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APOWERQUALITYSOLUTIONWITHLOWINVERTERRATINGS BY RAY-SHYANGLAI ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT OFTHEREQUIREMENTSFORTHEDEGREEOF DOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 1996 Inmemoryofmymother & TomywifeShwu-Yuann and mydaughterKaren ACKNOWLEDGMENTS Iwouldliketoexpressmydeepestappreciationtothechairmanofmycommittee, Dr.KhaiD.T.Ngo,forhisguidance,encouragement,patience,and,notlessimportant, financialsupportforaslowlearnerlikeme.Ireceivedmymaster'sdegreeunderDr. Ngo'ssupervision,too,sixyearsago.Ihavelearnedalotfromhimovertheseyears,andit hasgreatlyreshaped my life'spath. I also want to thankDr. Dennis P. Carroll,Dr. AlexanderDomijan,Dr.KennethK.O,andDr.LocVu-Quocfortheirparticipationonmy committee. MyspecialthanksgotoDr.Domijanforprovidingmetheequipmenttofinishmy experimentandMr.TariqAslamBuchhforcarryingtheextraheavyequipmentwithme.1 must thank Dr. Vu-Quoc for helping me cut the magnetic core in preparing the experiment. Therewillbealwaysawoundborneinmymindthatmymotherpassedaway whileIcouldnotstaywithherinherlastminutes,especiallyasithappenedonlyfourdays afterIhadvisitedherandreturnedtoUF.Therewouldnothavebeenanychanceof advancedstudyformewithoutherlife-longsacrifice. ThereisnowordtodescribemygratitudetowardmywifeShwu-Yuannforher supportandtolerance,especiallyasshegavemethepleasureofbeingababygirl'sfather notlongago. iii TABLEOFCONTENTS pages ACKNOWLEDGMENTS iii ABSTRACT vi CHAPTERS 1 INTRODUCTION 1 2 CONCEPTSANDEXISTINGSOLUTIONSOFHARMONIC ELIMINATIONANDACTIVEFILTERING 12 2.1 ActiveFilterinSerieswithPowerLineandShuntPassiveFilter 15 2.2 ActiveFilterinSerieswithShuntPassiveFilter 16 2.3 ActiveFiltersinSerieswithPowerLineandinSerieswith ShuntPassiveFilter 20 3 OPERATIONSOFPULSE-WIDTH-MODULATEDINVERTERS 28 3.1 IntroductiontoPowerElectronicConverters 28 3.2 OperationsofFull-BridgeInverter 32 3.3 OperationsofThree-PhaseVoltage-FedInverter 42 3.4 OperationsofThree-PhaseCurrent-FedInverter 52 4 THEPROPOSEDLOW-RATINGSPOWERQUALITYSOLUTION 81 4.1 ACurrent-FedInverterastheActiveFilter 82 4.2 AVoltage-FedInverterastheActiveFilter 98 4.3 LossMechanismsandPowerRatingsoftheActive/PassiveHybrid Filters 101 4.4TotalHarmonicDistortionErrorsDuetotheOpen-LoopControl 108 5 EXPERIMENTALRESULTSOFATESTSYSTEM 132 6 SUMMARYANDCONCLUSION 159 iv APPENDICES A PSPICE®FILES 161 B MATLAB®SCRIPTFILESANDOUTPUTS 188 REFERENCES 201 BIOGRAPHICALSKETCH 207 V AbstractofDissertationPresentedtotheGraduateSchool oftheUniversityofFloridainPartialFulfillmentofthe RequirementsfortheDegreeofDoctorofPhilosophy APOWERQUALITYSOLUTIONWITHLOWINVERTERRATINGS By Ray-ShyangLai December1996 Chairman:KhaiD.T.Ngo MajorDepartment:ElectricalandComputerEngineering The feasibility of a three-phase current-fed inverter for active filtering of distributionsystemharmonicswasinvestigatedsincemostofthepublishedworksinthis fieldhaveemployedvoltage-fedinverters. Thestudyshowedthatthevoltage-fedhybridfiltergenerallyhaslessconduction lossesthanthecurrent-fedonenomatterwhattypesofpassivefiltersareemployed.From thelossstandpoint,thecurrent-fedinverterissuitablefortheroleofactivefilteronly when it is in series with a parallel-resonantpassive filter tuned at the fundamental frequency,insteadofthepopularcompoundseries-resonantfilterstunedat5th,7th,and Uthharmonics.Thefunctionoftheparallel-resonantfilter,insertedinbetweenthepoint ofcommoncouplingandtheoutputcapacitorofthecurrent-fedinverter,istopreventhigh fundamentalcurrentfromcirculatingintheinvertercircuitry.Thepower(VA)ratingsof thesemiconductorcomponentsandpowerlossesoftheinverterthereforearereduced. vi Thestudyalsorevealedthat,incontrasttothevoltage-fedinverter-basedhybrid filter,whichrequiresboththeloadvoltageandthelinecurrenttobesensedandfunctions correctlyonlyunderclosed-loopfeedbackcontrol,thecurrent-fedinverter-basedhybrid filtercanperformactivefilteringsimplybyscalingthefundamentalremovedloadcurrent tobethedutyratiofortheinverter. Simulationworksconfirmedtheanalysis.Withtheduty-ratiodirectlyderivedfrom theloadcurrent,thecurrent-fedinverter-basedactivefilteringonaprototypeone-branch distribution system loaded with a 3kW full-bridge diode rectifier has been done successfully.Theresultsoftheexperimentareinaccordancewiththeanalysis.Using 3kWasthepowerbaseandselectingthesemiconductordeviceswithappropriateratings fortheinverters,theinverterlossesare0.82%p.u.forthecurrent-fed/parallel-resonant filter, 0.08% p.u. for the voltage-fed/parallel-resonant filter, and 2.09% p.u. for the current-fed/compoundseries-resonantfilterwiththesamecapacitance(150|J.F)ineach passivefilter.Increasingthecapacitanceincreasestheinverterlossesintheconventional hybridfilter, butdoes notincrease the inverterlossesin theproposedconfiguration. However,theVAratingsoftheparallel-resonantpassivefilteraretwicetheratingsofthe series-resonantfilter,andhighinductancesarerequired. Insummary,theproposedhybridfilter,consistingofathree-phasecurrent-fed inverterand aparallel-resonant passive filter tuned atthe fundamental frequency, is suitableforactivefilteringofdistributionsystemswithlowoveralllossesandlowVA ratingsoftheinvertercomponents.Thecontrolofthiscurrent-fedcombinationissimpler thanthecontrolofvariousvoltage-fedcombinations. vii CHAPTER 1 INTRODUCTION Theadvancesinmodemsemiconductordevicesdevelopmenthaveincreasedthe scopeofapplicationsforvariouselectroniccomponentsanddevices,suchasinHVDC[1, Chapter10],variable-speedmotordrives[2],andvariouspowerAC-to-DCandDC-to- ACconverters[3,4].Withtheincreasingapplicablerangesofvoltage,current,andpower, electroniccomponentsanddeviceshavebecometheessentialsincontrol,regulation,and powerdeliveryofelectricalpowersystemsnowadays.WithnonlinearV-Icharacteristics in nature, the growing application ofpower electronic devices increases the power demandsandpresentsaseriouspowerqualityproblemtopowersystems[5,6,7,8,9,10, 11],especiallyinthedistributionlevel[12,13]. Nonlinearloadscontributetothequalitydeteriorationofapowersourcethrough the generation ofharmonic and fundamental frequency reactive currents. The load- generated harmonic currents interact with the finite distribution/transmission line impedancetoproduceharmonicvoltagesateverybusalongthepowersystem,whichmay causeserioushazardstothepowersystemandotherpowerusers[1,Chapter10].The fundamental frequencyreactive current wastes the generatedpower andmaypose a stabilityproblem[1,Chapters2 and 11] tothecontrolofpowersystems.Thepower qualitycanbefurtherdeterioratedifthesagsandsurges,causedbyunbalancedthree- phasecurrents,subsynchronousfrequencycurrents,orlargeloadtransients,arepresented inthesupplyvoltageprofileofthesystem. Basedontherequirementthat,ideally,onlyin-phasevoltagesandcurrentsofthe fundamentalfrequencyshouldexistinthepowersystem,thequalityofapowersource canbecategorizedas •correspondingtotalharmonicdistortion(THD,expressedin%ofthefunda- mentalcomponent)ofloadvoltageandlinecurrent, •powerfactor,PF,and •%sagorsurgeoffundamentalvoltageofthepowersource. Thedefinitionsof%THD,powerfactor,andsag/surgecanbederivedfromFigure1.1. FromFigure1.1(a),die%THDoflinecurrent isdefinedas THD. = Vl s-I si hi TTTv (1-1) 1 sh %THD. = 100XTHDj whereiîisthefundamentalcomponentofi^,andIjandlî aretheroot-mean-square (rms)valuesof andiî,respectively.Thepowerfactor(PF)ofthepowersystemwitha nonlinearloadisdefinedas PF = y^cos(t) ss 1 (1.2) cose)) a/i+THDî whereVgisthermsvalueoflinevoltage andîsthephaseanglebetween andiîin Figure1.1(a).FromEquation(1.2),itisclearthattheharmoniccomponentsofanonlinear loadcurrentwill yieldalowerpowerfactor,andhencemake thepowersystemless efficient,comparedtoalinearloadwiththesamefundamentalcurrentandphaseangle 3 betweenthevoltage andcurrent.Figure 1.1 andEquation (1.2) assumethattheline voltagedoesnotcontainanyharmoniccomponent. Ifthereareharmonicsintheline voltagespectrum,Equation(1.2)isrewrittenas PF = -——cos<l)j ss (1.3) = Vl. +THd\:J•a/i+THD^COSCj)^ where V^j is the fundamental rms value ofv^; is the phase angle between the fundamentalvoltageandcurrent;andTHDyisthetotalharmonicdistortionofVj.When the loadisnonlinear, the currentharmonics are always accompanied by thevoltage harmonicsandtheefficiencyofthepowersystemisfurtherreduced. All major industrial standards impose power quality guidelines on electrical utilities,users,andelectricalequipmentmanufacturersinordertomaintainanacceptable powerqualityforallparties[14].Forpowerutilitiesandusers,theresponsibilitiescanbe simplyclassifiedasfollows: •tosuppressthe%THDofvoltagesuppliedtousersand •tostabilizethefundamentalfrequencyvoltage forthepowerutilities,and •tosuppressthe%THDofcurrentinjectedintoutilitiesand •tosustainanacceptablepowerfactor forthepowerusers.Thesplitoftheresponsibilitieshappensatthepointofcommon coupling(PCC),asshowninFigure1.2.