电子电路英文文献及翻译A sensorless and simple controller for VSC based HVDC systemsWord文档格式.docx
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Voltagesourceconverterhigh-voltagedirectcurrent(VSC-HVDC),Spacevectormodulation(SVM),Directpowercontrol(DPC),HVDCLight
1INTRODUCTION
Voltagesourceconverterhigh-voltagedirectcurrent(VSC-HVDC),controlledbypulsewidthmodulation(PWM),cansupplypowertobothactiveandpassiveelectricalsystems.TheintroductionofVSCandPWMmakespossiblefastandflexiblecontrolofpowerflowandmoreconvenientoperationofpowersystems.Besides,thisadvancement,comparedwithconventionalHVDC,mitigatesharmonicsinACcurrentandACvoltagegreatlyandimprovespowerfactorsoftheconnectedACsystems(LiGKetal.,2005).VSC-HVDCorHVDCLight,inrecentyears,havesuccessfullybeencommerciallycommissionedinsuchfieldsassupplyingpowertoremoteisolatedloads,empoweringurbancenters,connectingdistributedgenerationsources,linkingtwoasynchronouselectricalpowersystems,improvingpowerquality,andsoon(Asplund,2000;
Lietal.,2003).
TheadvantagesofaVSCbasedHVDCsystemare(Asplund,2000):
(1)onlyasmallfilterisrequiredtofilterhighfrequencysignalcomponents;
(2)thereisnocommutationfailureproblem;
(3)reactivepowercompensationisnotrequired;
(4)thereisnorestrictiononmultiplein-feeds;
etc.
TherearevariouscontrolmethodsforVSCbasedHVDCsystems.Zhangetal.(2002)usedtheinversesteadystatemodelcontrollertotracetheoperatingpointandadoptedtwodecoupledcontrollingloopstoeliminatethesteadystatedeviation.Chenetal.(2004)proposedasteady-statecontrollerdesignschemebasedondq0-axis.Zhangetal.(2002)andChenetal.(2004)assumedthatthetwoterminalsofVSC-HVDChavebeenconnectedtoaninfinitebussystem.ButoneterminalofVSC-HVDCmaybeconnectedtoageneratorand,asinAsplundetal.(1997),anHVDCLightsystemconnectsthegenerator(suchasanoffshorewindfarm)tothegrid.ThesestrategiesfocusoncontroloftheHVDCsystemitselfanddonotconsidertheinteractionbetweenACandDCsystems.Huetal.(2004)presentedanoptimalcoordinatedcontrolstrategybetweenthegeneratorexcitationandVSC-HVDC,whereasthederivationofcontrollawiscomplicated.Huetal.(2005)appliedageneticalgorithm(GA)tooptimizeparametersofthecontrollerafterdeterminingthem.OoiandWang(1991)andZhangandXu(2001)usedaphaseandamplitudecontrol(PAC)techniqueforVSCbasedHVDCapplications.LiGIetal.(2005)proposedanonlinearcontrolforanHVDCLightsystem.Thesemethodshaveusedvoltageandcurrentsensors.
Adirectpowercontrol(DPC)strategybasedonvirtualflux,calledVF-DPC,providessinusoidallinecurrent,lowerharmonicdistortion,asimpleandnoise-robustpowerestimationalgorithmandgooddynamicresponse(Rahmatietal.,2006).However,theVF-DPCschemehasthefollowingwell-knowndisadvantages(Malinowskietal.,2001;
2004):
(1)variableswitchingfrequency(difficultiesofLCinputfilterdesign),
(2)highsamplingfrequencyneededfordigitalimplementationofhysteresiscomparators,(3)necessityforafastmicroprocessorandA/Dconverters.
Therefore,thereisnotendencytoimplementVF-DPCinindustry.Alltheabovedrawbackscanbeeliminatedwhen,insteadoftheswitchingtable,spacevectormodulation(SVM)isapplied.
DPCisamethodbasedoninstantaneousdirectactiveandreactivepowercontrol(Malinowskietal.,2004).InDPCtherearenointernalcurrentcontrolloopsandnoPWMmodulatorblock.Moreover,theturn-onandturn-offcommandsofthestaticswitchesoftheconvertersaregeneratedbySVM.Useofspacevectormodulationcauseslowercurrentharmonics,relativelyhighregulationandstabilityofoutputvoltageandobtainsahighermodulationfactorrelativetosinusoidalmodulation(Malinowskietal.,2004).Also,itcaneasilybeimplementedinaDSPbasedsystem.
Doublesynchronousreferenceframephase-lockedloop(DSRF-PLL)basedonVFcausesthiscontrolsystemtoberesistanttothemajorityoflinevoltagedisturbances.Thisassuresproperoperationofthesystemforabnormalandfailuregridconditions.
InthispaperanewcontrolstrategyisproposedforVSC-HVDC.Inthisstrategy,thereactivepowerandoutputDCvoltageintherectifierstationandthereactiveandactivepowersintheinverterstationarecontrolled,separately.Also,theDPCrectifierequations(Malinowskietal.,2004)havebeendevelopedfortheinverter.Formoreaccuracyinhighpower,thesecondorderparameterisincludedintherectifierandtheinverterequations.Activeandreactivepowerfeedforwarddecouplingareusedforacceleratingthesystemresponse.Finally,DPCisappliedtotherectifierandinverterstationsofVSC-HVDC.
TheoperationofthiscontrolstrategyisverifiedinaSIMULINK/MATLABsimulationenvironmentforsteadystate,activeandreactivepowervariations,single-line-to-groundfaultsandunbalancedsourcesattherectifierandtheinverterstations.Also,thiscontrolstrategyisappliedtoa5kV·
Aprototypesystemwhichisverificationthatthiscontrolstrategyhasafastresponseandstrongstability.
2CONTROLofVSCBASEDHVDCSYSTEM
2.1VSCbasedHVDCsystem
VSC-HVDCinvolvestwovoltagesourceconverterswiththesameconfiguration,linkingwithadctransmissionlineorcable(Fig.1).Therearefourcontrolvariablesrepresentedby
and
forthissystem.Inthispaper,arectifierstationischosentocontrolDC-busoutputvoltageofrectifier(
).Also,reactivepower(
)andinverterstationaresettocontrolactivepower(
)andalsoreactivepower(
).Rcistheequivalentresistanceofthetransmissioncableandcanbepracticallyneglected.Thuswemaywrite
.
Fig.1AphysicalmodelforaVSCbasedHVDCsystem
2.2Virtual-fluxestimatorforrectifierandinverter
Fromtheeconomicalpointofview,andforsimplicity,morereliabilityandseparationofpowerstageandcontrol,AClinevoltagesensorsarereplacedbyafluxestimator(Malinowskietal.,2004).
ThebasicmodelofaVSCstationisshowninFig.2.IfDa,Db,andDcarethedutycyclesofSa,Sb,andScsignals,respectively,UdcistheconverterDCvoltage,anduLαanduLβarelinevoltageinα-βcoordinates,thentherelatedfluxofACvoltage,
canbewrittenas(Malinowskietal.,2004)
(1)
Also,theconvertervoltageequationsinα-βcoordinatesare:
(2)
(3)
Fig.2Basicmodelofavoltagesourceconverter
2.3Directpowercontrol
Activeandreactivepowerintherectifierandtheinverterstationsareestimatedusingthelinecurrentvectors
andestimatedvirtualflux
inα-βcoordinates(Malinowskietal.,2004):
(4)
(5)
2.4Rectifiercontroldesign
ThefullcontrolalgorithmoftheproposedcontrolsystemispresentedinFig.3.TheDPC-SVMusesclosed-looppowercontrol.Intherectifierstation,referencereactivepower(qrefr)issettozeroforunity-power-factoroperation.Inanidealcase,theactivepowerintherectifierstationandtheactivepowerintheinverterstationareequal,andnostorageelementsareneeded.Nevertheless,inrealsystemsdifferencesbetweentheseactivepowersareinevitable,andthesedifferencesareabsorbedbytheDClinkcapacitorandarereflectedinfluctuationsoftheDClinkvoltage.Thus,thereferenceactivepower(prefr)atthesideoftherectifieristhesumoftheouterproportional-integral(PI)dcvoltagecontrollerandestimatedactivepowerintheinverterstation(pi).
Fig.3ControlschemeforaVSCbasedHVDCsystemwiththerectifierandinverterstations
Accordingtothecurrentdirection,thelinevoltageuLrcanbeexpressedasthesumoftheinductorvoltageuIr,theresistorvoltageuRrandtherectifiervoltageuSr(Rahmatietal.,2006):
(6)
ByconsideringEq.
(1),theestimatedvirtualfluxesare:
(7)
(8)
2.5Invertercontroldesign
Intheinverterstation,referencereactivepower(qrefi)andreferenceactivepower(prefi)aresettonetworkdemand.Accordingtothecurrentdirection,theinvertervoltageuSicanbeexpressedasthesumoftheinductorvoltageuIi,theresistorvoltageuRiandthelinevoltageuLiatthesideoftheinverter.Theestimatedvirtualfluxesare(Rahmatietal.,2006):
(9)
(10)
3CONCLUSION
ThispaperproposesanewmethodforcontrollingaVSCbasedHVDCsystemwhichhasbeenconnectedbetweentwodistributionsystemswithdifferentfrequencies.Thismethodiseffectiveindampingsystemoscillationsquickly,andenhancespowerqualitywhenpowerflowisreversed.VFandDSRF-PLLcausethiscontrolsystemtoberesistanttothemajorityoflinevoltagedisturbances