Maximum Power Point Tracking

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    mppt:amaximumpowerpointtrackingphotovoltaicsystem.

    INTRODUCTIONomaximizeaphotovoltaic(PV)system'soutputpower,continuouslytrackingthemaximumpowerpoint(MPP)ofthesystemisnecessary.TheMPPdependsonirradianceconditions,thepanel'stemperature,andtheloadconnected.Maximumpowerpointtracking(MPPT)algorithmsprovidethetheoreticalmeanstoachievetheMPPofsolarpanels;thesealgorithmscanberealizedinmanydifferentformsofhardwareandsoftware.PVsystemsthatlackMPPTrarelyoperateatthemostefficient,MPP.Thisiswhytheratedpowerofthesolarpanelisalmostneverrealizedwhenconnectingaload.Thegoalofthisprojectwasto

    rapidlydevelop,construct,andtestaworkingsolutiontotheMPPproblemwithalimitedbudget.

    Thisprojectwasdevelopedfromthegroundupwithonlyafewreferences. wastheteam'sadvisorandprovidedinvaluableguidancethroughouttheproject'slifetime.Theteamconsistedoffiveundergraduatestudents: ,StephenTirador,NathanWilbanks,

    ,andmyself.

    RapiddevelopmentnecessitatedthattheteamfirstlookthroughexistinganalysisandpossiblesolutionstotheMPPproblem.Itbecameclearthattheperturbandobserve(P&O)techniquewaswidelyusedforitseaseofimplementation.Itisbasedonthefollowingcriterion:iftheoperatingvoltageofthePVarrayisperturbedinagivendirectionandifthepowerdrawnfromthePVarrayincreases,thismeansthattheoperatingpointhasmovedtowardtheMPPand,therefore,theoperatingvoltagemustbefurtherperturbedinthesame

    direction.Otherwise,ifthepowerdrawnfromthePVarraydecreases,theoperatingpointhasmovedawayfromtheMPPand,therefore,thedirectionoftheoperatingvoltageperturbationmustbereversed.

    TheteamchosetoimplementtheP&Oalgorithminsoftwaretoaddflexibilitytothesystemaswellassimplifythesystemtothepointwherethesystemcouldberapidlyconstructedandtestingcommenced.Thefollowingcomponentswereneededtodesignaminimalworkingsolution:solarpanel,currentsensor,voltagesensor,DCDCconverter,digitalcontroller,andthegluelogic/circuitrytoconnecteverythingtogether.

    PROJECTS PICTURES RESUME ABOUT

    CONTENTS

    HARDWARECOMPONENTSSOFTWAREANDALGORITHMSFUTUREWORK

    INTRODUCTION

    SOLARPANELDIGITALCONTROLLERDCDCCONVERTERSENSINGCIRCUITSDIGITALTOANALOGCONVERTER

    MPPTALGORITHMSEMBEDDEDSOFTWARE

    PROJECTCOSTFUTUREWORKCONCLUSION

    Dr.JaberAbuQahouq

    TravisGrantMattYork

    http://ece.eng.ua.edu/people/faculty_details.asp?ID=26mailto:tggrant@REMOVETHIS.crimson.ua.eduhttp://bryanwbuckley.com/about.htmlmailto:my3681@REMOVETHIS.gmail.comhttp://bryanwbuckley.com/resume/resume.htmlhttp://bryanwbuckley.com/pictures.htmlhttp://bryanwbuckley.com/projects.html

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    SOLARPANEL

    hesolarpanelusedinoursystemisanOEM40modelmanufacturedby andwasprovidedbytheDepartmentofElectricalandComputerEngineering's .Ithasapowerratingof40W,anopencircuitvoltage(Voc)of21Vdc,andashortcircuitcurrent(Isc)of2.68A.

    DIGITALCONTROLLER

    hemicrocontrollerprovidesthecontrolinoursystem.Thechoiceofmicrocontrollerforthesystemdictatesmuchofthecost,performance,andflexibilityoftheentiresystem.Takingintoconsiderationtheproject'sconstraints,theTexasInstruments modeldigitalsignalcontroller(DSC)waschosen.ThesinglechipC2000familyofmicrocontrollersistargetedtowardrealtimecontrolapplicationsthankstopowerful,highperformanceintegratedperipherals.Thecoreis"mathoptimized"andgivesdesignersthemeanstoimprovesystemefficiency,reliability,andflexibilitywhentheapplicationrequirescomplex

    SunWizeTechnologies,IncDr.TimHaskew

    TMS320F28335

    http://focus.ti.com/docs/prod/folders/print/tms320f28335.htmlhttp://ece.eng.ua.edu/people/faculty_details.asp?ID=12http://www.sunwize.com/

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    algorithms.Itfeatures:

    TheC2000'sdevelopmenttoolsareveryuseableandhelpminimizedevelopmenttime.Softwareforthecontrollercanbedeveloped,deployed,andtestedwitheasethankstotheprovided,easytouseIDEfeaturingaC/C++assembler/compiler/linkerinadditiontoapowerfuldebuggerandseamlessdeviceprogrammer.

    DCDCCONVERTER

    DCtoDCconverterisanelectroniccircuitwhichconvertsasourceofdirectcurrentfromonevoltageleveltoanother.Itisaclassofpowerconverter.ElectronicswitchmodeDCtoDCconvertersoperatebystoringtheinputenergytemporarilyandthenreleasingthatenergytotheoutputatadifferentvoltageandcurrent.Justlikeatransformer,theyessentiallyjustchangetheinputenergyintoadifferentimpedancelevel.Sowhatevertheoutputvoltagelevel,theoutputpowerallcomesfromtheinput;there'snoenergymanufacturedinsidetheconverter.Infactsomeenergyisusedbytheconverter

    circuitryandcomponentswhiledoingtheirjob.ItisthisprinciplethatmakesaDCDCConverteressentialforMPPT.

    Theconverterpresentsanelectricalloadtothesolarpanelthatvariesastheoutputvoltageoftheconvertervaries.Thisloadvariationinturncausesachangeintheoperatingpoint(currentandvoltagecharacteristics)ofthepanel.ThusbyintelligentlycontrollingtheoperationoftheDCDCconverter,thepoweroutputofthepanelcanbeintelligentlycontrolledandmadetooutputthemaximumpossible.

    TheDCDCpowerconverterusedinoursystemisaMicro24Vout,100WV28C24C100BLmodelmanufacturedby.Theinputvoltagerangeoftheconverteris936Vdc.Becausethevoltageprovidedbythesolarpanel(which

    servesastheinputvoltagetotheconverter)candropbelowtheconverter's9Vdcminimumandthuscausetheconvertertoshutdown,ourMPPTsystemisonlyoperationalwhenthevoltageprovidedbythesolarpanelisgreaterthanorequalto9Vdc.Theoutputvoltageoftheconvertercanbevariedbetween10%and110%ofitsnominal24Vdcoutput(i.e.2.426.4Vdc)viaareferenceinputvoltageattheSCpinwithrespecttotheOUTpinbetween0.1231.353Vdc.Theconverterhasthecapabilityoffunctioninginisolatedornonisolatedmodedependingonwhetherthegroundsoftheconverter(INandOUT)areseparateorconnectedtogether,respectively.

    SENSINGCIRCUITSVOLTAGESENSOR

    32BITFLOATINGPOINTCPU(

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    norderfortheMPPTcontrollertomeasurethevoltageprovidedbythesolarpanel,tworesistors,R1andR2,areemployedinparallelwiththesolarpaneltoactasavoltagedivider.ThevoltageacrossR2inthevoltagedividerisfedintoananalogtodigitalconverter(ADC)drivercircuit(opampinavoltagefollowerconfigurationthatfeedsintoalowpassfilter)beforebeingdeliveredtotheADCINA0channeloftheMPPTcontroller.BychoosingthevaluesofR1andR2as1.07Mand165k,respectively,themaximumamountofcurrentdivertedfromtheload,I2,issmallenough,eveninaworstcasescenario,tobeconsiderednegligible.

    TheallowablevoltagerangeforeachADCchanneloftheMPPTcontrolleris03Vdc.Therefore,thevoltageacrossR2(whichservesasascaleddownrepresentationofthesolarpanel'svoltage)shouldnotexceed3Vdc.BasedonthechosenvalueofR2as165k,themaximumvoltage,V(R2,max),senttotheADCdrivercircuit(andthusADCchannelADCINA0)is~2.81Vdc.

    CURRENTSENSOR

    norderfortheMPPTcontrollertomeasurethecurrentprovidedbythesolarpanel,asingleresistor(Rsense)isplacedinseriesbetweenthesolarpanelandtheDCDCconverter.ThevoltageacrossRsenseisfedintoanAD8215currentsensormanufacturedby whoseoutputvoltageisthenfedintoanADCdrivercircuit(opampinavoltagefollowerconfigurationthatfeedsintoalowpassfilter)beforebeingdeliveredtotheADCINA1channeloftheMPPTcontroller.BychoosingthevalueofRsenseas51m,themaximumvoltagedropacrossRsense,VRsense,issmallenough,eveninaworstcasescenario,tobeconsiderednegligible.

    Asstatedpreviously,theallowablevoltagerangeforeachADCchanneloftheMPPTcontrolleris03Vdc.Therefore,theoutputvoltageoftheAD8215currentsensor(whichservesasanequivalentvoltagerepresentationofthesolarpanel'scurrent)shouldnotexceed3Vdc.BasedonthechosenvalueofRsenseas51m,themaximumvoltage,Vout,senttotheADCdrivercircuit(andthusADCchannelADCINA1)is~2.73Vdc.

    COMPLETESENSINGCIRCUITWITHADCDRIVERCIRCUITS

    AnalogDevices

    http://www.analog.com/en/amplifiers-and-comparators/current-sense-amplifiers/ad8215/products/product.html

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    InordertoconditioneachofthevoltagesignalssenttotheADCchannelsoftheMPPTcontroller,TexasInstruments modelopampsareusedinvoltagefollowerconfigurationswitheachoftheiroutputsfedintoalowpassfilter.TheOPA340sprovidelowoutputimpedancetoeachoftheADCchannelswithoutmodifyingeachoftheoutputvoltagesbeingsentfromthevoltageandcurrentsensorcircuits.Ofaddedbenefitistheopamps'abilitytoprotecteachoftheADCchannelsfrombeingpermanentlydamagedbyaninputvoltagethatexceedsitsmaximumoperatingthreshold.Thisisaccomplishedbypoweringeachoftheopamps

    withthemaximumallowedvoltageoftheADCchannels:3Vdc.ThiseffectivelyclipsanypotentiallydamagingvoltagethatwouldotherwisebefedintotheADCchannelatasafevalueof3Vdc.ThevoltageandsensorcircuitsalongwiththeircorrespondingADCdrivercircuitsareallcombinedtoformthe"sensingcircuit"fortheMPPTsystem.

    DIGITALTOANALOGCONVERTER

    heDCDCconverteriscontrolledviaareferenceinputvoltageattheSCpinwithrespecttotheOUTpinbetween0.1231.353Vdc.Thedigitalsignalcontrollerutilizesonchippulsewidthmodulated(PWM)signalgeneratorstocreateanoutputsignalmeanttocontroltheDCDCconverter.AnanaloglowpassfiltercanremovethehighfrequencycomponentsofthePWMsignal,leavingonlythelowfrequencycontent.InthisMPPTapplication,asecondorderpassivefilterwasusedtoprovideadequatefilteringandresolution.ThePWMdutycycleis

    OPA340

    http://focus.ti.com/docs/prod/folders/print/opa340.html

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    controlledviasoftware;thedutycyclevaluesforwhichthereferenceinputvoltageoftheconverteris0.1231.353Vdcwereobtainedexperimentallyandthesoftwareensuresthatthesignalisneveroutofrange.This(PWMasDAC)solutionisalegitimatelowercostalternativetodedicatedoffchipDACs.

    MPPTALGORITHMSariousalgorithmsmayperformMPPT.ImportantfactorstoconsiderwhenchoosingatechniquetoperformMPPTaretheabilityofanalgorithmtodetectmultiplemaxima,costs,andconvergencespeed.

    Theirradiancelevelsatdifferentpointsonasolarpanel'ssurfacetendtovary.Thisvariationleadstomultiplelocalmaximapowerpointsinonesystem.Theefficiencyandcomplexityofanalgorithmdetermineifthetruemaximumpowerpointoralocalmaximumpowerpointiscalculated.Inthelatter

    case,themaximumelectricalpowerisnotextractedfromthesolarpanel.

    ThetypeofhardwareusedtomonitorandcontroltheMPPTsystemaffectthecostofimplementingit.ThetypeofalgorithmusedlargelydeterminestheresourcesrequiredtobuildanMPPTsystem.

    ForahighperformanceMPPTsystem,thetimetakentoconvergetotherequiredoperatingvoltageorcurrentshouldbelow.Dependingonhowfastthisconvergenceneedstooccurandyourtrackingsystemrequirements,thesystemrequiresanalgorithm(andhardware)ofsuitablecapability.

    PERTURBANDOBSERVE

    heconceptbehindthe"perturbandobserve"(P&O)methodistomodifytheoperatingvoltageorcurrentofthephotovoltaicpaneluntilyouobtainmaximumpowerfromit.Forexample,ifincreasingthevoltagetoapanelincreasesthepoweroutputofthepanel,thesystemcontinuesincreasingtheoperatingvoltageuntilthepoweroutputbeginstodecrease.Oncethishappens,thevoltageisdecreasedtogetbacktowardsthemaximumpowerpoint.Thispertubulancecontinuesindefinitely.Thus,thepoweroutputvalueoscillatesaroundamaximumpowerpointandneverstabilizes.

    P&Oissimpletoimplementandthuscanbeimplementedquickly.ThemajordrawbacksoftheP&Omethodarethatthepowerobtainedoscillatesaroundthemaximumpowerpointinsteadystateoperation,itcantrackinthewrongdirectionunderrapidlyvaryingirradiancelevelsandloadlevels,andthestepsize(themagnitudeofthechangeintheoperatingvoltage)determinesboththespeedofconvergencetotheMPPandtherangeofoscillationaroundtheMPPatsteadystateoperation.

    INCREMENTALCONDUCTANCE

    ncrementalconductanceconsidersthefactthattheslopeofthepowervoltagecurveiszeroatthemaximumpowerpoint,positiveattheleftoftheMPP,andnegativeattherightoftheMPP.TheMPPisfoundbycomparingtheinstantaneousconductance(I/V)totheincrementalconductance(I/V).OnceyouhavetheMPP,thesystemmaintainsthispowerpointunlessachangeinVorIoccurs(causedbyanexternalevent).Ifthishappens,thealgorithmwillfindthenewMPP.

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    ThistechniquehasanadvantageinthatitcanreachandmaintaintheMPPwithoutlosingsomeefficiencybyhavingtooscillatearoundthispoint.UnderrapidlychangingconditionsthisalgorithmtracksmoreaccuratelythantheP&Omethod.ThedisadvantageofthismethodisthatitcantakelongertoreachtheMPPbecausetheincreasedcomputationrequireddecreasesthenumberofperturbationstotheoperatingvoltageandcurrentpossibleinasetamountoftime.

    EMBEDDEDSOFTWAREoimplementthisintelligence,thegroupemployedtheaforementionedTF320F28335digitalcontroller.Withthisdevelopmenttargetinmind,thesoftwaredevelopmentbeganimmediately.Theteamfocusedongettingminimal"working"codeontothecontrollerassoonaspossible.TheteambecamefamiliarwiththeTIIDE,debugger,programmer,andmostimportantlytheheaderfiles.Throughthecourseofacoupleofweeksdifferentprojectsweredeployedtotheboard,rangingfromflashinganLEDtorunningprogramsfromtheflashmemory.EachperipheraltobeusedintheMPPTprojectwasinvestigatedandplayedwithin

    thesefirstfewweeks.Mostsignificantly,unittestsforthePWMandtheADC(usingDMA)weredeveloped.Eachunittestwasbuiltwithpredefinedpass/failcriteria.

    ForthePWMunittest,thedutycycleofthePWMwouldbeautomatically,continuouslyvariedthroughtheentireoperatingrangeofthefinalMPPTprojectasdefinedbytheDCDCconvertercontrolpinspecs.ThetestwouldpassifthePWMdutyneverexceededorfellunderthedefinedupperandlowerlimitsandifthePWMdutywascontinuouslyincreasingordecreasing.ThistestcouldrunwithoutinterfacingwiththeDCDCconverter;anoscilloscopewashookedintothePWMpinsofthecontrollerandthetestbehaviorwasobserved.

    FortheADCunittest,theresultofanADCconversionwouldbestoredinamonitoredvariable.Thereferenceanalogsignalwasvariedthrougharangeofvaluesandthevariablesvaluewasrecordedforeachanalogvalue.Theserecordedvalueswerecomparedtocalculated,expectedvaluestoverifyoperation.

    OncetheteamwasverycomfortableworkingwiththeC2000,workbeganonacustomMPPTalgorithmbasedontheP&Oalgorithmfrombefore.Thisalgorithmwasattemptedfirstsinceitwasthesimplestsolutionrequiringtheleastamountofeffortwhilestillfulfillingrequirements.Thesimplicityofthealgorithmcouldalsoaffordtheteammoretimetointegratethehardwareandsoftwareneartheendofdevelopmentifnecessary.Thissystemandalgorithmdoesnotrequireascheduler;ifthecontrollerneededtobemorereactivetomoreinputs(trackingtheMPPofmoresolarpanels)thenaschedulingmethodwouldlikelybenecessarytoensureaquickresponsetoallinputsandefficientutilizationoftheprocessor.

    Thissystemisdesignedtorunforalongtime;thesoftwareissinglepurposeandloopsforever.EachloopisaniterationoftheP&Oalgorithmdescribedearlier.Eachiterationstartsbysettingavariabletothecalculatedamountofpowerbeingsuppliedbythepanel(currentandvoltagevaluesknownfromADC).Theprogramthenswitchesonwhetherthepowerpointisincreasingordecreasinginvoltage.Insideeachcasetheiteration'spowervalueiscomparedtothepreviousiteration'sandthedutyofthePWMiseitherincreasedordecreasedtomoveclosertotheMPP;ifthePWMdutyisincreased,thevoltagewillbeincreasing,ifthePWMdutyisdecreased,the

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    voltagewillbedecreasing.Beforerepeatingtheloop,thepreviousiteration'spowervariableissettothecurrentiteration's.Pseudocodeforthisbehaviorisbelow.

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