Power electronics 2.docx

Power electronics 2.docx

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Powerelectronics2

Converters

Tableofcontents

Introductionp.3

1.Rectifierp.4

a.simplerectifierp.4

b.simplecotrolledrectifierp.5

c.Fullwaverectificationp.6

d.Controlledfullwaverectificationp.8

2.Inverterp.10

a.circuitdescriptionp.11

b.basicdesignsp.12

c.advanceddesign

Converters-Introduction

Powerelectronicsisabranchofelectricalengineeringthatrelatestodevicesforchangingtheformofelectricalenergy（converters）.

Wecandistinguishfourdifferenttypesofconverters :

∙ACtoDC（rectifier）

∙DCtoAC（inverter）

∙DCtoDC（DC-to-DCconverter）

∙ACtoAC（AC-to-ACconverter）

Powerelectronicsisabranchofelectricalengineeringthatrelatestodevicesforchangingtheformofelectricalenergy（converters）.

Powerelectronicsincludesthedesign,implementationandmaintenance:

-electroniccomponentsusedinhighpower;

-structuresofconverters;

-theorderoftheseconverters;

-industrialapplicationsoftheseconverters.

1.Rectifier

a.simplerectifier

Thistypeofrectifierisachievedbyreplacingthedioderectifierwhoisnotcontrolledbyathyristor.Thisismorethananeducationalassemblyactuallyusedexample.

V（t）Vs（t）

becauseThisconsideredlikeadiod.

istheroot-mean-squarevalueofoutputvoltage

isthemaximumvalueofv（t）

istheaverageoutputvoltage

b.Simplecontrolledrectifier

V（t）=sin（t）and

Thyristorbeginswithafiringdelaybetween0and

.Onceinitiated,itbehavesasadiode.

c.Fullwaverectification

AdiodebridgeorGraetzbridgeisanassemblyoffourdiodesbridge,single-phaseregime,rectifiesthealternatingcurrentintodirectcurrent,thatistosaynotflowinginonedirection.

Forlow-powerdevices,thesimplestfilterismade​​usingoneormorecapacitorsconnectedinparalleltotheoutput.Thecapacitorstoresenergywhenthecurrentthroughthediodesandthenreturns.

-Foragivenload,whenthecapacitanceofthecapacitorishigher,thefilteringismoreefficient.

-Foragivencapacitor,thelowertheloadcurrentcalls（Risbig）,themoreefficientthefilteris

-Generally,whentherectifiersuppliesaresistiveloadresistanceR,theproductRCishigher,thegreaterthefilteringwillbeeffective.

Thisshowsthatthehigherthecapacitanceofthecapacitoris,themorewelimitthecorrugations.However,increasingthecapacitycorrespondstoanincreaseincostandspaceandontheotherhand,toolargeacapacityleadstoverynarrowpeakcurrent,whichcausesharmonicpollutiononthedistributionnetwork.Finally,thetransientcurrentloadcurrentcapacitytopowertherectifiercancausedestructionofthelatterbyexceedingthemaximumallowableintensity.

Important :

Forloadswhichrequireahighcurrent,thevoltageofthefiltercapacitorisnotthemostefficient.Itisthenpreferredtoplaceaninductorinserieswiththeload.

d.controlledfullwaverectification

Whentriggerpulsesareavailable :

-AmongthethyristorsTh1andTh3,whohasthepotentialofitsmostpositiveanodeleads

-AmongthethyristorTh2andTh4,whohasthepotentialofitsmostnegativecathodeleads

-Whenthegatepulsenolongerexist,thyristorswhodrovebeforestoppingpulsescontinuedrivinguntiltheresumptionofpulses.

Theaveragevalueoftheoutputvoltagecanbecalculatedbytheequation:

ThenewperiodofoutputVs（t）is T/2.

N.B :

Inthediagram,wehaveacurrentsourceoutputtosimulatealargeinductance

Aswecansee,theinductancedoesn’tchangetheoutputvoltage.

2.Inverter

Aninverterisapowerelectronicdevicefordeliveringvoltagesandalternatingcurrentfromapowersourceprovidingadirectcurrent.Thisistheinversefunctionofarectifier.TheinverterisaDC/ACconverter.Aninverterisusedtoproduceanun-interrupted220VACor110VAC（dependingonthelinevoltageoftheparticularcountry）supplytothedeviceconnectedastheloadattheoutputsocket.TheinvertergivesconstantACvoltageatitsoutputsocketwhentheACmainspowersupplyisnotavailable.Principalapplicationsofinverterareportabledevicewhohavebatterywhoprovidesacontinuouscurrent,solargeneratingsystemswhoprovidescontinuouscurrentandsomeothers.

a.Circuitdescription

TheinvertersarebasedonaH-bridgestructureconsistingmostlyofelectronicswitchessuchasIGBTs,powertransistorsorthyristors.Forasetofswitchescontrolledappropriately（generallyapulsewithmodulation）,thesourceismodulatedtoobtainafrequencyACsignalthatwewant.

Therearetwotypesofinverters:

voltageinvertersandpowerinverters.

WealsodistinguishselfandnonautonomesUPSinverters :

Stand-aloneInverters

Anautonomousinverteroutputsavoltagewithafrequencyeitherfixedoradjustablebytheuser.Itdoesnotalwaysneedtorunelectricalnetwork,forexampleatravelconverterthatconnectstothecigarettelightersocketofacaruses12VDCvehicletogenerate120or230VAC50or60Hz;

TheinvertersareespeciallyusedforTVreceptioninmobilemode（satellitereceiverinamotorhomeforexample}）withoutinputlowvoltagepowersupply（~12V）.

Non-autonomousinverter

Anon-autonomousinverterisarectifiercircuitallthyristors（Graetzbridge）which,assistednaturalswitchingnetwork,whichitisconnected,enablesinverteroperation（egenergyrecoveryduringbrakingperiodsinelectricmotor）.AtthebaseofthedevelopmentofstaticvariablespeeddrivesforDCmotorsandACcyclo,powerinvertersforsynchronousandasynchronousmachines,uptopowersofseveralMW,thistypeofarrangementisgraduallysupplantedinfavorofGTOorIGBTconverters.

b.Basicdesigns

Inonesimpleinvertercircuit,DCpowerisconnectedtoatransformerthroughthecentertapoftheprimarywinding.AswitchisrapidlyswitchedbackandforthtoallowcurrenttoflowbacktotheDCsourcefollowingtwoalternatepathsthroughoneendoftheprimarywindingandthentheother.Thealternationofthedirectionofcurrentintheprimarywindingofthetransformerproducesalternatingcurrent（AC）inthesecondarycircuit.

Theelectromechanicalversionoftheswitchingdeviceincludestwostationarycontactsandaspringsupportedmovingcontact.Thespringholdsthemovablecontactagainstoneofthestationarycontactsandanelectromagnetpullsthemovablecontacttotheoppositestationarycontact.Thecurrentintheelectromagnetisinterruptedbytheactionoftheswitchsothattheswitchcontinuallyswitchesrapidlybackandforth.

Theswitchinthesimpleinverterdescribedabove,whennotcoupledtoanoutputtransformer,producesasquarevoltagewaveformduetoitssimpleoffandonnatureasopposedtothesinusoidalwaveformthatistheusualwaveformofanACpowersupply.UsingFourieranalysis,periodicwaveformsarerepresentedasthesumofaninfiniteseriesofsinewaves.Thesinewavethathasthesamefrequencyastheoriginalwaveformiscalledthefundamentalcomponent.Theothersinewaves,calledharmonics,thatareincludedintheserieshavefrequenciesthatareintegralmultiplesofthefundamentalfrequency.

Fourieranalysiscanbeusedtocalculatethetotalharmonicdistortion（THD）.Thetotalharmonicdistortion（THD）isthesquarerootofthesumofthesquaresoftheharmonicvoltagesdividedbythefundamentalvoltage:

c.Advanceddesigns

Therearemanydifferentpowercircuittopologiesandcontrolstrategiesusedininverterdesigns.Differentdesignapproachesaddressvariousissuesthatmaybemoreorlessimportantdependingonthewaythattheinverterisintendedtobeused.

Theissueofwaveformqualitycanbeaddressedinmanyways.Capacitorsandinductorscanbeusedtofilterthewaveform.Ifthedesignincludesatransformer,filteringcanbeappliedtotheprimaryorthesecondarysideofthetransformerortobothsides.Low-passfiltersareappliedtoallowthefundamentalcomponentofthewaveformtopasstotheoutputwhilelimitingthepassageoftheharmoniccomponents.Iftheinverterisdesignedtoprovidepoweratafixedfrequency,aresonantfiltercanbeused.Foranadjustablefrequencyinverter,thefiltermustbetunedtoafrequencythatisabovethemaximumfundamentalfrequency.

Sincemostloadscontaininductance,feedbackrectifiersorantiparalleldiodesareoftenconnectedacrosseachsemiconductorswitchtoprovideapathforthepeakinductiveloadcurrentwhentheswitchisturnedoff.TheantiparalleldiodesaresomewhatsimilartothefreewheelingdiodesusedinAC/DCconvertercircuits.

Hbridgeinvertercircuitwithtransistorswitchesandantiparalleldiodes

Fourieranalysisrevealsthatawaveform,likeasquarewave,thatisanti-symmetricalaboutthe180degreepointcontainsonlyoddharmonics,the3rd,5th,7th,etc.Waveformsthathavestepsofcertainwidthsandheightscanattenuatecertainlowerharmonicsattheexpenseofamplifyinghigherharmonics.Forexample,byinsertingazero-voltagestepbetweenthepositiveandnegativesectionsofthesquare-wave,alloftheharmonicsthataredivisiblebythree（3rdand9th,etc.）canbeeliminated.Thatleavesonlythe5th,7th,11th,13thetc.Therequiredwidthofthestepsisonethirdoftheperiodforeachofthepositiveandnegativestepsandonesixthoftheperiodforeachofthezero-voltagesteps.

Changingthesquarewaveasdescribedaboveisanexampleofpulse-widthmodulation（PWM）.Modulating,orregulatingthewidthofasquare-wavepulseisoftenusedasamethodofregulatingoradjustinganinverter'soutputvoltage.Whe