文献翻译Word格式.docx

文献翻译Word格式.docx

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HengchunMao,Member,IEEE,FredC.Y.Lee,Fellow,IEEE,DushanBoroyevich,Member,IEEE,andSilvaHiti,Member,IEEE

1.Introduction

Motivatedbytheforthcomingstringentpower-qualityregulations,power-factorcorrection（PFC）hasbeenanactiveresearchtopicinpowerelectronics.Thesingle-phasePFCisalreadyacommonpractice,andtheindustrialapplicationofthree-phasePFCtechniqueshasalsoemerged.Uptothispoint,theresearchofthree-phaseconvertershasbeenheavilyfocusedoninverterapplications.AlthoughmosttechniquesdevelopedintheinverterareacanbeusedinPFCapplications,aPFCcircuithasitsuniquecharacteristicsand,therefore,deservessomespecialtreatment.TheprimarydifferencesbetweenPFCandinverterapplicationsincludethefollowingaspects.

Specialattentionhastobepaidtothequalityofinputcurrenttoreducethepollutiontotheutility,usuallymeasuredbytheinputcurrenttotalharmonicdistortion（THD）.Althoughtherearenospecificlimitsontheinputcurrentdistortionofgeneralhigh-powerthree-phaseconvertersatpresent,itisacommonpracticetolimittheinputcurrentTHDofthree-phasePFCconvertersatleastbelow10%.Thismakescontroldesignmorecriticalthanininverters.

Theelectromagneticinterference（EMI）emissionsareagreatconcerninPFCapplications.Thehigh-speedswitchingactionofaPFCconvertergeneratesbothdifferentialmodeandcommon-modenoisesattheinputofthePFCconverterathighfrequencies.PassivefilteringiswidelyusedtoreducetheEMIemissionsintotheutility.

Highswitchingfrequenciesaredesirabletoreducethesizeandweightofreactivecomponents（especiallyinductors）andtoimprovecurrent-controlperformance.Whilea20-kHzswitchingfrequencyisdeemedsufficientinmostinverterapplications,aPFCconverterprefersamuchhigherswitchingfrequency,e.g.,50–100kHzintensofkilowattpowerlevel.Theeffectofsoftswitchingtechniquesis,therefore,veryprominentinPFCapplications.

Theinputcurrentsaregenerallyinphasewiththeinputvoltages,andbidirectionalpowerflowisusuallynotrequiredinPFCcircuits.Thesefactsprovidesomeflexibilitytodevelopsoft-switchingtechniquesandcontrolschemesspecifictoPFCconverters.

2.SimpleThree-PhasePFCCircuits

Toreducetheconvertercostandavoidthecomplexityoffull-bridgethree-phaseconverters,severalsimpletopologieshavebeenusedforthelowpowerendofthree-phasePFCapplications.

A.Three-PhaseRectifiersConsistingOfThreeSingle-PhaseConverters

AsimplewaytoimplementaPFCconverterinathree-phaseapplicationistocombinethreesingle-phaseboostrectifiersattheinputside,oneforeachphaseandeachfollowedbyadc–dcconverter.Thisconfigurationissimplifiedinbydirectlycouplingtheoutputsofthethreesingle-phasePFCconverters.Thissimplificationcouldresultinsignificantcostreduction,sinceonlyonedc–dcconverterisrequired.Theoutputcapacitorissharedbythethreeconverters,andthevoltageacrossitdoesnothavelow-frequencyrippleinbalancedconditions.Therefore,fastvoltagecontrolcanbeusedwithoutdistortingtheinputcurrentreferences.Themainproblemofthisconfigurationisthatthecurrentintoaphasemoduleisnotthesameasitsreturncurrent,whichcausescontrolinterferenceamongthethreephases.

B.Three-PhaseSingle-SwitchDCMRectifiers

Highpowerfactorcanbeeasilyobtainedwhenboost,buck-boost,flyback,Sepic,Cuk,andzetaconvertersareoperatedindiscontinuouscurrentmode（DCM）withconstantdutycycles.Thereareseveralsuchthree-phasePFCtopologiesdevelopedwhichrequireonlyoneactiveswitch.Thesingle-switchboostrectifier,isthemostpopulartopologyinthiscategory,duetoitssimplicityandrelativelygoodperformance.Usually,theconverteriscontrolledbyaslowvoltageloop,whichkeepsthedutycycleofthemainswitchpracticallyconstantoveralinecycle,soeachinputcurrenthasanenvelopeproportionaltoitscorrespondingphasevoltage.Thedutycycledeterminesthemagnitudeoftheinputcurrentsand,thus,theinputpower,whichprovidesameanstoregulatetheoutputvoltage.Althoughtheinputcurrentpeakisproportionaltothesinusoidalinputvoltageineachswitchingcycle,theaverageinputcurrentisdistortedbytheinductorcurrentduringthedischargingstage,thedurationofwhichisdeterminedbythedifferencebetweentheoutputandinputvoltages.Toreducethedistortion,theoutputvoltagehastobesufficientlyhigherthantheinputvoltagepeaktolimitthedurationofthedischargingstage.

Bychangingtheoutputstageoftheboosttopology,severalothertopologieshavebeenproposedin.IntheSepicandCuktopologies,thevoltagegaincanbereducedtofacilitatethedc–dcconverterinthesecondstage.Single-stagePFCcanbeachievedbyreplacingtheoutputinductorwithatransformerandinput–outputisolation.However,theswitchandtheintermediatebulkcapacitorareexposedtohighvoltagestress,andtheconductionlossisincreasedduetotheincreasedcirculatingcurrent.Intheso-calledditherrectifier,theinputinductorsarechangedtoflybacktransformers.Sincetheinputcurrentnowisonlytheinductorcurrentinthechargingstage,nearlyperfectsinusoidalcurrentcanbeobtained.Thedisadvantageofthisflybackconverteristhehighvoltagestressoftheswitchandthecomplexclampcircuitnecessarytoabsorbtheleakageenergyoftheflybacktransformers.

3.Three-PhaseBuckRectifiers

Abuckrectifierhassomeattractivefeaturescomparedtoaboostrectifier,suchasinherentshort-circuitprotection,easyinrushcurrentcontrol,andlowoutputvoltage.Inaddition,itsinputcurrentscanbecontrolledinopenloop,andmuchwidervoltageloopbandwidthcanbeachieved.Generally,abuckrectifierhashigherconductionlossthanitsboostcounterpart,becausemoresemiconductordevicesareinseries,andtheinputcurrentsarediscontinuous.However,abuckrectifierusuallyhaslowerswitchingloss,especiallyatlowlineconditions,wheretheboostrectifier’sswitchingloss（andalsoconductionloss）reachesitsmaximum.Theworstcasepowerlossofathree-phasebuckrectifierisnotnecessarilyhigherthanthatofathree-phaseboostrectifier.Atpresent,three-phasebuckrectifiersarenotusedaswidelyasthreephaseboostrectifiers,probablybecauseasingle-phasebuckrectifierisnotaviabletechniqueandthree-phasecurrentsourceinverters,whichusethesametopologyasthebuckrectifierswithoutthefreewheelingdiode,arenotpopular,exceptinveryhigh-powerSCRapplications.However,itispossiblethatthree-phasebuckrectifierscouldachievecertainperformance/costadvantagesoverboostrectifiersforsomeapplications,especiallyiftheperformanceofbidirectionalvoltagedevicescanbesignificantlyimprovedwiththedevelopmentofpowersemiconductortechniquesinthefuture.

4.ControlAndSystemIssues

A.ControlDesign

Controlofpowerconvertersusuallycanbedividedintothreefunctions:

modulation,currentcontrol,andregulationofanoutputvariable（theoutputvoltageinrectifiers）.Inthethree-phaseinverterapplications,thesystemdynamicsisusuallydominatedbytheslowelectromechanicaland/orlargereactivecomponents,sothattheinverterdynamicperformanceisnotverycritical.Additionally,accurateaccurrentcontrolisnotveryimportantinmanyinverterapplications（exceptforfield-orienteddrives）.Onthecontrary,high-qualitycurrentcontrol,withouttheuseoflargereactivecomponents,isthemajorobjectiveinPFCapplications.Withhighswitchingfrequencies,whicharemadepossiblethroughtheuseofsoft-switchingtechniques,highperformanceandverywidebandwidthcontrolcannowbedesigned.Thecontroldesignisfacilitatedbyrecentimprovementsinthemodelingofthreephaseconverters.

Allstandardmodulationtechniquesdevelopedforinverterscanbeusedinrectifierapplications.SinusoidalPWM（SPWM）iswellsuitedforanalogimplementation,butcauseshigherswitchinglossesandcurrentdistortion.Inboostrectifiers,SPWMcanbeusedwiththird-harmonicinjectiontodecreasetheminimumoutputvoltageby15%.Thesameeffectisautomaticallyachievedwithspace–vectormodulation（SVM）,whichalsosignificantlyreducesswitchinglossandhigh-frequencycurrentripple.Manyofthesoft-switchingtechniquesrequiretheuseofcompletelydifferentmodulationstrategiesormodificationsofthestandardPWMschemes,duetotherequirementofsynchronizingswitchturn-oninstantsinthethreephases.Inthebuckrectifiers,duetotopologicalrestrictions,threephasescannotoperateindependently.Thispreventsthedirectuseofhysteresisinputcurrentcontrollers.Instead,SVMormodifiedSPWMtechniquesareusuallyused.Excellentinputcurrentqualitycanbeachievedwithopenloopcontrol.

B.FilterDesignandSystemInteraction

SinceaPFCconverterhastomeetEMIspecifications,aninputfilterisusuallyrequired.Thefiltershouldprovideenoughhigh-frequencynoiseattenuation,whilekeepingalowinputdisplacementangleandshouldbeassmallandaslightaspossibletoincreasepowerdensity.AwaytopredicttheconductedEMIemissionofboostrectifiersispresentedin,whichcanbeeasilyextendedtobuckrectifiers.Aproperdampingnetworkisnecessarytoensuresystemstability.

5.Conclusions

Thispaperhasgivenacomprehensivereviewofrecentdevelopmentsinthree-phasePFCtechniques,especiallysoft-switchingtechniques.Itcanbeexpectedthatsoft-switchingconverterswillbeanincreasinglyviablealternativetotheconventionalhard-switc