2022-26-0010Published26May2022

DesignandSimulationofIsolatedAC-DCFlyBackConversionSystemforHighEnergyIgnitionUnitofGasTurbineEngines

AbarnaJ,PoonamKumari,andVishwanathaANGasTurbineResearchEstablishmentGTRE

Citation:Abarna,J.,Kumari,P.,andVishwanatha,A.N.,“DesignandSimulationofIsolatedAC-DCFlyBackConversionSystemforHighEnergyIgnitionUnitofGasTurbineEngines,”SAETechnicalPaper2022-26-0010,2022,doi:10.4271/2022-26-0010.

Received:22Nov2021Revised:24Feb2022Accepted:07Mar2022

Abstract

A

highenergyignitionsystemisusedinthegasturbineenginetoprovidedesiredheatenergywhichignitesthefuelandcompressedairpassingthroughthecombustionchamber.Thehighenergyoutputoftheignitionsystemdependsonthesuitableselectionofconvertermecha-nism.Therefore,anAC-DCflybackconvertermechanismisusedintheexciterunitwhichsupplieshighinputvoltage(3000V)tothegasdischargetube.Thisconverterdesign

providesinputandoutputsideelectricalisolationandassuresthesafetyofthesystem.Also,byincorporatingthiscircuitwiththeignitioncoil,thedesiredoutputof3-6joulesandsparkrateof200to350microsecondscanbeachieved.ThispaperdiscussestheproceduresinvolvedinestimatingtheparametersfordesignofflybackconversionmechanismforHEIunitandsimulatingthedesignedcircuitusingMATLAB/SIMULINKforperformanceanalysisandfurtheroptimization.

Introduction

A

gasturbineengineconsistsofthreemajorfunctionalblocksresponsibleforthesuccessfuloperationoftheentiresystem.Thethreemajorpartsarethecompressor,combustorandturbine.Themainareaoffocusisontheenginestartingsystem(HEI).Incombustion,theheatenergyisusedtoburnthehigh-pressureairandfuelmixtureisprovidedbymeansoftheignitionsystem

[1

].

Ignitionsystemprovidescontinuouscombustionprocessbysupplyingtheheatinputthatreleasesenergyintotheengineworkingcycle.Onceignited,fuelburnscontinuouslytilltheengineisshutoff.Toextinguishthecombustionprocess,thefuelsupplyiscut-off.Theelectricsparkgeneratedfromtheexciterisusedforignitingthefuel.Oncetheself-sustainingspeedisachievedtheignitionwillbecutoff.HEIunitnotonlystartstheenginebutalsohelpsinstandbyprotection.Therefore,thecontinuousignitionismandatoryfortheenginetorelightthefuel,preventingtheenginefromstoppingwhentheflameoutorunstableoperationofflightoccurs.Duringsuchconditions,oneormoreigniterplugsareoperatedcontinuouslywhichcausestheenginetorelightautomaticallyafterfuelcontrolcompensationoccurs.Whenignitionisusedasaprecautionagainstflameout,theprescribedignitionusetimelimitationsmustbeobserved.Thispreventstheignitionexciterunitcomponentsandenhancesthelifespanofthesystem[

2

].

TypesofElectricIgnition

System

AllgasturbineengineswillhavetwoHEIunitsforenginestarting,astheremightbeachanceofsystemfailureduetoexternalfactors.Electricignitionsystemhastwounits:ExciterandIgniterunits.Eachexciterunitfeedsaseparateigniterplug.Theexcitercircuitisacombinationofresistor,capacitorcircuitwithswitchingdevicesortransformertogeneratethehighvoltageoutput.TheinputtotheexciterunitmaybeeitherACorDCVoltagerespectively.Thevoltagegeneratedbymeansoftheelectrical/mechanicalswitchingisfedtotheGDTandthentotherespectiveignitioncoilviastorageandionizationcapacitor.Thehighvoltagesparkgeneratedattheignitioncoilwithrespecttothecharginganddischargingofthecapacitor(capacitordischargetypeignition)andpassedtotheigniterplugattachedtothecombustionchamber[

2

].Thedifferenttypesofelectricignitionsystembasedonitsmechanismisdiscussedbelow.

TremblerMechanism

Thebasictremblermechanismisswitchingofsupplywiththehelpofelectromechanical/electromagneticswitch.Thisdevelopsafluxchangeinprimarycoiloftheignition

TransformeroperatedHEIsystem[

7

].

2

DESIGNANDSIMULATIONOFISOLATEDAC-DCFLYBACKCONVERSIONSYSTEMFORHIGHENERGYIGNITIONUNIT

transformerwhichgeneratesthedesiredhighvoltageatthesecondaryofthetransformerasshownin

Figure1

.

Aninductioncoiltransformeroperatedbythetrembledactionchargesthestoragecapacitorthroughahighvoltagerectifier.WhenthecapacitorvoltageisequaltothebreakdownvoltageofGDT,theenergyisreleasedacrosstheplug[

2

].Achokeattachedtothismoduleextendsthedischargedurationandanyresidualenergystoredinthecapacitorisdischargedinthedischargeresistorwithin60secondsafterswitchoff.Asafetyresistorisusedtoensurethesystemisolationfromhightensionlead.

TransistorSwitching

Mechanism

ForHEIsystemwithDCvoltageasinput,thesemiconductordevicecanbeusedastheignitionswitchasshownin

Figure

2

.Whentheignitionswitchisin‘ON’conditionandtheswitchisinclosedstate,currentflowsfromthesupplytotheprimarywindingthroughthetransistordevice.Whentheswitchisopen,currentflowintheprimarywindingstops,causingthemagneticfieldgeneratedintheprimarywindingtocollapseandcutacrossthesecondarywinding.ThisinducesahighvoltageandcurrentinthesecondarywindingwhichchargesthestoragecapacitorconnectedwiththeGDT.OncethecapacitorvoltageisequaltotheGDTbreakdownvoltage,highvoltagesparkisgeneratedintheignitioncoil[

3

].

FIGURE1

TremblerMechanismCircuit[

7

].

FIGURE2

TransistoroperatedHEIsystem[

7

].

TransformerMechanism

MostoftheabovediscussedmechanismsaresuppliedwithDCvoltage.Byconsideringtheaerospaceapplications,theACpowersupplyisfedfromtheenginedrivenorselfdrivenelectricalmachineandsuppliedtotheHEIexcitersystemasshownin

Figure3

.Duetothesupplyofalternatingcurrenttothetransformerprimarywinding,changeinfluxoccursonthesecondarywindingwhichisusedtochargethestoragecapacitor.Similarcharginganddischargingactionofcapac-itorasinabovemechanismiscarriedouttogeneratethehighenergyoutput[

4

].

FlyBackConversion

Mechanism

TheDC-DCVoltageconversionprincipleisusedinhighenergyconversionsystems.However,theflybackconversionisusedinlowpowerapplications;thiscanbeusedforhighpowerapplicationbyidentifyingthesuitabledesignparam-eters.Thebasiccircuitofflybackconverterisshowninthe

Figure4

.ThemainadvantageofflybackconverteristhehighfrequencytransformerwhichreducesthesizeoftheunitaswellasprovidethegalvanicisolationbetweentheinputandoutputsideofHEIunit.

WhentheMOSFETswitchisconducting,thesupplyflowsfromthebridgerectifiertotheprimarywindingleadingtogradualincreaseofprimarycurrentwhichalsodependsontheinductanceoftheprimarycoilandtheenergystoredin

FIGURE3

FIGURE4

FlybackConversionSystem[

7

].

ConductionduringMOSFETTurnONTime[

7

].

DESIGNANDSIMULATIONOFISOLATEDAC-DCFLYBACKCONVERSIONSYSTEMFORHIGHENERGYIGNITIONUNIT

3

thetransformercoreduringtheturnONtime.FortheentireONtimeoftheswitch,thediodeonthesecondarysideofthetransformerwillbereversebiasedwithrespecttothedotconversionprincipleandthereisnoenergytransferfromtheinputsidetotheoutputside[

5

]asshownin

Figure5

.

WhentheMOSFETswitchisnotconducting,thentheenergyaccumulatedintheprimarycoilistransferredtothesecondarycoil.As,theinputsideisopencircuitedthereisnomeansofcurrentflowintheprimaryside,thevoltageisinducedinthesecondarycoilduetothedotconversionprin-cipleandthediodeisforwardbiasedanddeliveredtothestoragecapacitor.OncethecapacitorvoltageisequaltotheGDTbreakdownvoltage,highvoltagesparkisgeneratedintheignitioncoil[

5

]asshownin

Figure6

.

Theretwodifferentmodesofoperationinflybackconver-sion.Theyare

1.Continuousconductionmode

2.Discontinuousconductionmode

Incontinuouscurrentmode,alltheenergystoredintheprimaryisnotfullytransferredtothesecondary,thereissomeresidualenergypresentinit.ThiscausestheflowofcurrentthroughouttheONperiodandpreferredforhighpowerapplications.

Indiscontinuouscurrentmode,alltheenergystoredintheprimaryisfullydissipated,causingthesecondarycurrent

FIGURE5

FIGURE6

ConductionduringMOSFETTurnOFFTime[

7

].

FIGURE7

FlybackConverterWaveform.

toreachzerobeforetheswitchingtime.Thismodeofopera-tionispreferredinlowpowerapplications.Thewaveformfortheflybackconverter[

6

]isshownin

Figure

7.

ProposedArchitecture

Foraerospaceapplication,supplytotheignitionsystemwillhavehighfrequencyACsupply(400Hz).Oncethesignalfromtheenginecontrollerisreceived,thesupplywillbegiventotheignitionsystem.Fromtheabovediscussionitisclearthatthetransformeroperatedsystemissuitable.Byconsideringtheefficiency,highfrequencyofoperationandtemperatureconditions,combinationoftransistorandtransformeroperatedcircuitshouldbedesigned.Consideringtherequire-mentofcombinedconverterandaeroapplication,flybackconvertermechanismischosen;asitdoesnotrequireaddi-tionalisolationandprovidesthesafetyofinputandoutputsystemblocksasshownin

Figure8

.

Theinputsupplyvoltagefromstaticinverter/ignitionalternatorof110to115VsinglephaseACsupplyisgiventotherectifierunitinwhichACvoltageisconvertedtoDCsupplyusingthefullwavebridgerectifier.TherectifiedDCoutputvoltageisfedtothefilterblockwhichfilterstheripplesandnoisesgeneratedintheoutput.Thefilteredvoltageisgiventotheisolatedflybacktransformeroperatedwithsolidstatesemiconductingswitch(MOSFET).Theswitchingofsolid-statedeviceistriggeredwithPWMgatesignalwithrespecttotheoutputvoltageanditsswitchingfrequencyrangesfrom1.5kHzto2.5kHz[

5

].Theflybacktransformerusedhereisastep-uptransformerwhichstorestheenergyinthecoilunlessthesupplyiscut-off.Thestoredenergyinthecoilisdissipatedtothestoragecapacitor.Thischarginganddischargingofcapacitorgeneratehighvoltageintheignitioncoil.

ExciterCircuitwithFlybackConversionMechanism[

7

].

4

DESIGNANDSIMULATIONOFISOLATEDAC-DCFLYBACKCONVERSIONSYSTEMFORHIGHENERGYIGNITIONUNIT

FIGURE8

BlockDiagramoftheProposedArchitecture.

DesignConsiderationforFly

Back-ExciterCircuit

Ingeneral,theignitionexcitercircuitwillhaveatransistor-basedswitchwhichistriggeredwiththehelpoffeedbackcurrentcontrolcircuit.However,anadditionalisolationbetweentheinputandoutputmustbeprovided,asthecircuitoperateswithhighvoltageandvarioustemperaturecondi-tions.ToovercometheselimitationsandcircuitissimplifiedwithflybackconverteroperatedwithPWMgateswitchingconditions.Thecircuitshownin

Figure9

istheproposedarchitectureandthecircuitdesigndependsonthefollowingprocedures.

1.Determinationofpowersupplyspecifications.

2.Designandselectionofperipheralparts.

3.EvaluationofSelectedparameters.

Thepowersupplytotheenginestartingsystemismostimportantfactorindesigningthecircuit.Aspertheaerospaceapplications,itisdefinedthat110VAC,400Hzor28VDCissuppliedtothesystem.TheproposedarchitectureisforACinputsupplywhichisfedfromtheignitionalternatororaircraftbatterythroughstaticinverterrespectively.Theperipheralpartsarebridgerectifier,filtercircuit,highvoltage

dioderectifier,powerMOSFET,flybackinductioncoupledtransformer,gatedrivercircuit.Thesepartsshouldwithstandthehighvoltage,temperatureandvibrationconditionsaspertheaeromilitarystandards.Therefore,thecompletesystemisenclosedinsideahermeticallysealedcasing.

Thebridgerectifierhaving90%efficiencydelivers90-100VDCsupply.TheconstantDCsupplyispassedtotheflybackmechanismcircuitthroughRLCfiltertoeliminatetheharmonicspresentintherectifieroutput.Ahigh-powern-typeMOSFETperformstheswitchingoperationwithrespecttothecontrolsignalfromthePWMcontroller.Onthesecondarysideofthetransformerisahighvoltagedioderectifierwhichisolatestheflybackoperationfromthehighvoltageoutputofignitioncoiltransformer.Now,themostimportantpartoftheproposedsystemisthedesignofflybackmechanismcircuit.Whiledesigningthetransformer,itisnecessarytodeterminethemodeofoperationoftheswitchingdevice.ByConsideringalltheparametersdiscussedabove,themostsuitablemodeofoperationiscontinuousconductionmode.

DesignofFlyBackMechanism

forExciterCircuit

TheflybackconverterinvolvestheoperationofpowerMOSFETandcoupledtransformer.Thecoupledtransformerconsistsofprimary,secondaryandflybackcoilwhosevoltageandinductancetobedeterminedasfollows.

Theflybackvoltageisdeterminedbasedonthestabilitybetweenthethresholdvoltage(breakdownvoltage)ofthepowerMOSFETandhighvoltagediodeatthesecondarysideofthetransformer.

Vfb=n(Vo+Vs)(1)

BasedonthepowerMOSFETwaveform,thedesignvaluescanbefound.

Vds=Vfb+Vin(max)+Vsgm(2)

FIGURE9

MOSFETVdsWaveform.

DESIGNANDSIMULATIONOFISOLATEDAC-DCFLYBACKCONVERSIONSYSTEMFORHIGHENERGYIGNITIONUNIT

5

(3)

Vd=Vo+))|Vin(max)+Vsgd

Where,Vds-MOSFETDrainSourceVoltage(V)Vsgm-MOSFETGateSurgeVoltage(V)

Vfb-FlybackVoltage(V)

Vin(max)-ExciterInputVoltage(V)

Vs-FlybackTransformerSecondaryVoltage(V)Vd-VoltageacrossHVRectifierDiode(V)

Theturnsratioofthecoupledtransformerandthedutycycleoftheswitchingdevicearedeterminedfromtheflybackvoltage.

(4)

n==

Where,n-TurnsRatioofFlybackTransformerNp-NumberofTurnsinPrimaryCoil

Ns-NumberofTurnsinSecondaryCoilTurns

D=fb

(5)

V

infb

V+V

D-MOSFETDutyCycle

Basedonthemodeofoperation,theprimarywindinginductancecanbemanipulated.Forcontinuousmodeofoperation,theprimaryinductanceisgivenby

(6)

T=D

ONfsw

(7)

in

L=ViTNfsw

p2P

Where,TON-MOSFETTurnONTimePeriod(μs)

Lp-PrimaryCoilInductance(mH)

fsw-MOSFETSwitchingFrequency(Hz)

Pin-ExciterInputPower(VA)

Themaximumcurrentflowingthroughtheprimarywindingdependsontheprimaryinductance,maximumTondurationwithrespecttotheoverloadprotection.

FIGURE10

Vin(min)TON(max)

p

(8)

Ip(peak)=L

Where,Ip(peak)-PeakPrimaryCoilCurrent(A)

ThenumberofturnsinprimaryandsecondarysideoftransformercanbedeterminedusingIpandLp.

(9)

snBmAe

N=LpIp(peak)

(10)

Np=nNs

Where,Np-NumberofTurnsinPrimarySideNs-NumberofTurnsinSecondarySide

Thenumberofturnsintheflybackcoilcoupledwiththetransformerdependsontheinputvoltagetotheprimarycoilandtheoutputcurrentandcalculatedasfollows;

Nfb=|(VoVs)|Ns

(11)

(V)

Nfb-NumberofTurnsinFlybackCoil

Thedesignparametersforthesimulationofflybackconversionmechanismestimatedfrom

(1)

to

(11)

islistedin

Table1

.

ThePWMsignalisgeneratedandcontrolledbythe555-timercircuitandsuppliedasagatesignalintheMATLABcircuitsimulation.

MATLABSimulation

ResultsandDiscussion

Basedonthedesigndataestimated,theflybackexcitercircuitissimulated[

7

]asshownin

Figure11

andtheresultsaretaken.

Fromthesimulationresultobservationsshownin

Figure

12

,theconversionofAC-DCinputandoutputisachievedwiththebridgerectifier.Also,in

figure13

and

14

,themaximumvoltageof20000to25000Visgeneratedfromthesecondarysideoftheflybackconverter.So,thediodereversevoltageshouldbemorethanorequalto10kVtoprotecttheleakagecurrentorvoltagefromtheGDTorstoragecapacitortothetransformerwhosevoltagevalueisintherangeof2500to3000V.

Hence,thecapacitorchargingcanbeachievedwithinthedesireddurationof350μS.TheGDTstartsconductingoncethecapacitorchargestoitsbreakdownvoltageof3000Vconnectedtotheresistiveloadasobservedin

Figure15

.

SimulationDiagramofProposedArchitecture[

7

].

6

DESIGNANDSIMULATIONOFISOLATEDAC-DCFLYBACKCONVERSIONSYSTEMFORHIGHENERGYIGNITIONUNIT

TABLE1DesignParametersforSimulation

ConverterCircuitDesignData

Estimated

InputSpecifications

ACInputVoltage

ACInputPower

ACInputCurrent

InputFrequency

DiodeBridgeRectifier

OutputVoltage

Efficiency

Filter(RLC)

Resistance

Inductance

Capacitance

FlybackConverter

FlybackVoltage

MOSFETDrainSourceVoltage

MOSFETGateSurgeVoltage

VoltageacrossPrimarycoil

VoltageacrossSecondarycoil

VoltageacrosstheHVRectifierDiode

TurnsRatio

DutyCycle

TurnONperiod

MOSFETSwitchingFrequency

PrimaryCoilInductance

PeakPrimaryCurrent

NumberofTurnsinthePrimaryCoilNumberofTurnsintheSecondarycoilNumberofTurnsintheFlybackcoilPeakInverseVoltageofHVDiodeStorageCapacitor

GasDischargeTube

SparkVoltagegap

BreakdownVoltage

ResistiveLoad

90-115V,1?

230VA2.5-3A

400HZ

90-100VDC

90%

1.5KΩ

2.43mH

10pF

350V

490V

30V

100V

25KV

18KV0.0125

76-80%

304-320pS1.5-2.5KHZ5.02mH5.732A

2

160

1

10KV1.25pF

25V

2800V

3KΩ

Sl.No.

1

2

3

4

5

6

7

FIGURE11

FIGURE12

InputVoltageandRectifierOutputVoltage[

7

].

FIGURE13

MOSFETWaveform[

7

].

Conclusions

Thedesiredsimulationresult[

6

]fortheAC-DCflybackconverterisobtainedfromtheabove-mentioneddesignproce-durewiththemaximumoutputvoltageof22kV.Also,thesystemprovidesgalvanicisolationbetweenthesecondaryside

ISSN0148-7191

DESIGNANDSIMULATIONOFISOLATEDAC-DCFLYBACKCONVERSIONSYSTEMFORHIGHENERGYIGNITIONUNIT

7

FIGURE14

Voltage[

7

].

FlybackTransformerPrimaryandSecondary

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3.Mashkour,M.A.andAhmed,A.Y.,“InvestigationofSparkIgnitionEngineMathematicalModelUsingMATLAB(GUI),”AdvancesinNaturalandAppliedSciences11,no.11:36-50.

4.John,R.F.andSontag,B.,BothofJacksonville,Fla.,Frederick.IgnitionSystemofaTurbineEngine.Ed.Frusandetal.UnitedStatesofAmericaPatent5,561,350.

5.Holden,H.,“GasTurbineEngines,Aviation&RocketMotorExciters,”2014.

6.FujiElectricCo.Ltd.,“FlybackTransformerDeignInstructions,”July2016,DT_Fly-Back_TransformerAN-174ERev1.0.

7.MATLAB,“