Power Factor Correction Using a Series Active Filter

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Power Factor Correction Using a Series Active Filter

148IEEETRANSACTIONSONPOWERELECTRONICS,VOL.20,NO.1,JANUARY2005

PowerFactorCorrectionUsingaSeriesActiveFilter

ZhiguoPan,StudentMember,IEEE,FangZ.Peng,Fellow,IEEE,andSuilinWang

Abstract—Thispaperpresentsalow-costapproachtopowerfactorcorrection(PFC)ofsingle-phasedioderecti?ersusingaseriesactive?http://wendang.chazidian.comparingwiththetraditionalPFC,thepro-posedPFChaslowerrequirementsofpowerdeviceratings,whichleadstolowercost,higheref?ciency,andlowerelectromagneticinterference.ItalsocaneliminatethebulkyinductorneededinthetraditionalPFC.Thetopology,operationprinciple,andapplicationissuesoftheproposedPFCareanalyzedinthispaper.Thecontrolstrategyisdiscussedindetailandsimulationresultsareprovided.A1-kWprototypeisbuiltupandtheexperimentalresultsarepresentedtoverifytheanalysis.

IndexTerms—Adjustable-speeddrive,heating,ventilating,andairconditioning(HVAC),powerfactorcorrection(PFC),seriesac-tive?lter.

Fig.1.Traditionalpowerfactorcorrection.

simpleandeasytoimplement,ispresentedtoeliminatelinehar-monics.Simulationandexperimentalresultsaregiventoverifytheanalysisanddemonstratethecontrolperformance.

II.TOPOLOGYANDOPERATIONPRINCIPLE

I.INTRODUCTION

Ntherecentyears,moreandmorevariablespeedmotordrivesfedbysingle-phaseutilitypowerareusedinresiden-tialheating,ventilating,andair-conditioning(HVAC)systems.Mostofthemareusingdioderecti?erswithsmoothingdcca-pacitorsasthefront-end.Thedioderecti?erdrawspulsedcur-rentfromtheutilityline,whichleadstolowpowerfactor,lowef?ciency,andhighratingrequirementstotheswitchingde-vices.Asaresult,somestandardssuchasIEC1000-3-2havebeenbroughtforwardtolimittheharmoniccurrentoftheutilityline.Tocomplywiththesestandards,powerfactorcorrection(PFC)isneeded.Sofar,avarietyofpassiveandactivePFCtech-niqueshavebeenproposed[1]–[5].WhilethepassivePFCmaybethesimplestwayinthelowpowerapplications,theactivePFCareusedinthemajorityofapplicationsfortheirhighper-formance,compactsize,andlightweight.

Fig.1showsthemostcommonapproachtosignal-phaseac-tivePFC,whereaboostcircuitwithanadditional?lterinductorisusedtoshapethelinecurrent.Thebooststageforcesthedcbuscurrenttofollowthelinevoltage,whichresultsinanearlysinusoidallinecurrentwithoutphasedifference.However,itbringsinhighdcbusstress,highswitchingdeviceratings,highswitchinglosses,andhighelectromagneticinterference(EMI).Inthispaper,anewcompact,low-weight,low-costPFCusingaseriesactive?http://wendang.chazidian.comparingwiththetraditionalPFC,ithaslowerdeviceratings,lowercost,lowerEMI,andhigheref?ciency.TheinductorneededintheproposedseriesPFCisalsomuchsmallerthanthoseinthetraditionalPFC,andinthemostcases,thelineimpedanceoftheutilitylineisenoughforitsnormaloperation.Ahysteresiscontrolstrategy,whichis

ManuscriptreceivedAugust19,2003;revisedApril28,2004.RecommendedbyAssociateEditorH.duT.Mouton.

TheauthorsarewiththeDepartmentofElectricalandComputerEn-gineering,MichiganStateUniversity,EastLansing,MI48824-1226USA(e-mail:zpan@egr.msu.edu).

DigitalObjectIdenti?er

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10.1109/TPEL.2004.839819

Thedioderecti?erwithproposedPFCfortypicalresiden-tialHVACisshowninFig.2.Becausetheaclineis100Vandlow,avoltagedoublerisusedforrecti?cation,i.e.,twocapaci-torsarechargedalternativelyoveronecycle.Theoutputvoltageisdoubled,resultinginanapproximate250Vdclinkvoltage

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V).ThePFCcircuit,implementedbyafullbridge(

inverterwithanoptionalinductor,connectedinserieswiththedioderecti?er.Sincethedioderecti?erandthefullbridgein-verterbehavelikevoltagesources,thelinecurrentisdeterminedbythedifferenceamongthesethreevoltagesourcesandtheloop

andinductance,whichincludestheline

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impedances

theinductanceoftheoptional

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inductor.

Althoughfourswitchingdevicesareusedinthistopology,eachpowerdeviceonlyneedstosustainalowcapacitor

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voltage,,whichislimitedbelow50V.Thuslow-cost,high-ef?-ciency60-Vmetal-oxidesemiconductor?eldeffecttransistors(MOSFETs)canbeemployed.Thevoltageratingofeachdeviceisonly20%ofthevoltageratingoftheswitchingdevicethatareusedinthebooststageofthetraditionalPFC.There-fore,eventhoughfourMOSFETsareusedintheproposedtopology,itstotalcostofswitchingdevicesisstilllessthanthecostofthehighvoltagedeviceinthetraditionaltopology.Inaddition,http://wendang.chazidian.comparedwiththehightransferratioandisolatedpowersupplyneededinthegatedrivecircuitofthetraditionalboostPFC,theproposedtopologycanalsoreducethecomplexityandcostofthegatedrivesignaltremendously.

whileswitchingisalsoaTheEMIproblemdueto

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the

bigconcernofthePFCsystem.IntheproposedPFC,thevoltage

,whichislessthanstressontheswitchingdevicesis

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only

50V,whiletheswitchingdeviceinthetraditionalboostPFCneedstoswitchonandoffthewholedcbusvoltage,250V.Therefore,theEMIinterferenceduringswitchingwillbere-ducedsigni?cantly.Thevoltagedistortionbroughtintotheac

PANetal.:POWERFACTORCORRECTION

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149

Fig.2.ProposedPFC.

gridbytheproposedcircuitalsocanbeneglectedbecause1)

V)and2)theline

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theinverterdcvoltageisrelativelylow

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(

withtheinductanceoftheoptional

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inductor,impedance,

arenormallyfargreaterthanthesource

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impedance,.

Thesimpli?edequivalentcircuitisshowninFig.3,

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whereisthetotalline

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impedance,.Therelationshipbetweentheoutputvoltageoftheinverterandthestateofeachswitchcanbesummarized

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(1)

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where

isde?ned

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forisON

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andis

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OFFfor

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bothareONorOFF

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simultaneously

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forisOFF

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andisON

(2)

andarecomplimentarypairs,re-

Fig.3.Equivalentciruitofproposed

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PFC.

Note

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that

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spectively.

FromtheequivalentcircuitshowninFig.3,theline

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currentsatis?

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(3)

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where

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for

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for

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and

Fig.4.Hysteresiscontrol.

(4)

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?lter,andfedintoahysteresiscomparator.Theoutputofthein-verterwillswitch

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between

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andtolimittheharmoniccurrentwithinthehysteresisband.Inthiscontrolstrategy,thezerovoltageoftheinverterisnotutilized,whichresultsinhigherswitchingfrequencyandhigherswitchingripples.Inordertoreducetheswitchingfrequencyandthevoltageripple,thezerovoltagehastobeutilized.

Fig.4(b)showstheproposeddoublehysteresiscontrol,whichutilizesthezerovoltageoutputoftheinverter.Inthiscontrolstrategy,twohysteresiscomparatorswithdifferenthysteresis

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and,areused.Sincethetwophaselegsofthewidths,

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isfull-bridgeinverteraresymmetrical,wecanassume

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that

.Fig.5illustratesthecomparatorandinvertergreater

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than

andistherecti?ercapacitorvoltage.

andareconstant.Theline

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current,Assume

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that

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canbecontrolled

when,

issatis?ed.Thus,theseriesinverterwithenoughdcvoltageisabletomakethelinecurrentfollowthedesiredreference.

III.CONTROLSTRATEGY

A.HysteresisControl

Theeasiestwaytocontroltheproposedtopologyistousethehysteresiscontrol,asshowninFig.4(a).Inthiscontrol,thehar-moniccomponentoftheline

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current,isextractedbya

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notch

150IEEETRANSACTIONSONPOWERELECTRONICS,VOL.20,NO.1,JANUARY

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2005

Fig.5.Operationprincipleofthedoublehysteresiscontrol.

waveforms,

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where

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andare3Aand2A,respectively.

Whentheline

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voltage,

satis?

es,theoutputofcomparator1isalwayshigh,i.e.,

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switch

isturnedon.Thentheoutputoftheinverterswitchesbetween0

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and

,accordingtheoutputofcomparator2,toforcethelinecurrenttofollowthesinusoidalreference.Considering

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reachestheupperlimitofcomparator2andthe

outputofcomparator2?ipstohigh,thentheoutputofthein-verterbecomes0.Accordingto(3),thederivativeofthehar-monic

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current,

becomesnegative,

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andstartstode-crease.

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reachesthelowerlimitofcomparator2,thentheoutputoftheinverterchanges

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andstopsde-creasingandbeginstoincreaseagain.Therefore,

for

islimitedwithinthehysteresisbandofcom-parator2.

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Since

doesnotreacheitherlimitofcomparator1,theoutputofcomparator1willalwaysremainhighduringthis

period.

changesto

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negative,keepsde-creasingslowlyastheline

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voltage,

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approaches.

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When

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surpasses

atbecomespositiveaccordingto(3).

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Thenstartstoincreaseandreachestheupperlimit(2A)ofcomparator2again.Sincetheoutputofcomparator2isalready

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high,

willkeepincreasing

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untilreachesthelowerlimit

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(

A)ofcomparator1

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at,whentheoutputofcomparator1?ipstolow.

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After,theoutputoftheinverterswitchesbetween

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and

tocontroltheharmoniccurrentwithinthehysteresisbandofcomparator2.Similarly,wecananalyzetheoperation

for

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and.Inthiscontrolstrategy,

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switches

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andonlyswitchthreetimesperfundamentalcycle.Theswitchingfrequency

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switches

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andisapproximatelyhalfthatofthesinglehysteresiscontrolwiththesamehysteresiswidth.Therefore,thedoublehysteresiscontrolstrategytremendouslyreducestheswitchingfrequency.

B.VoltageControlofDCCapacitor

Intheformerdiscussion,weassumethatthedccapacitorvoltageoftheinverterisconstant,whichrequiresthenetav-erageactivepower?owingintothecapacitorduringone

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cycle

Fig.6.Blockdiagramofhysteresiscontrolwithdcvoltagecontrol.

tobezero.However,itcannotbeautomaticallyguaranteedintheactualcircuit.SoavoltagefeedbackcontrolshowninFig.6isaddedtoregulatethedcvoltageoftheinverter.

Inthiscircuit,theactualdccapacitorvoltageisdetectedandcomparedwiththereferencevalue,andtheerrorisfedintoaPIcontroller.Asinusoidalsignalinphasewiththelinevoltageisaddedtotheharmoniccurrentsextractedbythenotch?lter.TheamplitudeofthesinusoidalsignaliscontrolledbythePIcontroller.Byaddingthesinusoidaloffsettotheinputofthehysteresiscomparator,activepower?owingintothecapacitorwillbechanged,thusthedccapacitorvoltagecanbecontrolled.C.SwitchingFrequencyandVoltageRipple

Sincethehysteresiscontrolisusedtocontroltheswitchingdevices,theswitchingfrequencyisnotconstant.However,itisdesirabletoestimatetheaverageswitchingfrequencyfordesignpurposes.

Fig.7illustratestheswitchingpatternofthehysteresiscontrolwhentheline

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voltagesatis?

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es,andthe

outputoftheinverterswitchesbetween0

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and

.During

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period,theinverter

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voltageandtheline

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current

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satisfy

(5)(6)

Similarly,duringperiod

,we

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have

(7)(8)

Assumethattheswitchingfrequencyismuchhigherthanthefundamentalfrequency,

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thuscanbeassumedasaconstantvalueduringoneswitchingcycle,

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thenandcanbegiven

and

(9)(10)

Thedutycycleandswitchingfrequencycanbewritten

and

(11)(12)

PANetal.:POWERFACTORCORRECTION

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Fig.7.Switchingpatternofthehysteresiscontrol.

Similarly,

for

,thedutycycleand

switchingfrequencyaregiven

and

(13)(14)

Sincetheline

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voltage,variesovertimeinmagnitude,thedutycycleandfrequencyarealsochangingincorrespondingtothelinevoltage.Assumingthatthehysteresiswidthissmallenough,thelinecurrentwillfollowthesinusoidalwaveformwhenthe

line

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voltagesatis?

,andstayatzero

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whensatis?

es.Itcanbeseenthatthelinecurrent

isclosertoasinusoidalwaveform

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when

ishigher.Fig.8showsthewaveformsofthelinecurrentintheidealcase,thedutycycle,andtheswitchingfrequency,

where

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(15)(16)

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and

isthepeakvalueofthelinevoltage.Theactualswitchingfrequency,whichistheaveragefre-quencyoveronelinecycle,canbecalculated

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(17)

From(17),itcanbeseenthattheoptional

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inductorcanfur-therreducetheswitchingfrequencyandtheswitchinglosses.

However,thelineimpedancewillbeenoughtogetgoodperfor-manceformostcases.

Theline

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current,

charges,discharges,orbypassesthedccapacitordependingontheswitching

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state.Thedccapacitorvoltagecanbeexpressed

(18)

whereswitching

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stateisde?nedin(2).Fromtheaboveanal-ysisofthehysteresis

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control,iseither0or1

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whensat-is?

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es.Thedc

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capacitorkeeps

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discharging

151

Fig.8.Waveformoflinecurrent

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iintheidealcase,dutycycleDand

switchingfrequencyf.

duringthisperiodbecausethepolarity

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ofispositive,sameasthepolarity

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.alsokeepsdischarging

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whensatis?

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becauseisnegative

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andiseither0

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duringthisperiod.

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Similarly,keepscharging

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whenisgreater

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than.Thus,themaximumandminimumvalue

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ofoccurs

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and,respectively.Thevoltagerippleonthecapacitorcanbedeterminedbyintegratingthecurrent?owed

intothecapacitorover

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time

,which

is(19)

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where

isthepeakvalueofthelinecurrent.Therequiredcapacitanceoftheinverterforanacceptablevoltageripplecanbeobtainedfrom(19).

D.SwitchingFrequencyBalancing

Inthedoublehysteresiscontrol,theswitchingfrequencyof

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the

phaselegisonlythreetimesthefundamentalfre-quency,while

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the

phaseleghasmuchhigherfrequencyasexpressedin(17).Theunbalanceoftheswitchingfrequen-ciesoftwophaselegsresultsintheunbalanceofstressandpowerlosses.Fortunately,aswitchingfrequencybalancingcir-cuitcanbeeasilyimplementedbyasimpledigitallogiccircuit

152IEEETRANSACTIONSONPOWERELECTRONICS,VOL.20,NO.1,JANUARY

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2005

Fig.9.Schematicoftheswitchingfrequencybalancing

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circuit.

Fig.10.

Resultofswichingfrequencybalancing

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circuit.

Fig.11.Simulationresultofproposedcontrolstrategy.

byutilizingtheswitchingstatesredundancyofthezerovoltage

output.

Fig.9showstheschematicoftheswitchingfrequencybal-ancingcircuit.Italternatesthezerooutputbetweentwo

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states:

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andbothON

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and

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andbothON.TheresultofthebalancingcircuitisshowninFig.10,

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where

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andareunbal-ancedgatedrivesignals

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and

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andarebalancedgatedrivesignals.FromFig.10,itcanbeseenthattheswitchingfrequen-ciesofthetwophaselegsarewellbalancedandbecomehalfof

theoriginalfrequency

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whilegeneratingthesameoutputvoltage.

IV.SIMULATIONANDEXPERIMENTALRESULTS

Accordingtotheanalysisabove,aPFCcircuithasbeende-signed.Theparametersoftheloadandpowersourceare:output

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power

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kW,V(rms),60Hz.Thereferencevalue

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wassetto50V

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andto2A.Lettingtheripplevoltageofthedccapacitorbelessthan10%,theminimumca-pacitanceshouldbe4mFaccordingto(19).Assumethatthelineinductanceis0.2%p.u.,whichis

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53H.SimulationresultsareshowninFig.

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11.

Fig.12.Experimentalresultofproposedcontrolstrategy.V=100V,V

40V,

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V=114V,I

=20A,H=2

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A.Fig.13.Experimentalresultofproposedcontrolstrategy.V=100V,V

53V,

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V=100V,I=17A,H=2A.

Fromthesimulationresults,itcanbeseenthatthelinecur-rentisimprovedandbecomesnearlysinusoidal.Alsoitisshownthatthevoltageofthecapacitoriswellbalancedwithanaverage

valueof50.2V.Themaximumandtheminimumof

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the

are52.2and47.0V,respectively,andtheripplevoltageis5.2V,or10%,whichagreeswellwiththeanalyticalcalculation.The

switchingfrequencies

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andare180Hzand27kHz,re-spectively,whicharealsoconsistentwithourpreviousanalysis.AseriesPFCprototypewiththeproposedcontrolstrategyhasbeenimplemented.ExperimentalwaveformsareshowninFigs.12and13.Fig.12istheresultwhenthereferencevalue

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was40V,wherethecurrentTHDdroppedto24%aftercorrection,andthecurrentdisplacementwas1.2,resultinginapowerfactorof0.97.Fig.13istheresultwhenthereference

value

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was50V.Accordingtoourpreviousanalysis,thelinecurrentshouldbeclosertoasinusoidalwaveforminthiscase,whichwasveri?edbytheexperimentalwaveform.ThecurrentTHDdroppedto11%aftercorrection,andthecurrentdisplacementwas0.8,resultingapowerfactorof0.99.