THE NORTH SEA SUPER GRID A TECHNICAL PERSPECTIVE T.K. Vrana, R.E. Torres-Olguin, B. Liu, T.M. Haileselassie Norwegian University of Science and Technology NTNU, Trondheim, Norway,
[email protected] Keywords: North Sea Super Grid, Multi Terminal Grid, High Voltage Direct Current, Voltage Source Converter, Line Commutated Converter.
Abstract The North Sea Super Grid (NSSG) will include several independently planned projects, comprising a variety of different AC and DC technologies. The offshore clusters will most likely be AC based. Long distance transmission will be HVDC and nowadays Voltage Source Converter (VSC) technology is applied for offshore projects. However in the future, Line Commutated Converter (LCC) technology could become an interesting option. Hybrid HVDC systems that combine VSC and LCC technology, Multiterminal HVDC systems (MTDC) and parallel HVDC links will gain importance for the development of the NSSG. In this paper an overview over possible technologies is given and it is discussed how these can be utilised to realise the North Sea Super Grid.
The liberalisation of electricity markets and the increasing share of wind power in Europe demand stronger interconnections between the power systems of the UK, continental Europe and Scandinavia. Transnational power exchange capacity contributes to balance regional wind fluctuations, because the cross correlation of wind speeds all over Europe is much lower than within a region or a country. Furthermore, the tremendous value of the storage capacity and flexibility of Norwegian hydro power can only be utilized, if the interconnections between Norway and the rest of Europe are sufficient. The technology to realise the NSSG is not fully developed yet. Pure HVAC transmission is not possible, due to the long distances involved. HVDC has not been proven in multiterminal arrangements, and for MTDC a full grid might be a too advanced project to gather practical experience. If AC is used within offshore clusters and DC for long distance transmission, the benefits of both technologies can be combined. A possible NSSG topology is shown in Figure 1.
1 Introduction The current focus on offshore wind power, electrification of oil and gas rigs, and the pan-European electricity market implies that an electricity grid in the North Sea will be essential in the future [21]. Since this grid cannot be planned and built in one step, a modular approach is suitable, just like for onshore grids. To justify the tremendous investments, reliability, redundancy, robustness and flexibility will be of major importance. The construction of the NSSG may already have begun with the first wind park far from shore (Bard offshore 1). This grid will be built by several countries, involving many different TSOs and companies, combining projects with different goals and time horizons. Considering the number and scale of the planned offshore wind parks, integration via the NSSG has many advantages. An existing grid will also improve the possibility of adding smaller wind parks later, which would not have been feasible, if they needed a long separate cable connection to shore. If rigs can connect to an existing electricity grid in the North Sea, harvesting fossil fuels from remote and small reservoirs will be more efficient, attractive and economical. These platforms could also be supplied by nearby wind parks, avoiding long transmission distances and carbon dioxide emissions.
Figure 1: Possible NSSG topology
2 HVDC Converter Technology The two major converter types for HVDC systems are the VSC using IGBTs and the LCC using thyristors. An overview over the most important characteristics is given in Table 1. Converter VSC LCC
Cost Losses Power High High Low Low Low High
Flexibility Yes No
Table 1: Comparison of converter technologies
2.1 Voltage Source Converter
3 HVDC Links
VSC HVDC is the latest development in the field of HVDC technology and has gained more and more attention. It normally uses IGBT semiconductor elements with a switching frequency of approximately 1-2 kHz. The voltage polarity on the DC side is fixed, which makes it possible to connect several converters to the same DC bus. VSC stations are able to form their own AC voltage waveform and act as a true voltage source. This makes it possible to connect to weak grids or even passive networks. Therefore, without any auxiliary equipment, the VSC HVDC can be connected to electrical islands such as offshore wind farms or oil & gas installations. Furthermore, when the offshore AC network suffers a blackout, the connected VSC HVDC link is able to support grid restoration. A VSC has the capability to control the magnitude and direction of transmitted active power, and also to independently produce or consume reactive power. Power flow direction can easily be reversed by reversing the DC current. The basic two-level converter topology has been widely used in HVDC transmission systems. Meanwhile, numerous references like [15] mentioned multi-level converters, with lower switching losses and increased power quality. Those multi-level converters are based on either diode-clamped or floating-capacitor topologies. However, there are still some challenges for high power applications. As an example, the uneven loss distribution among the switching components causes difficulties for thermal system design. An actively clamped converter topology was proposed in [3], in order to avoid the mentioned uneven loss distribution. Furthermore, the voltage across the DC link capacitors needs to be well balanced to avoid voltage shifting at the neutral point. The features mentioned above and the compact design (it is about 60% of the size of a LCC station) make VSC technology ideal for offshore stations [2].
Considering point to point HVDC links and two possible converter technologies results into three options for HVDC links: VSC, LCC and hybrid HVDC systems. An overview over the most important characteristics is given in Table 2.
2.2 Line Commutated Converter LCC HVDC is a mature and established technology. The typical design includes capacitor banks for reactive power support, harmonic filters, transformers and the twelve-pulse thyristor bridges [23]. Typically, the reactive power consumption of a classic HVDC converter station is 50% of the active power transfer [13]. The applied thyristors have lower conduction losses than active devices like IGBTs and the switching frequency is as low as line frequency, leading also to low switching losses. LCC HVDC is preferred for bulk power transmission, in excess of 1000 MW, due to its lower losses, lower investment costs and its proven reliability and service life [2]. LCC HVDC technology is best suited for transmissions between relatively strong AC networks [13,23], where a STATCOM is not needed to provide voltage support to avoid commutation failures. If a STATCOM is applied, it could also replace some of the capacitors and filters, making the system more costly and complex but also smaller and therefore better for offshore stations.
Link VSC LCC Hybrid
Bidirectionality Yes Depends No
Losses High Low Medium
Power Low High Medium
Table 2: Comparison of HVDC links 3.1 VSC HVDC links In 1997, the first VSC HVDC system was installed by ABB for Hällsjön project in Sweden [25]. Since then, more and more VSC HVDC systems have been installed worldwide, for example the BorWin connection (2009) which is using 2 x 120km submarine cable to deliver up to 400MW offshore wind power from Bard Offshore 1 wind farm to Germany [25]. VSC HVDC can reach a maximal power rating of approximate 1200MW [25]. VSC converters operate with a constant DC voltage, which does not need to be reversed for a change of power flow direction. Therefore it is possible to use XLPE cables, which are advantageous but vulnerable towards voltage polarity changes. Due to these characteristics, VSC-HVDCs can offer a competitive mean to integrate offshore wind energy and oil rigs. 3.2 LCC HVDC links LCC based HVDC links can operate at high voltages and have therefore low transmission losses. In addition to the earlier mentioned low losses of the converter, the total efficiency of a LCC based system is superior. The power ratings achieved with LCC technology are larger than with VSC technology. A 1400MW HVDC cable connection is in operation in Japan [18], while a 3000MW connection between Java and Sumatra, which partly will be a HVDC cable, is under construction. LCC HVDC is less vulnerable towards DC faults (because of the absence of anti-parallel diodes, which operate as rectifier feeding power directly into the fault). The DC inductor also limits the fault current. DC faults can be cleared by operating both converters as inverter [24]. 3.3 Hybrid HVDC links Hybrid HVDC is composed by a VSC on one side and a LCC on the other side connected to the same DC link, as it was proposed in [24]. The LCC controls the DC voltage while the VSC controls the AC current and AC voltage (or directly the active and reactive power). Hybrid HVDC has some advantages for the connection of offshore power systems [24]. The compact VSC terminal fits on an offshore platform and can be connected to a weak
offshore AC system. The large LCC terminal is placed onshore and has lower cost and losses. In spite of the above mentioned advantages, the Hybrid HVDC cannot reverse the power flow easily because LCC requires changing the polarity of the DC voltage while VSC requires changing the direction of DC current. Power flow reversal in a hybrid system, requires to discharge the DC link and to reconnect with changed polarity at one terminal. If the LCC terminal is inverted, voltage polarity remains constant and XLPE cables could be used also for hybrid HVDC systems. For connection of wind farms, where power flow usually goes from wind farm to shore, this problem should not be of major importance. To avoid the complicated power inversion procedures, the hybrid link could be planned for unidirectional power flow. The development of this technology is still at an early stage and it has so far not been tested [6,11,20,24].
4 Arrangements of multiple HVDC links The NSSG will include many HVDC cables to integrate all offshore power systems and to cope with the large amounts of planned offshore generation. Multiterminal systems and multiple converters at the same bus will gain importance. 4.1 Multiterminal HVDC MTDC consists of several HVDC converters which are connected by a DC network with meshed, radial or ring structure [14]. The increasing number of electrical systems in the North Sea (wind farms and oil & gas platforms) indicates that multiterminal HVDC might be a potential attractive solution for the grid integration problem in the near future [4]. MTDC consisting of VSC only, LCC only and MTDC consisting of both types of converters (hybrid MTDC) have been suggested and studied in literature [7,9,12,17,19]. VSC based MTDC has been investigated most, due to the earlier mentioned advantages of VSC technology. Hybrid MTDC has gained little attention, but it will become important for the integration of already existing point to point HVDC connections of both types into the MTDC. A strong LCC onshore terminal could be connected to several weaker offshore VSC terminals. The Quebec-New England electric power connection is a three terminal HVDC system consisting of LCCs, while the Shin-Shinano HVDC connection in Japan is a three terminal HVDC consisting of VSCs [17,25]. These operational systems might indicate the possibility of establishing MTDC, but they do not fully reflect the technical capabilities, because converter construction and control technology have improved since then. In MTDC, power can be balanced instantaneously by the use of DC voltage droop control, which can be seen as the ability of MTDC system to compensate power imbalances in the DC grid at the cost of DC bus voltage deviations. This is analogous to the frequency droop control used in AC grids. It can be understood by comparing the characteristic curves of AC generators and HVDC converters as shown in Figure 2.
Figure 2: Characteristics of a grid connected synchronous generator and a MTDC connected HVDC terminal With all the new opportunities it provides, MTDC is still a technology under development and with some serious challenges, especially when it comes to system protection. Another practical challenge that needs attention is the standardisation of equipment ratings (DC voltage levels, maximum DC voltage droop...) and the determination of requirements for HVDC terminals, particularly in relation to fault handling. 4.2 Multiple Converter Control Due to the large amounts of power that need to be transferred from the offshore wind clusters towards shore, parallel HVDC links might be the only realistic solution. Two or more converter stations at the same busbar give possibility for new operational concepts. VSC based terminals could be built with increased power ratings, to be able to supply also reactive power to the nearby LCC terminals. Due to the independence of active and reactive power, a slightly overrated VSC could supply significant amounts of reactive power. It could also be possible to supply a part of the filtering required for the LCCs. In this case, the VSCs would be operated as inverter (or rectifier), filter and STATCOM at the same time. Reducing the requirements for capacitor banks and filters, the LCCs could possibly shrink significantly, making it more attractive for offshore applications. In this constellation even a LCC link could possibly be realised with XLPE cables, since voltage polarity changes are not needed. The transfer of power to the offshore cluster can be done with VSC links, while the LCC links are purely unidirectional to shore. VSCs provide bidirectional and flexible operation with black start capability, while the LCCs provide bulk power transmission at low cost and losses. The main draw backs of both technologies could be compensated. The coordinated operation of a VSC and LCC terminal does not even require that the HVDC links are parallel. Two separate HVDC links that have only one of their terminals at the same busbar could still exchange reactive and distortion power at that site. Also possible hybrid HVDC links could be controlled this way. The concept is applicable to three or more converter terminals as well. Due to the fact, that up to now no offshore facilities exist and will exist in the next few years, that exceed the power capabilities of a single HVDC link, the amount of literature
on parallel HVDC links is limited. Studies like [10] have proposed parallel HVDC links for increased power transfer capability, but the matter was rather treated from an economic point of view, than in technical detail. Future investigations of this subject are needed to avoid suboptimal grid solutions resulting from an uncoordinated approach.
5 Offshore AC Grids In regular onshore AC grids, the generation is mostly based on synchronous generators, and a significant share of the load is induction motors. All of those rotating machines have some inertia and add to the total system inertia, which usually is large resulting in a system time constant of several seconds. This rotating energy storage makes the entire system less vulnerable to power imbalances. The applied frequency control mechanisms can therefore be rather slow [14]. The operation of an offshore AC grid is challenging, since no classical rotating machines might be connected directly to the grid. The generation of the offshore AC grid will be based on wind turbines, where two different concepts, which today are applied to large wind turbines (5MW+), are likely to dominate in the future. Wind turbines with doubly fed induction generator (Repower, Bard) have the grid frequency and rotational speed decoupled. Wind turbines with a permanent magnet synchronous generator and a full back-toback converter (Multibrid, Enercon) have no direct coupling of the rotating machine to the grid at all. The load of an offshore grid is most likely based on inertialess HVDC converters, which transfer the generated power to shore. Neither the generation nor the load contributes in this case to the system inertia. Due to this lack of energy storage, power production, losses and consumption have to be balanced in real time. The only storage capacity is supplied by the capacitance of the cables and the filtering inductors of the converters. The problem caused by the absence of electrical machines can partly be overcome, if power converters are controlled as virtual synchronous machines. This control strategy can help to balance power in AC systems with a large number of power converters [22]. The grid frequency is not linked to the rotational speed of electrical machines. It is only determined by the controllers of the power converters and it could change instantaneously. A frequency determination method and the frequency itself have to be chosen, where several options are possible. 50Hz would of course be an easy solution, since a lot of existing devices could be applied, but other frequencies could also be possible. If the frequency is set by the largest converter, control is easier but vulnerable if that unit has a failure. If frequency is determined by all units (like in a regular AC grid) synchronism problems could arise. The operation of purely power converter based AC grids adds uncertainties to the implementation of the NSSG. The amount of operational experience, knowledge and literature available is very limited. Control schemes for the operation of parallel inverters have been developed [8], but have so far been applied only in small scale. The feasibility for larger systems will hopefully soon be demonstrated at Bard Offshore 1, but it is unlikely that important control details will be made public.
6 Implementation of the NSSG The starting point from where the NSSG could grow to finally connect all countries around the North Sea is the German Bight. There, the first far offshore wind park (Bard Offshore 1) is under construction, which is connected via HVDC. Several other wind farms are planned in that region, which makes interconnections between the wind farms feasible. This first stage of an offshore grid will probably only be used for the interconnection of wind farms. The Norwegian Valhall gas field will be connected via HVDC [25], which also could be seen as a step towards the electrification of the North Sea. This HVDC link is small though (78MW) and therefore its integration into an NSSG would be of minor importance. However, the region is rich of natural recourses, and if fuel prices continue to incline, a larger number of platforms could be constructed in that area, creating the need for stronger interconnections. England has huge long term plans for the Doggerbank. Several GW of wind farms are to be constructed there, but the projects are still in an early stage. The scale of the planned projects and the distance from the Doggerbank to the English coast indicate that it could be an important node in the NSSG. All other projects in the North Sea are rather close to shore and would therefore not benefit as much from a NSSG. The most realistic option might be a modular system that comprises all available technologies and utilises their advantages. Within offshore fields, AC could be used to link wind parks and offshore loads together to a grid. The operational experience with DC grids is still limited, which makes AC a more attractive technology for this task today. The feasibility of DC collection grids has been studied in literature and might gain importance in the future [16]. These offshore AC grids could be connected to each other or to shore via HVDC links. In the near future, those HVDC connections will be realised with VSC technology, since it is available today and offers flexible operation, compact design and black start capability. With increasing size of the offshore clusters, the demand for stronger interconnection will arise, where LCC technology might offer good solutions. It can either be applied as a regular LCC link or as a hybrid link. With an increasing number of separate offshore AC grids, MTDC will gain importance. If stabile and secure operation can be achieved with this technology, more cost efficient solutions can be realized than with conventional point to point connections. MTDC systems could be operated in parallel with regular point to point HVDC links, which gives important redundancy to a more risky project. The maximum power rating of the applied HVDC links is not only limited by the technical possibilities, but also by the primary reserves of the connected systems. While the primary control reserve of the UK national grid is about 1,5GW, HVDC links up to 10GW have been proposed in literature [5]. A failure of such an HVDC link would have unacceptable impact on the entire system and therefore the feasibility of such a link has to be questioned. Parallel HVDC connections could therefore be the only realistic option to connect several GW of offshore wind farms.
7 Conclusion The North Sea Super Grid can probably not be built as a planned and optimised structure. Independently planned projects can be coupled together, leading to a rather grown grid, comprising several DC and AC voltage levels and maybe even different AC frequencies. Due to a lack of operational experience with DC systems, AC will play a major role in the internal wind farm collection grids at medium voltage, as well as for high voltage wind farm cluster grids. Power frequency control might become a challenge for offshore AC grids, due to the absence of directly connected electrical machines, which add to the system inertia. In the future, DC technology might gain importance for this application. Long distance subsea transmission will be HVDC, where several technical options are possible. VSC technology will be the most promising option in the near future, and several recent offshore projects (Valhall, Troll, and Bard Offshore 1) are all utilizing it. However, LCC technology might also be applicable in future projects, because of its lower cost, lower loss, and higher transmission capability. Furthermore, some cutting edge technologies such as the hybrid HVDC, Multiterminal HVDC and parallel HVDC links will be interesting options for the development of the North Sea Super Grid, if certain technical challenges can be solved.
References [1] V.G. Agelidis, G.D. Demetriades, N. Flourentzou. “Recent advances in high-voltage direct-current power transmission systems”, Proc. IEEE Int. Conf. Ind. Technol. (ICIT), pp. 206–213, (2006) [2] C.D. Barker, N.M. Kirby, N.M. Macleod, R.S. Whitehouse. “Renewable Generation: Connecting the generation to a HVDC Transmission Scheme”, CIGRE Canada Conference on Power Systems, Toronto October 4-6, (2009) [3] B. Bijlenga. “HVDC device for converting between alternating voltages and direct current voltages”, U.S. Patent 6 480 403, (2002) [4] N. Fichaux, J. Wilkes. “Oceans of Opportunity”, European Wind Energy Association, page 34, September (2009) [5] F. Groeman, N. Moldovan, P. Vaessen. ”Ocean grids around Europe”, KEMA Briefing Paper, Novmber (2008) [6] L. Guangkai, L. Gengyin, L. Haifeng, and Y. Ming. “Research on hybrid HVDC”, PowerCon, volume 2, (2004) [7] T. Haileselassie, M. Molinas, T. Undeland. “MultiTerminal VSC-HVDC System for Integration of Offshore Wind Farms and Green Electrification of Platforms in the North Sea”, Nordic Workshop on Power and Industrial Electronics, June 9-11, (2008) [8] M. Hauck, H. Späth. “Control of a Three Phase Inverter Feeding an Unbalanced Load and Operating in Parallel with Other Power Sources“, EPE-PEMC Dubrovnik & Cavtat, (2002)
[9] R.L. Hendriks, G.C. Paap, W.L. Kling. ”Control of a multi-terminal VSC transmission scheme for connecting offshore wind farms”, European Wind Energy Conference (2007) [10] F. VanHulle. “Integrating Wind”, TradeWind project report, (2009) [11] Y. Iwata, S. Tanaka, K. Sakamoto, H. Konishi, and H. Kawazoe. ”Simulation study of a hybrid HVDC system composed of a selfcommutated converter and a linecommutated converter”, Sixth International Conference on AC and DC Power Transmission, pages 381–386, (1996) [12] B.K. Johnson, R.H. Lasseter, F.L. Alvarado, R. Adapa. “Expandable Multiterminal DC Systems Based on Voltage Droop”, IEEE Transactions on Power Delivery, Vol. 8, No. 4, pp. 1926-1932, October (1983) [13] T. Jonsson, P. Holmberg, T. Tulkiewicz. “Evaluation of Classical, CCC and TCSC Converter Schemes for Long Cable Projects”, EPE 09, Lausanne, Switzerland, (1999) [14] P. Kundur. “Power System Stability and Control”, McGraw-Hill, (1994) [15] A. Lindberg, T. Larsson. “PWM and control of three level voltage source converters in an HVDC back-toback station”, AC and DC transmission, London, UK, (1996) [16] L. Max. “Design and Control of a DC Collection Grid for a Wind Farm”, Doctoral thesis, Chalmers University, (2009) [17] T. Nakajima, S. Irokawa. “A control System for HVDC Transmission by Voltage Source Converters”, IEEE Power Engineering Society Summer Meeting, pp. 11131119, (1999) [18] H. Nakao, others. “The 1,400-MW Kii-Channel HVDC System”, Hitachi Review Vol. 50, No. 3, (2001) [19] W. Pan, Y. Chang, H. Chen. “Hybrid Multi-terminal HVDC System for Large Scale Wind Power”, PSCE Proceedings pp. 755-759, (2006) [20] R.E. Torres-Olguin, M. Molinas, T.M. Undeland. ”A model-based controller in rotating reference frame for Hybrid HVDC”, ECCE, (2010) [21] T. Trötscher, M. Korpås, J.O. Tande. “Optimal design of subsea grid for offshore wind farms and transnational power exchange”, Sintef Energy Research, (2009) [22] T.K. Vrana. “Analysis and Definition of Technical Specifications of Dispersed Inverter-Based Energy Conversion Units in Distribution Grids“, RWTHAachen University, Diploma thesis, (2008). [23] L. Xu, B.R. Andersen. “Grid Connection of Large Offshore Wind Farms Using HVDC”, Wind Energy. 9:371–382, (2006) [24] Z. Zhao, M.R. Iravani. “Application of GTO voltage source inverter in a hybrid HVDC link”, IEEE Transactions on Power Delivery, 9(1):369–377, 1994. [25] www.abb.com/hvdc accessed on July 5, (2010)
