HVDC Technology For Large Scale Offshore Wind Connections

Paper presented at Smartelec 2013, April 16, Vadodara, IndiaHVDC Technology for Large Scale Offshore Wind ConnectionsNandan Mahimkar, Gunnar Persson,Claes Westerlind,ABB AB, SE-771 80, Ludvika, Sweden,nandan.mahimkar@in.abb.com, com,tel: 46 240 782000, fax: 46 240 611159SummaryThe DolWin1 and DolWin2 projects, the first transmission links from the DolWin wind farm cluster inthe North Sea, Germany, combines the application of well proven transmission technology and vastproject experience with an unmatched realization of offshore wind integration further inland thanever before. Once completed, the 800 MW DolWin1 and the 900 MW Dolwin2 projects will integratea total amount of 1700 MW of wind power into the German grid using the latest Voltage SourceConverter (VSC) technology available, together with state of the art 320 kV polymeric cables,thereby reducing the overall losses while maintaining high availability and reliability.This article elaborates on the DolWin2 project and the latest developments in VSC technologythereby presenting a scalable solution to rightfully harness renewable energy generated by offshorewind farms.1. IntroductionThe DolWin2 project will be one of the largestoffshore HVDC grid connections for offshorewind farms worldwide. It uses modern HVDCtechnology with low losses to integrate theGode Wind II offshore wind farm and twofuture wind farms into the German HVACtransmission grid. This 135 km longtransmission link is based on voltage sourcedconverters technology, (VSC), by ABB calledHVDC Light , which use high powertransistors for the conversion between ACand DC. The DolWin2 transmission link isplanned to be commissioned in 2015.ThepredecessorstotheDolWin1 & 2 projects are the BorWin1, theTroll A Pre-compression and the ValhallDolWin BetaFigure 1: Principle Single Line Diagram of DolWin2Page 1 of 5

HVDC technology for large scale offshore wind connectionstransmission projects, which also employVSC technology. Throughout these projects,ABB has gained invaluable experience in thefield of offshore HVDC installations;something that will become vital in theexecution of the DolWin1 and DolWin2projects.2. System description and VSCtechnologyThe DolWin2 900 MW transmission link willconnect offshore wind farms located in thecluster DolWin (Gode Wind II wind farm,400 MW, and other wind farms) in the NorthSea to the German grid, located 45 km fromthe German North Sea coast, to the receivingstation at Dörpen West, 90 km inland fromthe sea shore.Gode Wind will have a total productioncapacity of 400 MW, and will eventuallyconsist of 80 wind generator units, each witha capacity of 5 MW. These units feed theirpower into a 33 kV AC cable system, fromwhich it will be transformed to 155 kV forfeeding the HVDC VSC offshore converterstation. At the receiving station at DörpenWest, the DC power is converted into ACpower at 380 kV.The HVDC VSC transmission link will operateat a DC voltage of 320 kV, the highestvoltage ever used for extruded cables, via apair of 45 km sea cables followed by a pair of90 km land cables. One reason for choosinga HVDC VSC link was the possibility toconnect the entire transmission system farinland, closer to the German backbone gridon high power cables, without addingoverhead lines. In this project, extrudedcables are used, simplifying installation onland and on sea, and allowing very short timefor cable jointing. The oil-free HVDC VSCcables minimize environmental impact on seaand on land.Furthermore, the DolWin2 project has aneven higher equipment redundancy level thannormal HVDC monopole systems: It hasparallel transformers, each capable ofhandling900 MW.Further,multipleredundant sea water pumps are provided onthe offshore platform.The high-voltage converter equipment weighsaround 2000 metric tons, and for the sendingstation is located on an offshore platform(refer to section 6). All equipment for theoffshore station is installed indoors while forthe onshore station all DC equipment isinstalled indoors. These choices ensure safeoperation and minimum environmentalimpact.3. Important benefits of using HVDC VSCtechnology for offshore transmissionThe HVDC technology makes the creationand connection of remote wind farmsfeasible, as there are no technical limitationsto the length of the cables that can be usedfor transporting the energy. In ACtransmission, the charging current of thecables takes a significant portion of thecurrent capability, which worsens withincreasing cable length. In HVDC there is nocharging current in the cables, since thevoltage polarity is not changing continuously.As the cables for HVDC are lighter than othercables, their installation onshore is moreeffective. Their lower weight per unit lengthallows the transport of longer sections inlorries, which translates into fewer cablejoints. In addition, as the cables are lesscomplex, the joints are also less complex andcan be executed faster, thereby reducing theinstallation time and hence the visual impacton the surroundings and eventual safetyissues.An indirect but decisive advantage of HVDCcables being less complex is that their costper MW and km will be lower than for anequivalent AC solution; partly because of thecables themselves, and partly on account ofof the easier laying and jointing.HVDC VSC converters have the inherentcapability of independent control of active andreactive power flow, with total control ofPage 2 of 5

HVDC technology for large scale offshore wind connectionspower from zero to full power without filterswitching. This enables and supports thesmooth and reliable operation of the offshorewind farm.The HVDC links for wind farm applicationshave an additional feature. As is known,during AC system faults onshore, theelectrical power goes to zero or close to,because the voltage collapses. To decouplesuch incidents from the wind farms,independently of the operation mode, fastenergy absorption capability is added into theHVDC VSC system, by a device named DCchopper. The purpose of the chopper is totemporarily absorb the excess energy fromthe wind park which cannot be transmitted totheACsystemduringtemporarydisturbances.The DC link decouples the wind park fromfaults, electrical transients and oscillationsthat may occur in the main grid, therebyreducing the mechanical stresses on theequipment in the wind turbines.The DolWin2 project features a refined typeof VSC technology based on many years ofexperience where two level converters arecascaded (see figure 2) with the aim ofmaintaining robustness while decreasinglosses (see figure 3) and stresses onequipment.Figure 2: Cascaded two-level converterThe original two-level converter valve,together with series-connected press-packinsulated-gate bipolar transistors (IGBTs),have been the base for the development ofABB’s cascaded two-level (CTL) converters.This solution consists of several two-levelbuilding blocks, as described above, alsocalled cells (see figure 2). The creation of anearly sinusoidal output voltage from theconverter is thus enabled.Figure 3: Losses in VSC systems [1]While high availability and reliability are keyrequirements on all power transmissionsystems this becomes even more evident inoffshore systems given the high costs ofmaintenanceandfulltransmissionredundancy. To achieve the best possiblereliability, it is therefore vital that the keycomponents, such as the valves, are of arobust design, with built-in redundancy, andwith a minimal number of mechanically activecomponents. Failure of one or even more ofthe IGBTs should not impose any limitation inthe operation of the system. In the solutionabove, this is achieved by basic redundancyin the number of components in each celltogether with short-circuit failure safe mode ofthe IGBT which, upon failure, safely goes intoa permanent conducting mode. This results inthe remaining units in the cell needing toaccept a slightly higher voltage, for whichthey are designed, with undisturbed operationas result.4. Expansion possibilities and VSCmodularityThe plan is to gradually expand thetransmission with additional HVDC links asthe DolWin cluster is populated with futurewind farms.Page 3 of 5

HVDC technology for large scale offshore wind connectionsEach wind farm has its own AC transformerplatform, on which the 33 kV cables arrivingfrom the wind turbines are connectedtogether. Using short high-voltage threephase cables, this platform is in turnconnected to the converter platform, whereAC power is rectified to DC. At the sametime, the converter station is able to cover thereactive power requirements of the offshorenetwork, and to regulate its frequency.Several of the converter platforms and henceHVDC VSC systems can be connected at seausing three-phase bus bars and cables,which enable additional transmission systemsto be successively integrated in parallel to thepresent one as the grid connection is in needof expansion in the future. Of course,integrating several platforms and HVDC VSCsystems in a new AC system offshorerequires advanced controls, but the presenttechnology can handle this challenge.Figure 4: Rating modularityIn terms of modularity the VSC solutionpresented in this article is fully scalable inorder to align with the specific needs of theproject. With cascaded two-level converters,the DC voltage will be dependent on thenumber of cells used. Currently; 80, 150and 320 kV are foreseen using extrudedcables making the solution reach from100 MW to above 1000 MW in powercapability (see figure 4). With massimpregnated cables, even higher ratings arepossible, since their present voltagecapabilities are higher than those of extrudedcables.5. Offshore transmission systemsThe BorWin1 project conclusively proved thatHVDC VSC is very suitable to meet offshoreoperating demands. The platform effectivelyprotects the high-voltage equipment from saltand humid air and due to the modular stationdesign is very flexible and adaptable todifferent platform designs as well as to futureexpansion.The high-voltage equipment for offshore isstandard equipment, which means that it isdesigned for outdoor placement. However, onthe DolWin2 platform all main circuitequipment including the transformers islocated indoors.The control system is advanced in its nature,but is standard with no need for a specialversion for the subjected application;applications such as a special fire protectionand detection system, due to strict offshoreregulations, are easily integrated into thestandard control system.The HVDC VSC converter stations have"black-start" capability – they can generate avoltage with amplitude and phase controlledas desired. This feature is especially usefulwhen it comes to starting up an offshorenetwork. The offshore station can initially actas a generator operating in voltage andfrequency control mode. It creates an ACvoltage with the desired amplitude andfrequency, which is ramped up smoothly toprevent transient over-voltages and inrushcurrents. As wind farms come on line, theyare synchronized into this voltage source.Additionally to the benefits for the offshorenetwork described above, a VSC system alsoprovides excellent voltage support and henceincreased stability for the onshore network.6. Platform design and installationThe high-voltage equipment, support systemsand safety systems are designed for longPage 4 of 5