Analysis of various control schemes for minimal Total Harmonic Distortion in cascaded H-bridge multilevel inverter

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ScienceDirect

JournalofElectricalSystemsandInformationTechnology3(2016)428–441

Analysis

of

various

control

schemes

for

minimal

Total

Harmonic

Distortion

in

cascaded

H-bridge

multilevel

inverter

Janardhan

Kavali

∗

,

Arvind

Mittal

EnergyCentre,MaulanaAzadNationalInstituteofTechnology,Bhopal,MadhyaPradesh462051,India

Received12August2015;receivedinrevisedform5January2016;accepted18January2016 Availableonline3August2016

Abstract

Multilevelinvertersarebecomingmorepopularinthepowerconversionsystemsforhighpowerandpowerqualitydemanding applications.TheMATLABbasedsimulationonSIMULINKplatformispresentedfortheSinglePhasefivelevelcascadedH-bridge MultilevelInverter(CHB-MLI)topologywithlessnumberofswitchesandwithdifferentcontrolschemesandSinusoidalPulse WidthModulation(SPWM)schemes.Adetailedcomparisonofvariouscontrolschemesispresentedinthispaperwithreference toTotalHarmonicDistortion(THD)intheoutputvoltageandutilizationfactorofthepowerdevices.Itisobservedthatamongall thecontrolschemes,theTHDisminimumintheSinusoidalPulseWidthModulation-PhaseDisposition(SPWM-PD)schemewith variablecarrierwavemagnitude.

Keywords:Multilevelinverter(MLI);Reducednumberofswitches;ControlschemesforCHB-MLI;SPWMcontrolschemes;MinimalTHD

1. Introduction

Multilevelinverters have gainmoreattention in the fieldof highvoltage andmedium power applicationsdue totheir advantages, such as low voltagestress on power semiconductor devices, low harmonic distortions, good electromagneticcompatibility, reducedswitching lossesandimprovedreliability onfaulttolerance.Therefore,the multilevelinvertersalsohavelowerdv/dtratiotopreventinductionordischargefailuresontheloads(Govindarajuand Baskaran,2010).Recentlythestudiesaregoingontoapplythemultilevelinvertersforlowvoltageapplicationssuch asintheuninterruptedpowersupply(UPS)andpowerinverterforsolarphotovoltaicsystem(PV)(KavaliandMittal, 2014).AsthenumberoflevelinMultilevelInverterincreases,theTotalHarmonicDistortionintheoutputvoltagegoes onreducing(Colaketal.,2011).Thedisadvantageofincreasingthenumberoflevelsisthatitincreasesthenumber ofdevices,hardwareandthecontrolcircuitbecomesverycomplicated.Hence,theobjectiveofthepresentworkisto

∗_{Corresponding}_author.

E-mailaddresses:janardhan.kavali@gmail.com,janardhan1989.nit@gmail.com(J.Kavali),am1970nit@gmail.com(A.Mittal).

PeerreviewundertheresponsibilityofElectronicsResearchInstitute(ERI).

http://dx.doi.org/10.1016/j.jesit.2016.01.007

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Fig.1. FivelevelcascadedH-bridgemultilevelinvertertopology(KavaliandMittal,2014;Lakshmietal.,2013).

reducetheTHDbyproperlyselectingthecontrolschemesbasedontheswitchingpatternofthedevices.Thedifferent controlschemesaresimulatedandcomparisonsaremadetochoosethebettertechnique,whichwillbeefficientand providestheoutputwithimprovedquality.

2. CascadedH-bridgemultilevelinvertertopology

AcascadedmultilevelinverterconsistsofaseriesofH-bridgeinverterunits.Thegeneralfunctionofthismultilevel inverteristosynthesize adesiredvoltagefromseveralseparate dcsources(SDCS), whichmaybe obtained from batteries,fuelcells,orsolarcells(Kangetal.,2005).ByaddingeachH-bridgemodule,wecanincreasethetwolevels inanoutputwaveform.NormallyforansinglephasecascadedH-bridgemultilevelinverter,numberofsemiconductor switchesrequiredis2(n−1),wherenisthenumberoflevels(KavaliandMittal,2014;Kangetal.,2005).Foranfive levelMLIeightpowerdevicesarerequiredandforsevenlevelfouradditionalpowerdevicesarerequired.Inproposed Topologyforfivelevel MLIonlysix powerdevices arerequiredandfromthenonwardsforincreasingtwo levels, onlyonemorepowerdeviceistobeaddedi.e.,forsevenlevelMLIsevenpowerdeviceandforninelevelMLIeight powerdevicesarerequired(KavaliandMittal,2014;Lakshmietal.,2013).Inthistopologyatatimeonlythreepower deviceswillconductforanylevelexcept0voltagelevel,whereonlytwopowerdeviceswillconductbutincaseof conventionalCHB-MLIbyincreasingthenumberoflevelsthenumberofconductingdevicesalsoincreases.Fig.1 showsthepowercircuitofsinglephasefivelevelcascadedH-bridgemultilevelinverterCHB-MLItopology.

3. Controlschemebasedonswitchingpattern

ByproperlycontrollingtheswitchingpatternofthepowerdevicestheTHDofMLIcanbedrasticallyreduced. Basedonthisswitchingpatternthreedifferentcontrolschemeshavebeendiscussedinthispaper.

3.1. ControlschemeI(α−α−2α−α−α)

Theconventionalcontrolschemeisdefinedas(α−α−2α−α−α)inwhichthedevicesareswitchedoninsucha mannerthatforαperiodtheoutputiszero,fornextαperiodtheoutputisVvoltsandfornext2αperiodtheoutputis2V voltsandsoonasshowninFig.2.ForsinglephasefivelevelMLI,thevalueofαis30◦.Itisthebasiccontrolscheme offivelevelmultilevelinverterandwiththiscontrolschemetheTHDcomesouttobe30.81%.Theswitchingofthe powerdevicestoobtainthesameoutputvoltagewaveformasshowninFig.2istabulatedinTable1.Theoutputvoltage andcurrentwaveformsandTHDspectrumforconventionalcontrolschemeIisshowninFigs.2and3respectively. 3.2. ControlschemeII(α−2α−4α−2α−α)

InordertoreducetheTHD,thesecondcontrolschemeproposedis(α−2α−4α−2α−α)inwhichthedevices areswitchedoninsuchamannerthatforαperiodtheoutputiszero,fornext2αperiodtheoutputisVvoltsandfor

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0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 -25 -20 -15 -10 -5 0 5 10 15 20 25 time (sec) voltage (volts) voltage current

Fig.2.OutputvoltageandcurrentwaveformsforcontrolschemeI(α−α−2α−α−α).

Table1

SwitchingtableoftheFivelevelCHB-MLITopologyforcontrolschemeI. S.No. Periodindegrees Statusofthepowerdevice

S1 S2 S3 S4 S5 S6 1. 0–30 1 1 0 0 0 0 2. 30–60 1 0 1 0 1 0 3. 60–120 1 0 1 0 0 1 4. 120–150 1 0 1 0 1 0 5. 150–180 1 1 0 0 0 0 6. 180–210 1 1 0 0 0 0 7. 210–240 0 1 0 1 1 0 8. 240–300 0 1 0 1 0 1 9 300–330 0 1 0 1 1 0 10. 330–360 1 1 0 0 0 0

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Fig.4. OutputvoltageandcurrentwaveformsforcontrolschemeII(α−2α−4α−2α−α).

Fig.5.THDspectrumforcontrolschemeII.

next4αperiodtheoutputis2VvoltsandsoonasshowninFig.4.ForsinglephasefivelevelMLIthevalueofαis 18◦.InthisproposedcontrolschemeII,theTHDisdrastically reducedto21.17%.Theoutputvoltageandcurrent waveformsandTHDspectrumforcontrolschemeIIisshowninFigs.4and5respectively.

3.3. ControlschemeIII(α−2α−6α−2α−α)

InordertostillreducetheTHD,thethirdcontrolschemeisproposedas(α−2α−6α−2α−α)inwhichthedevices areswitchedoninsuchamannerthatforαperiodtheoutputiszero,fornext2αperiodtheoutputisVvoltsandfor next6αperiodtheoutputis2VvoltsandsoonasshowninFig.6.Thevalueofαinthiscaseis15◦.Inthisproposed controlschemeIII,theTHDisfurtherreducedto17.07%whichisalmosthalfofthecontrolscheme-I.Itisoneofthe bestcontrolschemewithminimumhardwareandwithoutusingPWMtechniqueforreducingtheTHDintheoutput voltagewaveform.TheoutputvoltageandcurrentwaveformsandTHDspectrumforcontrolschemeIIIisshownin Figs.6and7respectively.

4. SinusoidalPulseWidthModulation(SPWM)

TheMultilevelInverter(MLI)controlschemescanbeclassifiedaccordingtoswitchingfrequenciesasfundamental switchingfrequencycontrolandhighswitchingfrequencyPWMcontrol (Mohanetal., 2013).Thehighswitching frequency PWMcontrol schemes canfurther be classifiedas SpaceVector Pulse Width Modulation (SV-PWM), SinusoidalPulse WidthModulation(SPWM)andSelectiveHarmonicEliminationPulse WidthModulation (SHE-PWM)(Chaturvedietal.,2006).TheSPWMcontroltechniqueismostpopularamongallandwidelyusedinindustrial applications.SPWMtechniqueusesseveraltriangularcarrierwavesandonereferencewaveperphase.Thenumber ofcarrierwavesis(n−1),wherenisthenumberoflevels(Balamuruganetal.,2013).Intheproposedsinglephase

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Fig.6.OutputvoltageandcurrentwaveformsforcontrolschemeIII(α−2α−6α−2α−α).

Fig.7.THDspectrumforcontrolschemeIII.

fivelevelcascadeH-bridgemultilevelinverter(CHB-MLI)topologytherewillbefourcarrierwavesandonereference wave.Themodulationindex(Ma)isgivenby

Ma= Am

(n−1)Ac

whereAmisthepeaktopeakamplitudeofreferencewave,Acisthepeaktopeakamplitudeofcarrierwaveandnis

thenumberoflevels(Mohanetal.,2013;KiranKumaretal.,2013).

ThegraphicalstructureofthedifferentcontrolschemesusedforcontrollingthemultilevelinverterisshowninFig.8. TheSPWMcontroltechniquecanfurtherbeclassifiedasSPWM-PhaseShift(SPWM-PS)andSPWM-LevelShift (SPWM-LS).Dependingonthedispositionofthecarriers,theSPWM-LevelShiftcanfurtherbeclassifiedasPhase Disposition(SPWM-PD),PhaseOppositionDisposition(SPWM-POD)andAlternatePhaseOppositionDisposition (APOD-SPWM)(Mohanetal.,2013).

5. SPWM-PhaseDisposition

WhenallthecarrierwavesareplacedinasamephasethenthatcontrolschemeiscalledPhaseDisposition(Kiran Kumaretal.,2013).ForsinglephasefivelevelMLI,fourcarrierwavesarecomparedwithareference/modulation signal(Colaketal.,2011).ThemodulationindexMaisvariedfordifferentcontrolschemesanditisobservedthatfor

mostofthecasestheTHDisminimumforamodulationindexof1.1.ThefrequencyratioPisdefinedastheratioof carriersignalfrequencytomodulationsignalfrequency(Elsheikhetal.,2011).ThevalueofPisalsovariedanditis observedthatatalmost250theoutputvoltagehasminimumTHD.Forabettercomparisonofvariouscontrolschemes, themodulationindexandfrequencyratioaretakenassameforallthecontrolschemes.

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Low SwitchingFrequency SHE SV-PWM 2D 3D Level Shied SPWM-LS Nearest Level SPWM-PD SPWM-POD PhaseShied (SPWM-PS) SPWM-APOD MULTILEVEL MODULATION

High Switching Frequency

Mul carrierSPWM _SHE-PW_M

Fig.8.Differentcontrolschemesusedforcontrollingthemultilevelinverter(Colaketal.,2011;Kangetal.,2005;Chaturvedietal.,2006;Kiran Kumaretal.,2013).

Fig.9.SIMULINKmodelfor1−fivelevelmultilevelinvertertopology.

5.1. SPWM-PhaseDispositionwithﬁxedcarrierwaveamplitude(Ac)

Thecarrierwavepeaktopeakmagnitudeissameforallcarrierwavesandtheslopeofthecarrierwaveisconstant throughoutthesimulationtime(Balamuruganetal.,2013;KiranKumaretal.,2013).TheSIMULINKmodelsfor powercircuitandcontrolschemeforSPWM-PDareshowninFigs.9and10respectively.InFig.10thecarriersignal andreferencesignalsaresuitablymodeledusingrelationaloperatorsandORlogicgatestogeneratethedesiredcontrol signals.

ThecarrierandmodulationwavesforfivelevelMLISPWM-PDcontrolstrategywithconstantcarrierwaveamplitude (Ac)areshowninFig.11.

TheoutputvoltageandcurrentwaveformsandTHDspectrumforSPWM-PDcontrolschemewithconstantcarrier waveamplitude(Ac)isshowninFigs.12and13respectively.

InSPWM-PDwithconstantcarrierwaveamplitude(Ac),theTHDhasreducedto8.57%ascomparedtoconventional

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Fig.10.SIMULINKmodelforcontrolschemeSPWM-PD.

Fig.11.CarrierandmodulationwavesforSPWM-PDcontrolstrategywithconstantAc.

5.2. SPWM-PhaseDispositionwithvariablecarrierwaveamplitude(Ac)

Thecarrierwavemagnitudeissameforallcarrierwavesbutisvaryingwithtimeinbetweenminimumtomaximum andtheslopeofthecarrierwaveisalsovaryingatregularintervalsoftimesthroughoutthesimulationtime.Thecarrier andmodulationwavesforSPWM-PDcontrolstrategywithvariablecarrierwaveamplitude(Ac)areshowninFig.14.

TheoutputvoltageandcurrentwaveformsandTHDspectrumforSPWM-PDcontrolstrategywithvariablecarrier waveamplitude(Ac)isshowninFigs.15and16respectively.

InSPWM-PDwithvariablecarrierwaveamplitude(Ac),theTHDhasconsiderablyreducedto5.69%ascompared

toSPWM-PDwithconstantcarrierwaveamplitude(Ac)of8.57%.Henceforthforallcontrolschemesthevariable

carrierwaveamplitudeisconsideredforcomparisonofthevariouscontrolschemes.

6. SPWM-PhaseOppositionandDisposition

Whenthecarrierwavescorrespondingtonegativehalfofreferencewaveare180◦outofphasetothepositivehalf carrierwavesthenitiscalledasPhaseOppositionandDisposition(POD)(Balamuruganetal.,2013;KiranKumar

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Fig.12.OutputvoltageandcurrentwaveformsforSPWM-PDcontrolstrategywithconstantAc.

Fig.13.THDspectrumforSPWM-PDcontrolstrategywithconstantAc.

Fig.14.ThecarrierandmodulationwavesforSPWM-PDcontrolstrategywithvariablecarrierwaveamplitude(Ac).

etal.,2013).ThecarrierandmodulationwavesforSPWM-PDcontrolstrategywithvariablecarrierwaveamplitude (Ac)areshowninFig.17.

The output voltage andcurrent waveformsand THDspectrum of aSPWM-POD control strategy is shownin Figs.18and19respectively.

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Fig.15.OutputvoltageandcurrentwaveformsforSPWM-PDcontrolstrategywithvariableAc.

Fig.16.THDspectrumforSPWM-PDcontrolstrategywithvariablecarrierwavemagnitudeAc.

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Fig.18.OutputvoltageandcurrentwaveformsforSPWM-PODcontrolstrategy.

Fig.19.THDspectrumforSPWM-PODcontrolstrategy.

The THDisslightly increasedfrom 5.69%to5.75%incaseof SPWM-PODas comparedto SPWM-PDwith variableAc.

7. SPWM-AlternatePhaseOppositionandDisposition

WhenthecarrierwaveisopposedtotheadjacentcarrierwavesthenthatcontrolschemeiscalledasAlternatePhase OppositionandDisposition(APOD)(Balamuruganetal.,2013).ThecarrierandmodulationwavesforSPWM-APOD controlstrategywithvariablecarrierwaveamplitude(Ac)areshowninFig.20.

TheoutputvoltageandcurrentwaveformsandTHDspectrumofaSPWM-PODisshownFigs.21and22respectively. TheTHDobtainedinSPWM-APODis5.73%whichisinbetweenSPWM-PDwithvariableAcandSPWM-POD

andalmostalltheobtainedTHDswithvariableAclevelshiftingissame.

8. SPWM-PhaseShift(SPWM-PS)

SinusoidalPulseWidthModulation-PhaseShift(SPWM-PS)issameasSPWM-PDbutheretwomodulationsignals willbeplacedwithaphasedifference(Balamuruganetal.,2013;KiranKumaretal.,2013).Forthisthephasedelay betweentwomodulationsignalsare45◦.Modulationindex(Ma)andfrequencyratio(P)correspondstoeachmodulation

signalissameasSPWM-PD.ThecarrierandmodulationwavesforfivelevelMLISPWM-PScontrolstrategywith variablecarrierwaveamplitude(Ac)areshowninFig.23.

The output voltage and current waveforms and THD spectrum for SPWM-PS control strategy is shown in Figs.24and25respectively.

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Fig.20.ThecarrierandmodulationwavesforSPWM-APODcontrolstrategy.

Fig.21.OutputvoltageandcurrentwaveformsforSPWM-APODcontrolstrategy.

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Fig.23.ThecarrierandmodulationwavesforSPWM-PScontrolstrategy.

Fig.24.OutputvoltageandcurrentwaveformsforSPWM-PS(45◦)controlstrategy.

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0 5 10 15 20 25 30 35 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

Diﬀerent control techniques

THD in % Control Schem-I Control Schem-II Control Schem-III SPWM-PD with constant Ac SPWM-PD with variable Ac SPWM-POD with variable Ac

SPWM-APOD with variable Ac SPWM-PS with variable Ac

Fig.26.ComparisonofTHDsfordifferentSPWMcontroltechniques. Table2

Comparisonoftheutilizationfactorofpowerdevices.

S.No. Controlscheme Utilizationfactor

S1 S2 S3 S4 S5 S6 1. Controlscheme-I 66.67 66.67 33.33 33.33 33.33 33.33 2. Controlscheme-II 60 60 40 40 40 40 3. Controlscheme-III 58.33 58.33 41.67 41.67 33.33 50 4. SPWM-PDwithConstantAc 59 59 41 41 43.5 38.5 5. SPWM-PDwithvariableAc 57.2 57.2 42.74 42.74 35.532 49.95

6. SPWM-PODwithvariableAc 57.51 57.51 42.49 42.49 35.13 49.85

7. SPWM-APODwithvariableAc 57.245 57.245 42.755 42.755 30.34 55.17

8. SPWM-PS 58.67 58.67 41.33 41.33 40.01 42.65

TheTHDisincreasedto7.42%incaseofSPWM-PS,whichishigherascomparedtoallcontrolschemesofLevel ShiftSPWMwithvariablecarrierwaveamplitude.

9. Comparativeanalysis

Thevarious control schemesbasedon switching pattern andusing SPWM techniqueswere simulated andthe comparativebarchartofTHDinoutputvoltagewaveformisshowninFig.26.

ItisobservedthattheTHDismuchlowerinalltheSPWMtechniqueswhencomparedwithconventionalswitching pattern.ThelevelshiftSPWMtechniqueswerefoundtobemuchbetterascomparedtophaseshiftSPWMtechnique. WithinlevelshiftSPWM,itisveryclearfromthebarchartthatSPWM-PDhasaminimumTHDlevelof5.69%.From theTHDcurves,itcanbeeasilydepictedthatthelowerorderharmonicsareconsiderablyreducedincaseoflevelshift SPWMtechniquesandevenharmonicsarealmostnegligibleincaseofSPWM-PD.

Theutilizationfactorofthevariouspowerdevicesisalsocalculatedforeachcontrolschemeandistabulatedin Table2.

FromthetableitcanbedepictedthattheutilizationfactorofthepowerdevicesisimprovedinthecaseofSPWM controltechniquesanditisalmostbestinSPWM-PDwithvariableAc.

10. Conclusion

In thispaper different control schemesare simulated andcomparisons are made tochoose a noveltechnique, whichwillbeefficientandprovidestheoutputwithimprovedpowerquality.Amongallthecontrolschemesbasedon switchingofdevices,thecontrolschemeIII(α−2α−6α−2α−α)givestheminimumTHD.AmongalltheSPWM controlschemesSinusoidalPulseWidthModulation-PhaseDisposition(SPWM-PD)hasthelowestTHDof5.69%

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atafrequencyratioandmodulationindexof250and1.1respectively.Alsothelowerorderharmonicsarereduced considerablyandevenharmonicsarealmostnegligibleinthiscontrolscheme.Thecomparisonismadebasedonthe utilizationfactorofthepowerdevicesanditisobservedthatitisimprovedforSPWMcontroltechniques.

Acknowledgment

Theauthorsare verygrateful andwould liketoacknowledgethe Departmentof EnergyandDirector,Maulana AzadNationalInstituteofTechnologyformotivatingandprovidingallpossiblefacilitiesforcarryingoutthisresearch paper.

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