Power Electronics - Thyristor Commutation

This article gives brief overview on the different commutation techniques of SCR.

1. THYRISTORS
-COMMUTATION -
ER. FARUK BIN POYEN
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
[email protected]

2. Contents:
1.Commutation
2.Natural Commutation
3.Forced Commutation
4.Class A Commutation: Self Commutation by Resonating Load
5.Class B Commutation : Self Commutation by L –C Circuit
6.Class C Commutation : Complementary Commutation
7.Class D Commutation : Impulse Commutation
8.Class E Commutation : External Pulse Commutation
9.Class F Commutation : Line Commutation
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3. Commutation –Turning Off SCR
Toturnonathyristor,alowvoltage,shortdurationpulseisappliedtothegate(typically
4V,100µs).
Oncethethyristoristurned-on,thegatelosescontrolandthethyristorwillonlyturnoff
whentheloadcurrentfallsvirtuallytozero,orthethyristorisreversebiased.
ThethyristorwillturnoffnaturallywithA.C.suppliesasthevoltagereverses(whichis
calledasNaturalCommutation).
NosuchreversaloccurswithD.C.suppliesanditisnecessarytoforceavoltage
reversaliftum-offistooccur.ThisprocessiscalledForcedCommutation.
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4. Commutation
TheprocessofturningOFFSCRisdefinedas"Commutation".
Inallcommutationtechniques,areversevoltageisappliedacrossthethyristorduringthe
turnOFFprocess.
ByturningOFFathyristorwebringitfromforwardconductingtotheforwardblocking
mode.
TheconditionstobesatisfiedinordertoturnOFFanSCRare:
I
A<I
H(Anodecurrentmustbelessthanholdingcurrent)
AreversevoltageisappliedtoSCRforsufficienttimeenablingittorecoveritsblocking
state.
TherearetwomethodsbywhichathyristorcanbeturnedOFF.
NaturalCommutation
ForcedCommutation
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5. Natural Commutation
InACcircuit,thecurrentalwayspassesthroughzeroforeveryhalfcycle.
Asthecurrentpassesthroughnaturalzero,areverseVoltagewillsimultaneouslyappear
acrossthedevice.
ThiswillturnOFFthedeviceimmediately.
Thisprocessiscalledasnaturalcommutation,sincenoexternalcircuitisrequiredforthis
purpose.
ThismethodisonlyapplicableforA.Csupply.
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6. Forced Commutation
ToturnOFFathyristor,theforwardanodecurrentshouldbebroughttozerofor
sufficienttimetoallowtheremovalofchargedcarriers.
IncaseofDCcircuits,theforwardcurrentshouldbeforcedtozerobymeansofsome
externalcircuits.
Thisprocessiscalledasforcedcommutation.
WithD.C.supply,weuseexternalcircuitandactive/passivecomponentstoreduce
passingcurrent’svaluebelowholdingcurrent.
Thecircuitinvolvedinthisprocedureiscalled“commutationcircuit”.
Thecomponentsusedarecalled“commutatingcomponents”
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7. Natural Commutation vs Forced Commutation
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8. Class A: Self Commutated by a Resonating Load
ClassAisoneoffrequentlyusedthyristorcommutationtechniques.
Ifthyristoristriggeredorturnedon,thenanodecurrentwillflowbychargingcapacitorC
withdotaspositive.
Thesecondorderunder-dampedcircuitisformedbytheinductororACresistor,capacitor
andresistor.
IfthecurrentbuildsupthroughSCRandcompletesthehalfcycle,thentheinductor
currentwillflowthroughtheSCRinthereversedirectionwhichwillturnoffthyristor.
ThecapacitorvoltageisatitspeakwhentheSCRturnsoffandthecapacitordischarges
intotheresistanceinanexponentialmanner.
TheSCRisreverse-biasedtillthecapacitorvoltagereturnstothelevelofthesupply
voltageV.
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9. Class A: Self Commutated by a Resonating Load
Afterthethyristorcommutationorturningoffthethyristor,thecapacitorwillstart
dischargingfromitspeakvaluethroughtheresistorisanexponentialmanner.
Thethyristorwillbeinreversebiasconditionuntilthecapacitorvoltagereturnstothe
supplyvoltagelevel.
ThetimeforswitchingOFFtheSCRdependsontheresonantfrequencywhichisafactor
ofLandCcomponents.
Themethodisreliable,simpleandforhighfrequencyapplicationabove1000Hz.
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10. Class A: Self Commutated by a Resonating Load
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11. Class B: Self Commutated by an L-C Circuit
ThemajordifferencebetweentheclassAandclassBthyristorcommutationtechniques
isthattheLCisconnectedinserieswiththyristorinclassA,whereasinparallelwith
thyristorinclassB.
BeforetriggeringontheSCR,thecapacitorischargedup(dotindicatespositive).
IftheSCRistriggeredorgiventriggeringpulse,thentheresultingcurrenthastwo
components.
TheconstantloadcurrentflowingthroughtheR-Lloadisensuredbythelargereactance
connectedinserieswiththeloadwhichisclampedwithfreewheelingdiode.
IfsinusoidalcurrentflowsthroughtheresonantL-Ccircuit,thenthecapacitorCis
chargedupwithdotasnegativeattheendofthehalfcycle.
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12. Class B: Self Commutated by an L-C Circuit
ThetotalcurrentflowingthroughtheSCRbecomeszerowiththereversecurrent
flowingthroughtheSCRopposingtheloadcurrentforasmallfractionofthenegative
swing.
Iftheresonantcircuitcurrentorreversecurrentbecomesjustgreaterthantheload
current,thentheSCRwillbeturnedOFF.
Thiscommutationtechniqueismostlyusedforchoppercircuits.
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13. Class B: Self Commutated by an L-C Circuit
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14. Class C: C or L-C Switched by another Load
Carrying SCR
IntheabovethyristorcommutationtechniquesweobservedonlyoneSCRbutinthese
classCcommutationtechniquesofthyristortherewillbetwoSCRs.
OneSCRisconsideredasmainthyristorandtheotherasauxiliarythyristor.
InthisclassificationbothmayactasmainSCRscarryingloadcurrentandtheycanbe
designedwithfourSCRswithloadacrossthecapacitorbyusingacurrentsourcefor
supplyinganintegralconverter.
IfthethyristorT2istriggered,thenthecapacitorwillbechargedup.
IfthethyristorT1istriggered,thenthecapacitorwilldischargeandthisdischarge
currentofCwillopposetheflowofloadcurrentinT2asthecapacitorisswitched
acrossT2viaT1.
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15. Class C: C or L-C Switched by another Load
Carrying SCR
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Mainlyusedinsingle–phaseinverterswithcentretappedtransformers.
Viz.McMurrayBedfordinverter.
Itisusefulevenatfrequenciesbelow1000Hz.

16. Class C: C or L-C Switched by another Load
Carrying SCR
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17. Class D: L-C or C Switched by an Auxiliary SCR
TheclassCandclassDthyristorcommutationtechniquescanbedifferentiatedwiththe
loadcurrentinclassD:
ClassD:OnlyoneoftheSCR’swillcarrytheloadcurrentwhiletheotheractsasan
auxiliarythyristorwhereasinclassCbothSCRswillcarryloadcurrent.
Theauxiliarythyristorconsistsofresistorinitsanodewhichishavingresistanceof
approximatelytentimestheloadresistance.
BytriggeringtheTa(auxiliarythyristor)thecapacitorischargeduptosupplyvoltageand
thentheTawillturnOFF.
Theextravoltageifany,duetosubstantialinductanceintheinputlineswillbedischarged
throughthediode-inductor-loadcircuit.
IftheTm(mainthyristor)istriggered,thenthecurrentwillflowintwopaths:
commutatingcurrentwillflowthroughtheC-Tm-L-Dpathandloadcurrentwillflow
throughtheload.
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18. Class D: L-C or C Switched by an Auxiliary SCR
IfthechargeonthecapacitorisreversedandheldatthatlevelusingthediodeandifTais
re-triggered,thenthevoltageacrossthecapacitorwillappearacrosstheTmviaTa.
Thus,themainthyristorTmwillbeturnedoff.
UsedininvertersandJoneschoppeercircuit.
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19. Class D: L-C or C Switched by an Auxiliary SCR
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20. Class E: External Pulse Source for Commutation
FortheclassEthyristorcommutationtechniques,atransformerwhichcannotsaturate(as
itishavingasufficientironandairgap)andiscapabletocarrytheloadcurrentwithsmall
voltagedropcomparedwiththesupplyvoltage.
IfthethyristorTistriggered,thenthecurrentwillflowthroughtheloadandpulse
transformer.
Anexternalpulsegeneratorisusedtogenerateapositivepulsewhichissuppliedtothe
cathodeofthethyristorthroughpulsetransformer.
ThecapacitorCischargedtoaround1Vanditisconsideredtohavezeroimpedancefor
theturnoffpulseduration.
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21. Class E: External Pulse Source for Commutation
Thevoltageacrossthethyristorisreversedbythepulsefromtheelectricaltransformer
whichsuppliesthereverserecoverycurrent,andfortherequiredturnofftimeitholdsthe
negativevoltage.
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22. Class E: External Pulse Source for Commutation
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23. Class F: AC Line Commutated
InclassFthyristorcommutationtechniques,analternatingvoltageisusedforsupplyand,
duringthepositivehalfcycleofthissupply,loadcurrentwillflow.
Iftheloadishighlyinductive,thenthecurrentwillremainuntiltheenergystoredinthe
inductiveloadisdissipated.
Duringthenegativehalfcycleastheloadcurrentbecomeszero,thenthyristorwillturn
off.
Ifvoltageexistsforaperiodofratedturnofftimeofthedevice,thenthenegativepolarity
ofthevoltageacrosstheoutgoingthyristorwillturnitoff.
Here,thedurationofthehalfcyclemustbegreaterthantheturnofftimeofthyristor.
Thiscommutationprocessissimilartotheconceptofthreephaseconverter.
Letusconsider,primarilyT1andT11areconductingwiththetriggeringangleofthe
converter,whichisequalto60degrees,andisoperatingincontinuousconductionmode
withhighlyinductiveload.
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24. Class F: AC Line Commutated
IfthethyristorsT2andT22aretriggered,theninstantaneouslythecurrentthroughthe
incomingdeviceswillnotrisetotheloadcurrentlevel.
Ifthecurrentthroughtheincomingthyristorsreachestheloadcurrentlevel,thenthe
commutationprocessofoutgoingthyristorswillbeinitiated.
Thisreversebiasingvoltageofthyristorshouldbecontinueduntiltheforwardblocking
stateisreached.
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25. Dynamic Turn OFF Switching Characteristics
ThetransitionofanSCRfromforwardconductionstatetoforwardblockingstateiscalled
asturnOFForcommutationofSCR.AsweknowthatoncetheSCRstartsconducting,the
gatehasnocontroloverittobringbacktoforwardblockingorOFFstate.
ToturnOFFtheSCR,thecurrentmustbereducedtoalevelbelowtheholdingcurrentof
SCR.
VariousmethodsarediscussedtoturnOFFtheSCRinwhichSCRturnOFFisachieved
byreducingtheforwardcurrenttozero.
ButiftheforwardvoltageappliedimmediatelyafterthecurrentzeroofSCR,itstarts
conductingagainevenwithoutgatetriggering.
Thisisduetothepresenceofchargecarriersinthefourlayers.
Therefore,itisnecessarytoapplythereversevoltage,overafinitetimeacrosstheSCRto
removethechargecarriers.
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26. Dynamic Turn OFF Switching Characteristics
HencetheturnOFFtimeisdefinedasthetimebetweentheinstanttheanodecurrent
becomeszeroandtheinstantatwhichtheSCRretainstheforwardblockingcapability.
TheexcesschargecarriersfromthefourlayersmustberemovedtobringbacktheSCRto
forwardconductionmode.
Thisprocesstakesplaceintwostages.
Inafirststageexcesscarriersfromouterlayersareremovedandinsecondstageexcess
carriersintheinnertwolayersaretoberecombined.
Hence,thetotalturnOFFtimetqisdividedintotwointervals;reverserecoverytimetrr
andgaterecoverytimetgr.
t
q= t
rr+ t
gr
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27. Dynamic Turn OFF Switching Characteristics
ThefigurebelowshowstheswitchingcharacteristicsofSCRduringturnONandOFF.
Thetimet1tot3iscalledasreverserecoverytime;attheinstantt1theanodecurrentis
zeroandbuildsupinthereversedirectionwhichiscalledasreverserecoverycurrent.This
currentremovestheexcesschargecarriersfromouterlayersduringthetimet1tot3.
Atinstantt3,junctionsJ1andJ3areabletoblockthereversevoltagebut,theSCRisnot
yetabletoblocktheforwardvoltageduetothepresenceofexcesschargecarriersin
junctionJ2.
Thesecarrierscanbedisappearedonlybythewayofrecombinationandthiscouldbe
achievedbymaintainingareversevoltageacrosstheSCR.
Hence,duringthetimet3tot4,therecombinationofchargestakesplaceandattheinstant
t4,junctionJ2completelyrecovers.
Thistimeiscalledgaterecoverytimetgr.
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28. Dynamic Turn OFF Switching Characteristics
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29. Dynamic Turn OFF Switching Characteristics
FromthefiguretheturnOFFtimeisthetimeintervalbetweenthet4andt1.
Generally,thistimevariesfrom10to100microseconds.ThisturnOFFtimetqis
applicabletotheindividualSCR.
Thetimerequiredbythecommutationcircuittoapplythereversevoltagetocommutate
theSCRiscalledthecircuitturnOFFtime(tc).
Forasafetymarginorreliablecommutation,thistcmustbegreaterthanthetqotherwise
commutationfailureoccurs.
TheSCRswhichhaveslowturnOFFtimeasinbetween50to100microsecondsare
calledasconvertergradeSCRs.
Theseareusedinphasecontrolledrectifiers,cycloconverters,ACvoltageregulators,etc.
TheSCRswhichhavefastturnOFFtimeasinbetween3to50microsecondsareinverter
gradeSCRs.
Thesearecostliercomparedtoconvertergradeandareusedinchoppers,force
commutatedconvertersandinverters.
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30. References:
1.Power Electronics A to Z: Turning Off SCR (Commutation)
2.http://www.electronicshub.org/scr-turn-off-methods/
3.www.pantechsolutions.net/powerelectronics-tutorials
4.www.elprocus.com/classification-of-thyristor-commutation-methods/
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