Control strategy without phase-locked loop based on coordinate transformation fo

IETPowerElectronicsResearch

Article

Controlstrategywithoutphase-lockedloopbasedoncoordinatetransformationforthree-phaseAC/DCconverter

XinxinZheng1,2?,LanXiao1,ZilongWang1,YunLei1,ChuyangWang1

1

ISSN1755-4535

Receivedon5thAugust2014Revisedon2ndFebruary2015Acceptedon25thMarch2015doi:10.1049/iet-pel.2014.0583m.kkreddy.com

Abstract:Anovelcontrolstrategywithoutphase-lockedloop(PLL)forthethree-phaseAC/DCconverterisproposedinthisstudy.Thereisnoneedtodetectthephaseangleofthegridandthedefectofinaccuratephase-lockingcanberemedied.Thefrequencyofthegridcanbedetectedbyafrequencytrackingmethod.Alinearchangedanglefrom?πtoπwiththedetectedgridfrequencyisgeneratedbytheDSPcontrollertotakepartinthecoordinatetransformation.Thegridfrequencytrackingmethodisrealisedbynegativefeedbackofreactivepower.Itcanensurethatthefrequencyofthelinearchangedangleisthesameasthegridfrequency.Theprincipleoftheproposedcontrolstrategyisanalysedindetails.ComparedwiththecontrolstrategywithPLL,theproposedcontrolstrategydoesnotneedcomplexPLLanditcanrealisehighpowerfactorandlowtotalharmonicdistortioninthenon-idealgrid.Thestructureoftheproposedgridfrequencytrackingmethodisestablishedandtheparametersofitaredesigned.An18kWDSP-basedbidirectionalthree-phaseAC/DCconverterisestablished.Simulationandexperimentalresultsverifytheoreticalanalysis.

1Introduction

Withthedevelopmentofthenewenergytechnologies,thedemandforthepowerconvertersisbecomingmoreurgent.Three-phasepulsewidthmodulation(PWM)controlledAC/DCconvertercanrealisebidirectionaloperationandhighpowerfactor(PF).Itiswidelyappliedinthe?eldsofwindpowerandelectricvehicles[1–5].Tosimplifythecontrolsystemandobtainlowertotalharmonicdistortion(THD),theimprovementofthecontrolstrategyhasgreatsigni?cance[6–11].

Thethree-phaseACsignalscanbechangedintotwo-phaseDCsignalsthroughcoordinatetransformation.Itprovidesatime-invariantmodeloftheAC/DCconverterwhichmakesthecontroldesignsimilarwiththatoftheDC/DCconverters[12–14].Thegridphaseangletakespartinthecoordinatetransformation.Itshouldbedetectedbyphase-lockedloop(PLL).TheconventionalPLListhesynchronousreferenceframe-PLL(SRF-PLL).Itcontainsonlyonetrackingloopandthecontrolstructureissimple.However,thegridisnotideal,thedistortionandimbalanceofthegridvoltagemayexist,whichwouldaffecttheaccuracyofPLL[15].Tosolvetheproblem,manyadvancedSRF-PLLmethodshavebeenproposed.Aconventionalthree-phasePLLfollowedbyaPI-controlledmovingaverage?ltertogetherwithaphase-lockingalgorithmisproposedtocapturethefundamentalphaseangleandangularfrequencyofthegrid[16].ThePI-controlledaveragingblockandthephase-lockerblockareadded,whichwouldcomplicatethecontrolsystem.Asystematicandef?cientapproachtodesignthecontrolparametersoftheSRF-PLLwithpre-?lteringstageisproposedin[17].Themultiplecomplexcoef?cient?lters(MCCF)isthepre-?lteringstage.Toprovideaccuratedetectionunderdistortedgridconditions,theparametersoftheMCCFarenoteasytodesign.Acomprehensivedesign-orientedstudyoftheadvancedPLLsispresentedin[18,19].SeveraladvancedPLLshavebeenanalysed,suchasthemultiplereferencesframe-basedPLLandthedualsecond-ordergeneralisedintegrator-basedPLL.AllthesePLLsmaycomplicatethecontrolsystem.What’smore,therobustnessofthePLLstothenon-idealgridhasrelationshipwiththeparametersofthePLLs.ThedesignofthePLLsisdif?cult.

BesidestheimprovedPLLbasedontheconventionalSRF-PLLmentionedabove,thefastFouriertransform(FFT)-PLLandthelow-passnotch?lter-PLLbasedonFFT-PLLalsohavebeenproposed[20].Thesemethodsdonotrequirethecoordinatetransformation.However,thedetectionofthegridfrequencyhasnotbeenconsidered.Intherealcondition,thegridfrequencyischangingintheallowedrangeandisnot?xed.Thedistortionandimbalanceofthegridvoltageincreasethedif?cultyofgridfrequencydetection.

ExcepttheconventionalPLL,alltheotherPLLsmentionedabovearenotsimple.What’smore,therobustnessofthePLLstothenon-idealgridcannotbeguaranteed.Inrecentyears,controlstrategywithoutPLLhasbeenproposedandhasattractedplentyofattention.ThecontrolstrategywithoutPLLforgrid-connectedinvertersbasedonpassiveHamiltoniantheoryisproposedin[21].ThePLL-freereferencecurrentgenerationalgorithmbasedonsynchronousrevolutioncoordinateframecanremedythedefectofinaccuratephaselockingbyPLL.Howeverthefrequencysetbythecontrollercannottrackthegridfrequency.In[22,23],theinternalcharacteristicsofthethree-phaseAC/DCconverteraresimilarwiththatoftherotationalsynchronousmachines.Thecontrolleroftheconverterisdesignedaccordingtothemathematicalmodelofsynchronousgeneratorswhichareoperatedinthecompensatormode.Asaresult,PLLisneedless.However,thepremiseofthismethodisthatthephaseangleoftheACsidevoltageofthebridgelegisthesameasthatofthegrid.ThatistosaytheAC-sideoftheconvertershouldbecontrolledasthevoltagesourceandthecirculatingcurrentmayoccur.

Fortheabovereasons,anovelcontrolstrategywithoutPLLforthree-phaseAC/DCconverterisproposedinthispaper.Theproposedcontrolstrategycanreducetheimpactofthenon-idealgrid.Alinearchangedanglegeneratedbythedigitalsignalprocessing(DSP)controllertakespartinthecoordinatetransformation.Asaresult,thephaseangleofthegriddoesnotneedtobedetected.TheproposedcontrolstrategyandtheprincipleofreactivepowernegativefeedbackaredescribedinSection2.TheerrorregulatorsinthecontrolsystemareillustratedindetailinSection3.SimulationresultsarepresentedtoverifythetheoreticalanalysisinSection4.The

IETPowerElectron.,2015,Vol.8,Iss.9,pp.1701–1709&TheInstitutionofEngineeringandTechnology2015

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