7631 FFWD Sum04.qxd - ABB

enewablesPeter Jones, Sector Manager – Renewables ABB PT (UK), outlines the key technical issuesrelating to the AC grid connection of offshore wind farms and explains how reactive powercontrol via SVC (Static VAr Compensation) can enhance system stability and reliability.Building resilient and reliable ACoffshore wind farm grid connectionsWith very large offshorewindfarm arrays about tobecome more commonplace,new challenges are beingplaced on the transmissionsystem operators to maintainsystem stability and limitdynamic voltage variations.SYSTEM CONNECTIONIn the past, wind turbine unitstypically had a small power outputrating when compared to thestrength of the connecting electricalnetwork, so a simple control systemthat disconnected the wind farmswhenever a network disturbanceoccurred was sufficient.With the larger windfarmspresently being planned, this designphilosophy becomes questionable.Windfarm connections must bedesigned so that the wind turbinesare capable of continuous,uninterrupted operation during theprotection clearance times for thefaulted, adjacent, networkcomponents (‘ride-throughcapability’).STABILITY AND RELIABILITYFor large wind farms there area number of stability and reliabilityissues that need to be addressedduring the design stages.Conventional inductiongenerator units and doubly-fedinduction generator (DFIG) windturbines may disconnect from thetransmission system for low voltageconditions caused by system faultsmore quickly than conventionalexisting synchronous generatorpower plants.An induction generator has thepotential to over-speed beyond itspullout torque, at which point themachine races away anddisconnects from the grid. ForDFIG induction generators, thereare issues relating to the controland protection of the voltage onthe converter DC bus that can leadto the tripping of the unit, whicha conventional synchronousgenerator could normally endure.If these issues are not addressedat an early planning stage, theperformance of the windfarm maybe in violation of system security,planning and availability criteria –resulting in a requirement for anincrease in spinning reserve. Inmore serious situations it may leadto the grid system experiencing acascading power outage.VOLTAGE CONTROLReactive power control isnecessary to address these networkstability and reliability issues. Withsynchronous generators, reactivepower control is achieved by meansof the exciter system. However, thisis not possible for basic inductiongenerators. Instead, a Static VArCompensator (SVC) positioned atthe grid connection point can actas a central exciter system but withthe advantage that reactive powercan be controlled even when nopower is generated.The transmission systems towhich offshore windfarms may beconnected are usually designed todistribute power from the main gridto remote customers. These remotesystems are, in many cases, weakand a change in power flowdirection will affect voltage levels.Mechanically switched capacitorbanks (MSC) are often used to dealwith voltage level problems.However, power production, andthus reactive power consumption,in windfarms varies with windspeed. The resulting frequentswitching of MSC deterioratespower quality and decreases thelifetime of the MSC. An SVC, withcontinuously variable susceptance,offers a cost efficient alternative toseveral small MSC units.Several phenomena associatedwith power produced from windintroduce voltage flicker on theconnecting node – generator startand stop, wind speed variations,and tower shadow effects. Thisflicker has a detrimental effectupon other components connectedto the grid causing complaintsfrom power consumers. Byconnecting an SVC at the gridconnection points, this flicker canbe mitigated.SVC IMPLEMENTATIONSVCs are available in twodifferent versions. The first SVCapproach is based on conventionalcapacitor banks together withparallel thyristor-controlledinductive branches, which consumethe excess of reactive powergenerated by the capacitor bank.This type of equipment can bedirectly connected to theintermediate voltage bus, whichinterconnects the wind farms (upto 36kV). When needed, it is alsopossible to connect the SVC to thehigh-voltage network via adedicated transformer.The second alternativeimplementation of the SVC makesuse of a power electronic voltagesource converter (VSC). Theconverter utilises semiconductorshaving turn-off capability. Theconverter can inject or consumereactive power to or from the buswhere it is connected. Thisapplication of VSC technology isusually referred to as STATCOM(Static Compensator). Thisalternative has the benefits of asmaller footprint, as large air-coredinductors are not used. Anotheradvantage stems from the fact thata smaller parallel capacitor bankcan be used, as the converter itselfmay contribute reactive power.By combining the two types ofschemes, a cost-effective dynamiccompensator can be achieved, ratedfor a high dynamic yield during ashort time and a lower yield forsteady-state operation.SUMMARYThe UK has generally benefitedfrom a stable and reliabletransmission grid system based ontraditional sources of generation.SVC technology will have anincreasingly vital role to play inensuring that networks with largeamounts of windfarm connectionsremain resilient and that small scalelocal network faults do not escalateinto more serious widespreadtransmission outages.Summer 2004 >> FFWD 09