A gridconnected photovoltaic power conversion system with singlephase multilevel inverter.docx

A gridconnected photovoltaic power conversion system with singlephase multilevel inverter.docx

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Agridconnectedphotovoltaicpowerconversionsystemwithsinglephasemultilevelinverter

Agrid-connectedphotovoltaicpowerconversionsystemwithsingle-phasemultilevelinverter

ErsoyBeser

BirolArifoglu

SabriCamur

EsraKandemirBeser

DepartmentofElectricalEngineering,KocaeliUniversity,UmuttepeCampus,41380Kocaeli,Turkey

Received7May2010.Revised26September2010.Accepted28September2010.Availableonline20October2010. Communicatedby:

AssociateEditorNicolaRomeo.

http:

//dx.doi.org/10.1016/j.solener.2010.09.011,HowtoCiteorLinkUsingDOI

CitedbyinScopus（4）

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Abstract

Thispaperpresentsagrid-connectedphotovoltaic（PV）powerconversionsystembasedonasingle-phasemultilevelinverter.TheproposedsystemfundamentallyconsistsofPVarraysandasingle-phasemultilevelinverterstructure.First,configurationandstructuralpartsofthePVassistedinvertersystemareintroducedindetail.Toproducereferenceoutputvoltagewaves,asimpleswitchingstrategybasedoncalculatingswitchinganglesisimproved.Bycalculatedswitchingangles,thereferencesignalisproducedasamultilevelshapedoutputvoltagewave.Thecontrolalgorithmandoperationalprinciplesoftheproposedsystemareexplained.OperatingPVarraysinthesameloadconditionisaconsiderablepoint;thereforeasimulationstudyisperformedtoarrangethePVarrays.AfterdeterminingthenumberandconnectiontypesofthePVarrays,thesystemisconfiguredthroughthearrangementofthePVarrays.Thevalidityoftheproposedsystemisverifiedthroughsimulationsandexperimentalstudy.Theresultsdemonstratethatthesystemcanachievelowertotalharmonicdistortion（THD）ontheoutputvoltageandloadcurrent,anditiscapableofoperatingsynchronousandtransferringpowervalueshavingdifferentcharacteristictothegrid.Hence,itissuitabletousetheproposedconfigurationasaPVpowerconversionsysteminvariousapplications.

Keywords

Photovoltaic;

Photovoltaicpowerconversion;

Grid-connected;

Single-phasemultilevelinverter

Nomenclature

Ck

capacityofthekthcapacitorgroup（μF）

f

gridfrequency（Hz）

I

invertercurrent（A）

IPM

maximumpowercurrentvalue（A）

Ls

inductanceoftheselfinductor（H）

m

numberofthelevelmodules

n

outputlevelnumber

PLMk

powerofthekthlevelmodule（W）

Pref

referencerealpower（W）

P0

systemoutputpower（W）

Qk（t）

thekthswitchingsignal

Qref

referencereactivepower（VAr）

Rs

resistanceoftheselfinductor（Ω）

S

sumoftheenergyratios

t

instantaneoustimevalue（s）

tmax

maximumvalueofthesampletime（s）

tsample

valueofthesampletime（s）

Vb

voltageofthebaselevelmodule（V）

Vbus

busvoltage（V）

Vbusref

thereferencebusvoltage（V）

VCk

voltageofthekthcapacitorgroup（V）

Vg

gridvoltage（V）

Vmax

maximumvalueoftherequiredvoltage（V）

VPM

maximumpowervoltagevalue（V）

Vref

referencevoltage（V）

Vz

voltageoftheselfinductor（V）

V0

outputvoltage（V）

WCk

energyofthekthcapacitorgroup（J）

WLMk

energyofthekthlevelmodule（J）

XLs

reactanceoftheselfinductor（Ω）

Zs

impedanceoftheselfinductor（Ω）

Δt

timebetweentiandti+1（s）

δ

anglebetweenV0andVg（°）

δref

referenceanglebetweenVrefandVg（°）

θ

theanglebetweenRsandXLs（°）

φ

powerangle（°）

ω

angularspeed（rads−1）

1.Introduction

Recently,renewableenergyresourceshavebeenbecomingpopularduetothedecreaseoffuelsourcesandtheirdamagestotheenvironment.Asaresultofthenegativeeffectsofglobalwarmingandclimatechanges,theinterestofrenewableresourceshasbeenincreasedgradually.

Solarenergyisoneofthesealternativeenergyresources.Itisconvertedtotheelectricalenergybyphotovoltaic（PV）arrays.PVarraysdonotgenerateanytoxicorharmfulsubstancesthatpollutetheenvironment（[Kangetal.,2005a] and [Kangetal.,2005b]）.Anotherconsiderablefeatureofthemistherequirementoflowmaintenance.Dependingonthedevelopmentinphotovoltaictechnologies,theefficiencyofthePVarrayshasbeenimproved.Therefore,studiesonPVsystemshaveincreasedgradually.

PVsystemsareoccasionallyoperatedinstand-alonemodeandtheyfeedfixedloadsbystand-alonePVinverters（[Myrzik,2001],[Kangetal.,2005a],[Kangetal.,2005b],[Daheretal.,2008],[Lalounietal.,2009] and [SaravanaIlangoetal.,2010]）.PVsystemsarealsointerconnectedtothegrid.InterconnectingaPVsystemtothegridhasbeenthepopulardesigntrendandgrid-connectiontypesofPVinvertershavebeenproposed（[Calaisetal.,1999],[Myrzik,2001],[Kuoetal.,2001],[Alonsoetal.,2003],[Yuetal.,2005],[Wuetal.,2005],[Patcharaprakitietal.,2005],[Leeetal.,2008],[Hassaineetal.,2009] and [Rahimetal.,2010]）.ThereforevariouspowerelectronicstechnologiesareimprovedtoconvertthedctoacpowerforPVapplications.

Inaddition,itisimportanttooperatePVenergyconversionsystemsnearthemaximumpowerpointtoincreasetheoutputefficiencyofPVarrays（[Kuoetal.,2001],[Alonsoetal.,2003],[Yuvarajanetal.,2004],[Yuetal.,2005],[Patcharaprakitietal.,2005] and [Leeetal.,2008]）.Thus,powerelectronicsinvertersarerequiredformaximumpowerpointtracking（MPPT）algorithm,whichprovidesmaximumPVpower.TheyarealsoneededfortransferringthePVpowertoaloadortothegrid.

Multilevelinvertersaresuitablechoicesforrealizingthisobjective.Variousmultilevelinvertertopologieshavebeenintroducedandstudiedintheliterature.Themostconsiderableofthesetypesarethediodeclamped,theflyingcapacitor,thecascadedH-Bridge,themagneticcoupledandthefullbridgewithcascadedtransformersinverters.Theremarkablefeatureoftheseinvertersisgeneratinglessharmoniccomponentsonbothoutputvoltageandloadcurrent.Byincreasingthenumberofoutputlevels,thequalityoftheoutputvoltageandloadcurrentisincreasedstepbystep（[Calaisetal.,1999],[Rodriguezetal.,2002],[Kangetal.,2005a],[Kangetal.,2005b],[Daheretal.,2008],[Rahimetal.,2010] and [Beseretal.,2010]）.Duetotheproductionoflessharmoniccomponents,thePVpoweristransferredtotheloadortothegridinahigh-qualityformbymultilevelinverterstructures.

Relatedtothesedevelopments,thisstudypresentsaPVassistedmultilevelinvertersystemfortheconversionofPVpowertotheelectricalpower.TheproposedstructureoftheinvertersystemisquitesuitablefortheuseofPVarrays.Owingtotheuseofamultilevelinverterstructure,moresinusoidalshapedoutputvoltagewavesareobtained.Therefore,theTHDoftheoutputvoltageisconsiderablyreduced.PlacementofthePVarraysinthesystemisthesignificantpointofthestudybecauseofloadingthearraysinthesamecondition.Toaccomplishthis,asimulationstudyisperformedandthenumberandconnectiontypesofthePVarraysinthesystemaredetermined.AccordingtotheorderofthePVarraysaconfigurationisformedforthePVassistedinvertersystem.Thesystemissimulatedwhileoperatingsynchronousandtransferringvariouspowervalueshavingdifferentcharacteristicstothegrid.Thevalidityoftheproposedsystemisalsoverifiedthroughanexperimentalstudy.ThemeasuredTHDvaluesoftheoutputvoltageandcurrentarequitelow.ThesystemprovidesgoodperformanceasaPVenergyconversionsystem.

2.ProposedPVassistedsingle-phasemultilevelinvertersystem

2.1.Configurationandstructuralpartsoftheproposedsystem

Fig.1showsthebaseconfigurationoftheproposedPVassistedmultilevelinverter.ItconsistsofPVmodules,levelmodules（LM）andaconventionalH-Bridgemodule.Thebaseconfigurationofthemultilevelinvertergeneratesa7-levelshapedoutputvoltagewave.However,theproposedsystemcanbeeasilyexpandedandthenumberofoutputvoltagelevelsisincreasedbyaddinglevelmodulestothesystem（Beseretal.,2010）.

Fig.1. ConfigurationoftheproposedPVassistedsingle-phasemultilevelinvertersystem.

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Theproposedinverterstructureprovidesanadvantageinpointofswitchingelementnumbercomparedtosomeinvertertypesintheliterature.Aswitchnumbercomparisonrelatedtooutputlevelnumber（n）ismadebetweendifferentinvertertypesinTable1.

Table1.Switchnumbercomparisonrelatedtooutputlevelnumber（n）.

Invertertype

Switchnumber

Diodeclamped

2（n − 1）

Flyingcapacitor

2（n − 1）

CascadedH-Bridge

2（n − 1）

Magneticcoupled

4 log3（n）

Full-bridgewithcascadedtransformer

4 log3（n − 2） + 4

Proposed

2 log2（n + 1） + 2

Full-sizetable

ItcanbeseenfromTable1that60switchesareusedindiodeclamped,flyingcapacitorandcascadedH-Bridgeinverterstoobtain31-leveloutputvoltage.Themagneticcoupledinvertertypeuses12switchesandshapes27-levelvoltagewave.Thefullbridgewithcascadedtransformerinvertertypeuses16switchesandforms29-levelshapedoutputvoltage.However,theproposedinverterachieves31-leveloutputvoltagebyusingonly12switches.

Anotherconsiderablefeatureoftheproposedinverteristhatthesystemconfigurationallowsoperatingregenerative.Sothisfeatureprovidestotransferbothactiveandreactivepowertothegrid.

2.1.1.PVmodule

Fig.2showstheconfigurationofthePVmodule.ItcanbeseenfromFig.2thatitconsistsofPVarraysandacapacitorgroup.

Fig.2. ConfigurationofthePVmodule.

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Dependingontheincreaseoftheoutputlevelnumber,PVarraysaresuitablyconnectedseriallyandPVmoduleisexpandedaccordingtothenumberoflevelmodules.TheconfigurationoftheexpandedPVmoduleisshownin