described as: L = L ; L = L ; L3 = L3 With the two AC systems with opposite phase angle: L L3 L L3 a = e = a = a = a = a j L L L L The voltages of opposite polarity may be achieved by using a transformer with a tap at the middle of the secondary windings (Figure 5) or by connecting two transformers with anti-parallel windings (Figure 6). The latter are common practice in combination with power converters. At the adjacent end of the cable identical transformers are to be connected in the same manner. Figure 5: two single core cables supplied by the secondary windings of a transformer with middle tap (only one pair of cables shown) U -U

3 3 Trafo Trafo Yy Yy Yy6 Figure 6: six-phase bipolar cable system, supplied by two conventional AC transformers in Yy and Yy6 configuration y l Figure 7 shows the pair of cables L- and L-, buried close to each other. Obviously, the current I in the conductor of the first cable (L-) running to the load returns via the conductor of the second cable (L-). A single pair of cables according to Figure 7 represents a two phase AC system transmitting instantaneous power pulsating with twice the frequency of the connected power system. The instantaneous power of the resulting bipolar sixphase AC system, however, is constant and just equals the active power. With n representing the nominal voltage of the individual 3 phase system, the resulting transmitted power S according to Figure 6 is: S 3 I. = n III. COLLECTIE OFFSHORE TRANSMISSION CONCEPT The configuration being assessed below consists of the following elements (see Figure 7): - platforms with offshore substations of individual offshore wind farms (); - the collection platform (); - and the offshore transmission line to the connection point with the onshore grid (3). The concept uses the 4 k AC cable design proposed by Nexans with integrated return conductor []. For the assumed transmission capacity of MW a copper cross section of mm would be required. Compensation is applied at both ends of the transmission line only. Alternatively, the two DC concepts (voltage source converter SC using IGBTs and current source converter CSC using thyristor valves) may be considered. The distances y and l in Figure 7 are assumed being 7 km and km, respectively. Küste shoreline Figure 7: collective offshore bipolar AC transmission concept 3 A. Losses The weighted average losses of the alternatives have been assessed and are summarised in Table. The calculation of averages is based on typical conditions for offshore projects in the German economic zone [3], [4]. TABLE : AERAGE LOSSES OF 4 K BIPOLAR AC TRANSMISSION AND DC ALTERNATIES (ALL MW, KM) Loss category Cable: Dielectric oltage related Current related Converter (incl. HAC bipolar, 4 k, 6** mm transformer) Transformer oltage related Current related HDC SC, 3 k, 4*5 mm Average losses [MW] 3.9 *% 4 HDC CSC, 3 k, 4*5 mm 3.9 *.8% Compensation 5. Total The figures in Table indicate that the conversion losses associated with the DC concepts clearly exceed those of the bipolar AC concept. Figure 8 generalizes these findings for varying distances. The figure indicates that the DC technologies are superior to the bipolar AC concept from a loss perspective only for distances above 35 5 km.

4 4 P 5 MW 4 3 HDC light SC HDC classic CSC HAC m. ZK K Mio. 5 5 HDC light SC HAC m. ZK HDC classic CSC km ( km 5) y (l) Figure 8: average losses for offshore transmission technologies as function of distance (dashed line for HAC bipolar: including compensation at platform half way) The net present value of the losses over a years period (assumed interest of 5%, average revenue of 7 /MWh) is estimated to be about M. The values for the AC, HDC SC and HDC CSC alternatives are 34 M and 9 M B. Cost implications For an economic comparison of the options not only the net present value of the losses but also the investment costs need to be considered. By nature, it is difficult to acquire representative data on component prices and implementation costs [6]. Hence, the outcomes show a significant uncertainty and the comparison is indicatively only, though still allowing conclusions. The (SC) DC options may become competitive for offshore distances above km. Still, the number of project being planned so far from shore in Europe is quite limited, at least in the current stage of development. Those remote projects are faced to significant additional risks (accessibility) and technology challenges (e.g. water depth). From the financing perspective also technology risks related to the transmission concept itself have to be taken into account. From that perspective the AC concept clearly offers advantages as it relies exclusively on components and technologies proven in decades of industrial operation. For the DC concepts performance at the required scale and under offshore conditions still has to be demonstrated. The manner, how those additional risks are valued by investors should not be underestimated km (7 9 3 km 5) y (l) Figure 9: comparison of transmission options with respect to costs (investments plus net present value of transmission losses) depending on distance to shore I. SUMMARY AND OUTLOOK The proposed transmission system uses six single core cables connected to two conventional AC systems with opposite polarity. Those six-phase bipolar AC systems offer the opportunity to lay one pair of cables simultaneously in one trench. Because the sum of the currents in one pair of cables is zero, the resulting magnetic field is negligible and, hence, the three pairs of cables are magnetically decoupled. This concept allows usage of maximum voltages and conductor cross sections. This results in substantially enhanced capacities for offshore AC connections. The losses of the proposed configuration will be significantly lower than those associated with conventional AC submarine cables [6] or alternative DC technologies. The extra effort is limited. On the one hand, an additional or modified transformer is required on both ends of the cable. On the other hand, compared to conventional configurations, the amount of compensation capacity required doubles. As with conventional single core cables, three trenches are required, each for a one pair of cables, and the distance between the trenches has to be approximately the water depth. Of course, the construction effort and time are significantly higher than in the case of a single three core AC cables. However, both configurations are hardly comparable because of their dramatically differing capacity. At a distance of several kilometers the armourings of a pair of cables have to be connected electrically, in order to allow the capacitive and dielectric currents compensating each other. In this way, these currents do not run via the lead shield. For the connection of the shields, pre-fabricated joints for corrosions protection can be used.

5 5 The extra effort is justified by the following benefits: The proposed AC concept allows using single core cables, with the associated possibilities to apply highest, technically feasible voltages (> 5k) and conductor cross sections. Respective cable technologies are proven and rely on conventional designs. The option of offshore collectors may become technically and economically feasible and would be based on conventional, commercial and reliable HAC components. The AC option seems to be superior to alternative DC concepts from an efficiency as well as from a cost perspective. Further analysis of costs and cost trends is recommended in order to reduce uncertainties in this respect. Magnetic fields along the cable route are negligible because of the vicinity of the conductors. Compasses and other instrumentation of ships are not affected and impact on marine flora and fauna is extremely unlikely. Of course, there is a variety of open questions. All collective connection options imply co-ordinated planning of projects, synchronisation of timing, possible early investments and, hence, the risk of stranded investments. From that perspective, they will require a supportive regulative and policy framework. Otherwise the barriers for implementation, in particular from the perspective of individual project developers may be prohibitive. On the other hand, collective connections in the GW range require strong nodes where connecting with the onshore networks. This may restrict application in certain areas or, on the other hand, may require strategic planning and development of the onshore transmission network. Planning implications as well as the potential synergies may even have dimension affecting neighbouring control areas and, hence, requires a European scope. The implications of strong connections for design and system operation need to be analysed more in detail, in particular in the light of security of supply standards. As the dynamic characteristics of the technology options clearly differ, also aspects fault response and dynamic stability have to be analysed more in detail. Additionally, the benefits of collective connections may be related to completely different aspects than their costs and drawbacks, e.g. reduced spatial requirements for offshore cable routes versus investment risks and energy market regulation. The question how to balance these different aspects and to derive balanced regulative framework conditions is a challenge for policy makers.. REFERENCES [] G.E. Balog e.a, Energy transmission on long three core/three foil XLPE power cables,jicable 3 [] D. Wensky, Gleichstrom-Netzanbindung großer Offshore-Windparksew (5) H. 9, S [3] AREA, Überlegene elektrische Lösungen für den Windenergiemarkt [4] H. Brakelmann, F. Richert, Bemessung der Energiekabel zur Netzanbindung von Offshore-Windfarmen, ew 4, H.4, S [5] H. Brakelmann, F. Richert, Bemessung der Landkabel für die Netzanbindung von Windfarmenerscheint im Bulletin des SE (5) [6] H. Brakelmann, Drehstrom-Netzanbindung großer Offshore-Windparks Wirkungsgrade und Grenzen, H. WIND-KRAFT Journal () H. 5, S [7] H. Brakelmann, K. Burges, M. Jensen and T. Schütte: Bipolar transmission systems with XLPE HAC submarine cables, 6. Int. Workshop on Large Scale Integration of Wind Power and Transmission Networks for Offshore Windfarms, October 6, Delft, pp [8] B.R. Oswald, ergleichende Studie zu Stromübertragungstechniken im Höchstspannungsnetz, FORWIND-Studie, 5, [9] Econnect Study on the Development of the offshore grid for Connection of the Round Two Wind Farms, Department of Industry and Trade, London, January 5 [] Schütte, Th.; Ström, M; Gustavsson, B.: Erzeugung und Übertragung von Windenergie mittels Sonderfrequenz. In: Elektrische Bahnen /, S , mit Berichtigung in Elektrische Bahnen -/, S. 74. [] Brakelmann, H. Steinbrich, K.: Frequenzreduzierte Energieübertragung und -verteilung mit Kabeln, Bull. SE () H., S [] Jensen, M.: Konzepte zum projektübergreifenden Anschluss großer Offshore-Windparks an das Übertragungsnetz, Dena-Kongreß, Berlin, May 5 [3] Offshore Forum Windenergie: orschlag zu einem Offshore-Grid in der Nordsee, March 7 [4] Brakelmann, H.: Bipolare HAC- und HDC-Hochleistungs- Übertragungssysteme mit PE-isolierten See- und Landkabeln, Energiewirtschaftliche Tagesfragen, (to be published, June 6)