Dual converters (1)

its about duel converter

1. Dr. Tejas N. Zaveri
Pro. Rashmi K. Patel
DUAL CONVERTER

2. INTRODUCTION OF DUAL CONVERTER
•DualConverterisanElectronicDeviceorCircuitmadebythe
combinationoftwobridges.OneofthemworksasRectifier(Converts
A.C.toD.C.)andotherbridgeworksasInverter(convertsD.C.into
A.C.).Thusanelectroniccircuitordeviceinwhichtwoprocessestake
placeatsametime,isknownasDualConverter.

3. PRINCIPLE OF DUAL CONVERTER
•Inthissimplifiedrepresentation,assumptionismadethatthedualconvertersareideal
andtheyproducepureDCoutputterminals.Asshowninfig.1,eachtwo-quadrant
converterisassumedtobeacontrollabledirectvoltagesource,connectedinseries
withadiode.DiodeD1andD2representtheunidirectionalcurrentflow
characteristicsoftheconverters.Thecurrentinloadcircuitcan,however,flowin
eitherdirection.

4. SINGLE PHASE DUAL
CONVERTER
•Adualconvertermaybesinglephaseor3phasedevice.Thesimplediagram
givenaboveisofsinglephasedualconverter.
Fig. (2) Single phase Dual converter

5. •Thedifferencebetweensinglephaseandthreephasedualconverter
isjustthatinThreephaseweusesthreephaserectifieratfirststage,
whileinsinglephasedualconverterwemakeuseofsinglephase
rectifiercircuitatfirstbridge.
•Asexplainedabovethatinsinglephasedualconverterweusessingle
phaserectifiercircuitforconvertingsinglephaseA.C.intosteadyD.C.
BridgeNo.1consistsofRectifier.thentherectifiedD.C.fedtoafilter
whichremovespulsesfromrectifiedD.C.andconvertsittoapure
D.C.byfiltering.Afterthat,thispureD.C.isfedtoloadandfromload
itisgiventoinvertercircuitwhichconvertsthisD.C.toA.C.andfinally
thisA.C.ofinvertertakenasoutput.

6. THREE PHASE DUAL CONVERTER
•Inthreephasedualconverter,wemakeuseofthreephaserectifier
whichconverts3phaseA.C.supplytoD.C.Therestoftheprocessis
sameandsameelementsareused.Theoutputofthreephase
rectifierisfedtofilterandafterfilteringthepureD.C.isfedtoload.At
lastthesupplyfromloadisgiventolastbridgethatisinverter.Itdo
theInvertprocessofrectifierandconvertsD.C.into3phaseA.C.
whichappearsatoutput.
Fig. (3) Three Phase Dual Converter

7. PRACTICAL DUAL CONVERTERS
•Ifthefiringanglesofthetwofullconverterareadjustedsuchthatα1+α2=
180º,andboththeconvertersoperatesimultaneously,thentheywill
producethesameaverageterminalvoltages,whichwillbeequaltothe
averagedcloadterminalvoltage.
•Duetothisvaluesoffiringangles,oneconverterwillbeintherectification
modeandtheotherintheinversionmode.
•Howeverinthe“NonIdealDualConverter,”(Practical)eachconverter
producearipplevoltagesattheoutputsofthetwoconvertersaremostlyout
ofphase.Inthewayeventhoughtheaverageoutputvoltagesofthetwo
convertersareequal,theirinstantaneousvaluesarenodequal.
•Duetothisfactwhentheoutputsofthetwoconvertersareconnected
togethertheinstantaneousvoltagedifferenceattheterminalsofthetwo
converterswillproducealargecirculatingcurrentbetweenthetwo
converters.
•Thiscurrentwillnotflowthroughtheload.Itisnecessarytocontrolthis
circulatingcurrentbysomemeanstoprotecttheSCRsconnectedinthe
convertercircuits.

8. •Twomethodsthatarecommonlyusedtocontrolthecirculatingcurrent
areasfollows:
1.Operationwithoutcirculatingcurrent.
2.Operationwithcirculatingcurrent.
1. Operation without circulating current(Non circulating current
mode)
In this mode of operation the flow of circulating current is not
allowed at all by operating only one converter at a time. In this mode
the converter which is operating will carry the entire load current. The
other converter is temporarily kept in its OFF state.(firing pulse are
not applied to it).

9. Fig:4 Dual Converter in Circulating Current Mode
2. Operation with circulating current(Circulating current mode):
Inthismodeofoperation,themagnitudeofcirculatingcurrentis
controlledbelowasafevalue.Thisisachievedbyconnectinga
circulatingcurrentreactorbetweentheoutputterminalsofthetwo
convertersasshowninfigure.

10. DUAL CONVERTER WITH CIRCULATING
CURRENT
•In this mode of operation, both the converters are turned on simultaneously, the
firing angles are adjusted by the control voltage Vcin such a way that (α1+ α2) = 180º
•The average voltage produced by both the converters will be same, but their
instantaneous values will be different. The circulating current will flow due to this
potential difference.
•The circulating current is limited by a circulating current reactor.
Advantages
•Both the converters remain in conduction continually, irrespective of whether the
load current is continuous or not..
•The dual converter responds fast if the converters are in continuous conduction.
Disadvantage
•The disadvantage is that reactor has to be used. The reactors are costly and bulky
at high power levels.

11. Waveforms for circulating current mode

12. Fig:5 Waveforms for circulating current mode

13. OPERATION
•Theoperationofthedualconverterwithcirculatingcurrentisexplainedwiththe
followingassumptions:1.Therectorislossless.2.Thefiringanglesofthetwo
convertersarecontrolledsothattheirsumis180º.[(α1+α2)=180º].
•Thewaveformsforthecirculatingcurrentmodeareasshowninfig.(5)Thefiring
anglefortheconverterIisα1=60º,thereforefiringangleforthesecondconverter
α2=180-60=120º.
•Becauseofthecirculatingcurrent,boththeconvertersarekeptintheonstate,at
thenoloadandwithloadconditions.Hencethevoltagewaveformsarewelldefined,
asshowninfig.(5)
•Thesupplyvoltageandtheconverteroutputvoltagesareasshowninfig.(5)The
instantaneousdcoutputvoltageistheaverageoftheinstantaneousconverter
voltages.
•Thisoutputvoltagehasashapedifferentfromthatoftheconverteroutputvoltages.
Buttheaveragevalueoftheoutputvoltageisthesameastheaveragevalueof
individualconvertervoltage.
•Thevoltageacrossthecirculatingcurrentreactor,asshowninfig.(5)isthedifferent

14. DUAL CONVERTER WITHOUT CIRCULATING
CURRENT MODE
•Inadualconverterwithoutcirculatingcurrentoperatingmode,theflowof
circulatingcurrentiscompletelyinhibitedthroughautomaticcontrolofthe
firingpulses,sothatonlythatconverterwhichcarriestheloadcurrentisin
conductionandtheotherconverteristemporarilyblocked.Sinceonlyone
converteroperatesatatimeandtheotherisinblockingstate,noreactoris
requiredbetweentheconverters.
•Ataparticularinstant,supposeconverter1isoperatingasarectifierandis
supplyingtheloadcurrentwhilepulsestosecondconverterareblocked.For
theinversionoperation,converter1isfirstblockedbyremovingitsfiring
pulsesandloadcurrentisreducedtozero.Converter2ismadetoconduct
byapplyingthefiringpulsestoit.Thecurrentinconverter2wouldnowbuild
upthroughtheloadinthereversedirection.Solongasconverter2isin
operation,converter1isintheblockingstatesincethefiringpulsesare
withdrawnfromit.

15. •Thepulsestoconverter2areappliedafteradelaytime(current-freesafetyinterval)
of10to20ms.Thisdelaytimeensuresreliablecommunicationofthrustersin
converter1.Iftheconverter2istriggeredbeforetheconverter1hasbeen
completelyturned-off,alargecirculatingcurrentwouldflowbetweenthetwo
converters.
•IrregularjumpsintheleveloftheDCterminalvoltageatthepointofcurrent
reversalmustbeavoidedinordertoachieveasmoothchangeoverofcurrentfrom
oneconvertertotheother.Thusthefiringpulsecontrolshould,ideally,besuchthat
themeanDCterminalvoltageoftheconverter2,attheinstantofcurrentreversal,is
thesameasthatoftheconverter1.
•Fromtheabovediscussion,itbecomesclearthatsuchamodeofoperationrequires
sophisticatedcontrolsystemwhichautomaticallyblocksandunblockstheindividual
convertersinaccordancewiththedirectionofloadcurrentsuitablywithasafety
intervals.

16. APPLICATIONS OF DUAL CONVERTER
•Dualconvertersaremostlyusedatindustrieswherewerequires
reversibleD.C.GenerallyDualConvertersareusedforSpeedControl
ofD.C.Motorsetc.