电气工程及其自动化专业英语第三章ection-3-3

Chapter3PowerElectronicTechnologySection3DC-ACConvertersTextNewWordsandExpressionsExercisesEndTransitionofpartofspeechSection3DC-ACConvertersIntroductionDC-ACconvertersareconventionallycalledtheinverters.Suchconvertersareverypopularinthebattery-operatedpowersystemssuchastheuninterruptiblepowersupplies(UPS)forhospitals,andACmotordrives.Lowpowerlevelinvertersareusuallysingle-phasetypeandmediumandhighpowerinvertersarethree-phasetype.WewilllookintotheperformanceofDC-ACconvertersofseveralusefultopologies.Thefocuswillbeontheinverteroperationforlow-frequencyoutputSection3DC-ACConvertersvoltagessuchas50～60Hzor400Hzinaircraftsystems.AnalysesofDC-ACconvertercircuitsatlowandhighfrequenciesareverysimilarexceptthatathighfrequenciestheparasiticcapacitancesandinductances,chargestorage,andheatlocalizingproblemsinswitchingdevicesmustbetakenintoaccount.ControlledinversionTheDCtoACinvertersystemisshowninFig.3-5(a).InputisfromaDCsourceandtheoutputisdesiredtobeasinusoidalvoltageorcurrentwithSection3DC-ACConverterszeroDCcomponent,single-phaseorthree-phasesignal.TheloadisaR-L-Cload,anACvoltageoranACcurrentsink.Controlparametermaybe,asshallbeseenlater,anangle,apulsewidth,avoltageoracurrentsignal.Theconverterconsistsofswitches,reactivecomponentssuchasL,C,transformersandresistances.Switchesincludingthediodesareassumedtobeidealandunidirectional,thatis,theyhavezeroon-statelosses,zerooff-statelosses,zeroswitchinglossesandunrestrictedvoltageorcurrentcarryingSection3DC-ACConverterscapabilities.Switchingdevicesinapracticaldesignexamplemustbeselectedwiththesedesiredcharacteristicsinmind.Inductors,capacitorsandtransformersareassumedtobeideal.Fig.3-5(a)ADC-ACinvertersystem;(b)operationalquadrantsontheUo-IoplaneSection3DC-ACConvertersElectricalloadscanbeofthreetypes:impedanceload,ACvoltagesinkloadorACcurrentsinkload.Examplesofimpedanceloadareelectromagneticacrelays,electromagneticpickupandliftequipments.Theloadcomprisesofalargevalueinductor.Examplesofacvoltagesinkaretheconstantspeeddrivesforinductionandsynchronousmotors.ExamplesofACcurrentsinkareconstanttorquedrivesfortheinductionandsynchronousmotors.Section3DC-ACConvertersGenerallyspeaking,theinputcurrentisnon-sinusoidalifthesourceisaDCvoltagesourceandviceversa.Theoutputvoltageandtheoutputcurrentontheotherhand,canbenon-sinusoidalsimultaneously.Hence,inputandoutputlow-passfiltersaregenerallyemployedtoreducehighfrequencycomponentsofthenon-sinusoidalsignalsoneitherside.Aseriesinductorisacurrentsmootherandaparallelcapacitorisavoltagesmoother.Section3DC-ACConvertersOutputcurrentgenerallydiffersinphasewiththeoutputvoltage.Hence,inaswitchingperiod,thepolaritiesofcurrentandvoltagevaryoverallfourquadrants,seeFig.3-6.Powerflowsfromtheconvertertotheloadinthefirstandthirdquadrantsandinthereversedirectioninthesecondandfourthquadrants.Aunidirectionalswitchintheinverters,therefore,hasaninverseconnecteddiodeinparalleltoallowbidirectionalcurrentflow.Section3DC-ACConvertersFig.3-6OutputcurrentandoutputvoltagewaveforminaninverterSection3DC-ACConvertersInvertersderivedfromvoltagesource(VSI)areideallysuitedforsupplyingpowertoaresistiveandinductiveloadandtheapplicationsthatrequireconstanttorqueoperationofACmotors.CurrentSourceInverters(CSI)areideallysuitedforsupplyingpowertolargelycapacitiveloadsandACvoltagebuses.Inverterscanbefurtherclassifiedonthebasisofthemodeofcontroloftheoutputvariable:voltageorcurrent.Invertercontrolinpracticeconcernsthreeaspects:fundamentalfrequency,amplitude,harmonicprofile.Section3DC-ACConvertersFundamentalfrequencyiseitherequaltotheswitchingfrequencyoftheinverter(square-waveandPWMinverters)oritssimplefraction(1/3,1/5,andsoon)suchasinMultiplePWMorsinePWMinverters.Powerelectronicapplicationsrequiretheamplitudecontrolofoutputvoltageoroutputcurrent,calledVoltage-ModecontrolandCurrent-Modecontrolrespectively.AnexampleofVoltage-Modecontrolistheadjustablespeeddrivesofinductionandsynchronousmotors.AnexampleofCurrent-ModeSection3DC-ACConverterscontrolisaconstanttorquedriveforinductionmotors.OutputamplitudeinbothmodescanbecontrolledbyvaryingtheDCsourcevoltageorbyvaryingthepulse-width(pulse-widthmodulation,PWM).TheinvertersbasedonthecontrolofDCsourcevoltagearealsotermedDC-LinkInverters.TheseinvertersuseanACtocontrolledDCconverterontheinputside.Theinverterissquare-wavecontrolled.ThisarrangementisexpensivebutexhibitsafasterresponsetimethanPWMinverters.Thecontrolcircuitryisalsosimpler.Section3DC-ACConvertersHarmonicprofilingisanextrafeatureaddedovertheamplitudecontrol.Theobjectiveofharmonicprofilingistobolstertheamplitudeofthefundamentalcomponentoftheoutputvoltageandtheeliminationorreductionofhighorderharmonics.Oneofthetechniquesinvolvesthenotchingoraddingsteppulsestoabasepulse.Theothertechniqueinvolvesmultiplepulsespercycleofthefundamental,eachpulse-widthmodulatedtoachievedesiredwave-shapingoftheoutputwaveform(Sine-PWM).Section3DC-ACConvertersInvertertopologiesTheDC-ACinvertercanbethoughtofasathree-positionswitchasshowninFig.3-7(a).Theloadisconnectedtothepoleoftheswitchandthefirsttwopositionsar