The nonlinear internal control of STATCOM theory and application
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ReviewThenonlinearinternalcontrolofSTATCOM:theoryandapplicationFengLiua,*,ShengweiMeia,QiangLua,YixinNib,FelixF.Wub,AkihikoYokoyamacaDepartmentofElectricalEngineering,TsinghuaUniversity,Beijing100084,People’sRepublicofChinabDepartmentofElectricalandElectronicEngineering,TheUniversityofHongKong,PokfulamRoad,HongKong,People’sRepublicofChinacDepartmentofElectricalEngineering,SchoolofEngineering,TheUniversityofTokyo,Tokyo113-8656,JapanAccepted26September2002
AbstractInthispaper,atwo-levelcontrolstructureofStaticSynchronousVarCompensator(STATCOM),whichconsistsofinternalandexternalcontrolsystem,isdescribed.Thecontroltargetsoftwolevelsandrelationsbetweenthemarealsostated.ThesuggestedconfigurationofcontrolsystemscanbenefitthesystemicdesignforSTATCOMcontroller.H1controlapproach,whichconsidersimpreciseparametersandexternaldisturbances,isappliedtodesigntheinternalcontrollerofSTATCOM.Withtheproposedinternalcontrol,themodelofSTATCOMcanbesimplifiedtobeafirst-orderinertiablock,whichcansignificantlysimplifythedesignofsystemcontrol.Afterthat,someimprovementsandoptimizationsbasedonthedynamicalanalysisofSTATCOMhavebeendevelopedtoachievebetterdynamicalperformanceandhighertrackingaccuracy.SimulationsarecarriedouttoverifytheperformanceoftheproposedcontrollercomparingwiththatoftheconventionalPIcontroller.Theresultsshowthattheproposedcontrollerhasfasterdynamicresponse,higheraccuracyoftrackingthereference,andstrongerrobustnesstoparametersvariationandexternaldisturbances.q2003ElsevierScienceLtd.Allrightsreserved.Keywords:StaticSynchronousVarCompensator;DirectFeedbackLinearization;Robustcontrol;H1control
1.IntroductionFlexibleACtransmissionsystems(FACTS)devices,whicharebasedonvoltage-sourceorcurrent-sourceinverters,suchasStaticSynchronousVarCompensator(STATCOM),StaticSynchronousSeriesCompensator(SSSC),andUnifiedPowerFlowController(UPFC),havebeenusedtoflexiblepowerflowcontrol,secureloadinganddampingofpowersystemoscillations[1,2].AsonekindofthetypicalFACTSdevices,STATCOMisplayingmoreandmoreimportantrolesinreactivepowerprovisionandvoltagesupportbecauseofitsattractivesteadystateperformanceandoperatingcharacteristics,whichhasbeenwellstudiedinthepastyears.Moreover,lotsofworkaboutitsdynamiccharacteristicsandapplicationstoimprovethetransientstabilityofpowersystemsalsohavebeendone[3,4].Asweknow,propercontrolstrategiescorrespondingtothecontrolobjectivesarenecessaryinordertoachievefullutilizationofSTATCOM.Sofar,variouscontrolapproacheshavebeenappliedtoSTATCOMdynamicalcontrolandachievemanygoodresults[5–9].Nevertheless,mostoftheseliteratureshavenotclearlydocumentedtheholisticconfigurationofcontrolsystemofSTATCOM,whichisobstructionofthesystemicdesigningSTATCOMcontrollersinindustrialpractice.Usually,whenonedesignsSTATCOMcontrollers,twousualwaysareutilizedasfollows:1.AdoptingadetailmodelofSTATCOMwithoutdis-tinguishingtheinternalandexternalcontrol.Thedesignprocessisobviouslyextremelycomplexanddifficulttohandlesincethecontrollermustconsidertheelectro-mechanicaldynamicsofpowersystemsandinternalelectromagnetictransienceofSTATCOMitself,whichismuchfasterthantheformerone,atthesametime.Furthermore,inthedesignprocess,themodelofSTATCOMinvolvestheelectromagnetictransiencecausedbyinductancesandcapacitors,whilethemodelofpowersystemsdoesnotconsidersuchtransiencebutonlythesinesteadystateofvoltagesandcurrents.Itisdifficulttoexplainwhydifferentwaysareadoptedtohandlethedynamicsfromtheviewpointofphysical0142-0615/03/$-seefrontmatterq2003ElsevierScienceLtd.Allrightsreserved.PII:S0142-0615(02)00129-
1ElectricalPowerandEnergySystems25(2003)421–430www.elsevier.com/locate/ijepes
*Correspondingauthor.Tel.:þ86-10-62785-490;fax:þ86-10-62794-778.E-mailaddress:liuf@ps.tsinghua.edu.cn(F.Liu).realityandmathematicalreasoning.2.Adoptingasimplifiedmodelwithoutdistinguishingtheinternalandexternalcontrol.Suchwaysignificantlysimplifiesthedesignprocessbecausesimplermodelisadopted.Whereas,therearetwopotentialproblems:(a)Whatisthetheoreticalfoundationofthesimplification?(b)HowtodealwiththeinternalelectromagnetictransienceofSTATCOM?Inmostliteratures,thesetwoproblemsarenotdocumented.Basically,thedifficultiscausedbydifferenttimescaleofexternalelectromechanicaldynamicsandinternalelectro-magnetictransience.Forthesakeofovercomingthisdifficult,onenaturalwayistodecouplethecontrolsystemintotwolevels:internalcontrolandexternalcontrol(orsystemcontrol),whichisshowninFig.1.Eachofthemhasitsspecificcontroltargets:1.Theinternalcontroller:operatestheinverterpowerswitchestogenerateafundamentaloutputvoltagewaveformwiththedemandedmagnitudeandphaseangleinsynchronismwiththeACsystem.Besides,theinternalcontrol,thevoltageontheDCcapacitorshouldbeconstantinaPWMbasedSTATCOM.2.Theexternalcontroller:figuresoutreferenceinputsoftheinternalcontrolleraccordingtosystemdynamics,orrather,‘tells’theinternalcontrollerthathowmuchreactivecurrenttheSTATCOMshouldgenerateorabsorbinordertomeettherequirementofsystems.TheinverterofstaticgeneratorcomprisesalargenumberofGTOorIGBT.Thegatingpulsesofthesesemiconductorpowerswitchesaregeneratedbytheinternalinvertercontrolinresponsetothedemandofreferencesignals.Fromtheviewpointofinjectingcurrent,aSTATCOMcanberegardedasacontrollertogenerateorabsorbademandedreactivecurrentfromtheconnectedsystem.ThereferencesignalsofSTATCOM(alsotheinternalcontroller)areprovidedbytheexternalcontroller,whichdeterminetheoperatingmodesofSTATCOMinresponsetothedemandoftheACsystem.Besides,theinternalcontrolshouldalsomeettherequirementsofsecureoperationofSTATCOMitself.SoanothertargetofinternalcontrolistokeepthevoltageontheDCcapacitortobeconstantinaPWMbasedSTATCOM.Additionally,therealcurrentexchangebetweenthesystemandSTATCOMshouldbelimitedtoo.Therearealsomanyothertasksthattheinternalcontrolexecutes,suchastokeepthemaximumvoltageandcurrentwithinasafelimit.However,theyarenotthemostimportantproblemsfromtheviewpointofcontrolsystems.Itismoresuitabletodiscusssuchissuesinthedesignofprotectionsystems.Sowewillnotdiscusstheseproblemsinthesubsequentparts.Inindustrialpractice,theinternalcontrollerisanintegralpartoftheinverter.Then,iftheinternalcontrolstrategyiswelldesigned,thewholeSTATCOMsystemwithitsinternalcontrollercanbesimplifiedtobeafirst-orderinertiablockwhendesigningtheexternalcontroller(systemcontroller).Thenthecontrolvariableoftheexternalcontrollerischangedtobethereactivecurrent,whichisalsothereferencesignalofinternalcontrol.ItisdeterminedbythefunctionaloperationsofSTATCOM,theoperatorinstructionsandsystemvariables.Therefore,thedesignofexternalcontrollercanevadethecomplexityofinternaldynamicsofSTATCOMitself,andthedesignofsystemcontrolcanemphasisonthedynamicsbetweentheACsystemandSTATCOM.Fig.1showstheconfigurationofthetwo-levelcontrolsystemofSTATCOM.BecauseofthesimilarstructuresofFACTSdevicesbasedoncontrollableinverterbridge,thiscontrolsystemconfigurationcanbeextendedtomostFACTSdevicesbasedonvoltagesourceinverter(VSI).Basedonthetwo-levelconfigurationofSTATCOMcontrolsystem,H1controlapproach,consideringimpreciseparametersandexternaldisturbances[10,11]isappliedtodesigntheinternalcontrollerofSTATCOM.Withthiscontroller,STATCOMcanbesimplifiedtobeareactivecurrentsourcewithafirst-orderinertiablockduringthedesignofexternalcontroller.BasedonthedynamicalanalysisofSTATCOM,someimprovementsandoptimiz-ationshavebeendevelopedtoachieveabetterdynamicalperformanceandtrackingaccuracy.Sincetheemphasesofthispaperfocusesontheinternalcontrol,thedesignofexternalcontrollerwillnotbediscussedhere.Simulationsarecarriedouttotesttheperformanceofthiscontroller.Theresultsshowthattheproposedcontrollercantracktheoutputreferencewithfastdynamicresponse.Therobustnesstoparameters’variationanddisturbancesalsohasbeenverified.ThecomparisonofperformancesbetweentheproposedcontrollerandconventionalPIcontrollershowstheadvantageoftheformer