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GRID-CONNECTED SOLAR PV SYSTEMSNO BATTERY STORAGE
Design Guidelines for Accredited Installers
January 2013
(Effective 1 February 2013)
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These guidelines have been developed by Clean Energy Council. They represent latest industry best practice for the design and installation
of grid-connected PV systems. © Copyright 2013
While all care has been taken to ensure this guideline is free from omission and error, no responsibility can be taken for the use of this information in the
installation of any grid-connected power system.
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GRID-CONNECTED SOLAR PV SYSTEMS (no battery storage)
Design guidelines for accredited installers
Last update: January 2013 1 of 18
CONTENTS
1 GENERAL 2
2 DEFINITIONS 2
3 DESIGNANDINSTALLATIONSTANDARDS 3
4 LICENSING 3
4.1 ExtraLowVoltage(ELV) 3
4.2 LowVoltage(LV) 3
5 DOCUMENTATION 4
6 RESPONSIBILITIESOFSYSTEMDESIGNERS 4
7 SITE-SPECIFICINFORMATION 5
8 ENERGYYIELD 5
8.1 EnergyYieldFormula 6
8.2 Specificenergyyield 10
8.3 Performanceratio 10
9 INVERTERSELECTION 11
9.1 Multipleinverters 11
9.2 Invertersizing 11
9.3 ArrayPeakPower 11
9.4 Arraypeakpower-invertersizing 12
9.5 Arrayde-ratingformula 13
9.6 Matchinginverter/arrayvoltage 14
9.7 Minimumvoltagewindow 15
9.8 Maximumvoltagewindow 17
9.9 InverterDCinputcurrent 18
9.10 Effectsofshadows 18
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GRID-CONNECTED SOLAR PV SYSTEMS (no battery storage)
Design guidelines for accredited installers
Last update: January 2013 2 of 18
Theobjectivesoftheseguidelinesareto:
→ improvethesafety,performanceandreliabilityofsolarphotovoltaicpowersystemsinstalled inthefield
→ encourageindustrybestpracticeforalldesignandinstallationworkinvolvingsolar photovoltaicpowersystems
→ provideanetworkofcompetentsolarphotovoltaicpowersystemsdesignersandinstallers
→ toincreasetheuptakeofsolarphotovoltaicpowersystems,bygivingcustomersincreased confidenceinthedesignandinstallationwork.
Theperformanceofareliableinstallationthatfulfilscustomerexpectationsrequiresbothcarefuldesignandcorrectinstallationpractice.Compliancewithrelevantstatehealthandsafetyregulationsisalsonecessary.
ThisdocumentusesthesameterminologyasoutlinedinAS/NZS5033.Twoimportantdefinitionsare:
2.1.1 Wheretheword“shall”isused,thisindicatesthatastatementismandatory.
2.1.2 Wheretheword“should”isused,thisindicatesthatastatementisarecommendation.
1 GENERAL
2 DEFINITIONS
NOTE:Theseguidelinesalonedonotconstituteafullydefinitivesetofrulesand aretobereadinconjunctionwithallrelevantAustralianstandards.Where theseguidelineshaveadditionalrequirementsabovethatstatedinthe Australianstandardsthentheseguidelinesshouldbefollowed.
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Design guidelines for accredited installers
Last update: January 2013 3 of 18
Accreditedinstallersshallcomplywiththefollowingstandardswhereapplicable:
3.1.1 Thegrid-interactiveinvertershallbetestedinaccordancewiththeAS4777(parts2and3) andlistedontheCleanEnergyCouncil’sapprovedinverterlist.
3.1.2 Thesystemshallcomplywiththerelevantelectricalserviceandinstallationrulesforthe statewherethesystemisinstalled. (NOTE:thelocalelectricitydistributormayhaveadditionalrequirements.)
3.1.3 Theseguidelinessetadditionalrequirementstothestandards.Anaccreditedinstaller orsupervisorisexpectedtofollowtheseguidelinesinadditiontotherequirementswithin therelevantstandards.
3.1.4 Theseguidelineswillbecomemandatoryon1February2013.
3 STANDARDSFORINSTALLATION
AS/NZS3000 WiringRules AS4777.1 Gridconnect-Installation
AS/NZS5033 InstallationofPhotovoltaic(PV)Arrays
AS/NZS1768 LightningProtection
AS/NZS4509.2 Stand-alonePowerSystems-Design
AS/NZS3008 Selectionofcables
AS1170.2 WindLoads
4.1 ExtraLowVoltage(ELV)
4.1.1 Allextralowvoltagewiringshouldbeperformedbya‘competent’person,whichis definedbytheAustralianStandardAS/NZS4509.1stand-alonepowersystemsas: “apersonwhohasacquiredthroughtraining,qualifications,experienceoracombination ofthese,knowledgeandskillenablingthatpersontocorrectlyperformthetaskrequired.”
4.2 LowVoltage(LV)
4.2.1 Alllowvoltagework:>120VDCor>50VACshallbeperformedbyalicensedelectrician.
4.2.2 Alicensedelectricianisrequiredtoberesponsibleforthesafetyofthesystemwiringprior toconnectionofthesystemtothegrid.IfthesystemcontainsELVwiringinstalledby anon-licensedperson,thenaminimumlevelofinspectionbytheelectricianpriorto closingthePVarrayisolatorswouldinclude:anopencircuitvoltagetestoneachPV stringandonthetotalarray.AvisualinspectionofanopenPVjunctionbox(randomly selected)andthemasterarrayjunctionboxisrequiredtocompleteajob.
Theseinspections/checksshallconfirm:
→ thearrayvoltagesareasdesignedandspecified
→theappropriatecables(CSAandinsulation),junctionfittingsand enclosureshavebeenused.
Boththenon-electricianELVinstaller,aswellasthelicensedelectrician, areexpectedtocarryoutthechecksontheELVwiring.
4 LICENSING
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Design guidelines for accredited installers
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Thedesignerisrequiredtoprovidethefollowingdocumentationtotheinstaller:
→ Alistofequipmentsupplied.
→ Alistofactionstobetakenintheeventofanearthfaultalarm.
→ Theshutdownandisolationprocedureforemergencyandmaintenance.
→ AbasicconnectiondiagramthatincludestheelectricalratingsofthePVarray,andtheratingsof
allovercurrentdevicesandswitchesasinstalled.
→ Site-specificsystemperformanceestimate.
→ Recommendedmaintenanceforthesystem.
→ Maintenanceprocedureandtimetable.
Systemdesignersmustcomplywiththefollowingresponsibilities.
→ Providefullspecificationsofthesystemincludingquantity,makeandmodelnumberofthesolar modulesandinverter.
→ Provideasitespecificfullsystemdesignincludingallshadingissues,orientationandtilt,alongwith thesystem’ssite-specificenergyyield,includingaveragedailyperformanceestimateinkWhfor eachmonthofsolargeneration.
→ Ensurearraydesignwillfitonavailableroofspace.
→ EnsurearraymountingframeinstallationwillcomplywithAS1170.2.
→ Ensurearrayconfigurationiscompatiblewiththeinverterspecification.
→ Ensureallequipmentisfitforpurposeandcorrectlyrated.
→ Obtainwarrantyinformationonallequipment.
→ Whendesigningagridconnectbatterybackupsystemthedesignshallbeperformedbyaperson(s) withCECgridconnecteddesignaccreditationandCECstand-alonedesignaccreditation.
5 DOCUMENTATION
6 RESPONSIBILITIES OFSYSTEMDESIGNERS
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Todesignasystemthefollowingsite-specificinformationisrequiredasaminimum:
→ occupationalsafetyrisksofthesite(scaffolding,fallprotect,elevatedworkplatformrequired)
→ whethertheroofissuitableformountingthearray
→ solaraccessforthesite
→ whetheranyshadingwilloccuranditsestimatedeffectonperformance
→ orientationandtiltangleoftheroof
→ wheretheinverterwillbelocated
→ locationofACswitchboards
→ whetheranyswitchboardormeteringalterationsarerequired.
Australiansystemsaretypicallysoldbasedonpriceorthesizethatwillfitontotheavailableroofspace.Oncethesize,inkWp,isselectedthenthedesignershalldeterminethesystem’senergyoutputoveroneyear(knownastheenergyyield).
7 SITE-SPECIFICINFORMATION
8 ENERGYYIELD
Therearemanycommercialtoolsavailabletoassistincalculatingenergyyield,forexamplePV-GC,SunEye,PVSyst,SolarPathfinder,etc.Someofthesemakeanallowanceforshading.
Itisrecommendedtouseoneofthesetoolsonthesitevisittoprovideaccurateestimatesofenergyyield.
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Design guidelines for accredited installers
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8 ENERGYYIELD
NOTE:Theaboveformulaforenergyyieldcouldberearrangedtodetermine thesizeofthearray,ifthesystemistobedesignedtoprovidea predeterminedamountofenergyperyear,forexamplewhena customerwantsasystemthatmeetstheirtotalannualenergyusage.
8.1 ENERGYYIELDFORMULA
Theaverageyearlyenergyyieldcanbeestimatedasfollows:
Esys=Parray_STCxƒmanxƒdirtxƒtempxHtiltxη pv_invxη invxη inv_sb
where:
8.1.1 Manufacturer’soutputtolerance
TheoutputofaPVmoduleisspecifiedinwatts,withamanufacturingtoleranceand isbasedonacelltemperatureof25°C(STC). Example: A160Wmodulehasamanufacturer’stoleranceof±3%.The“worstcase”adjusted outputofthePVmoduleistherefore160Wx0.97=155.2W.
8.1.2 De-ratingduetodirt
TheoutputofaPVmodulecanbereducedasaresultofabuild-upofdirtonthesurface ofthemodule.Theactualvalueofthisde-ratingwillbedependentontheactuallocation butinsomecitylocationsthiscouldbehighduetotheamountofpollutionintheair.Ifin doubt,anacceptablede-ratingwouldbe5%.
Example: Thede-ratedmoduleof155.2Wwouldbede-ratedbyafurther5%duetodirt: 155.2Wx0.95=147.4W.
Esys = averageyearlyenergyoutputofthePVarray,inwatt-hours
Parray_STC = ratedoutputpowerofthearrayunderstandardtestconditions,inwatts
ƒman = de-ratingfactorformanufacturingtolerance,dimensionless(refertosection4.2.1)
ƒdirt = de-ratingfactorfordirt,dimensionless(refertosection4.2.2)
ƒtemp = temperaturede-ratingfactor,dimensionless(refertosection4.2.3)
Htilt = Yearly(monthly)irradiationvalue(kWh/m2)fortheselectedsite(allowingfortilt,orientation)
η pv_inv = efficiencyofthesubsystem(cables)betweenthePVarrayandtheinverter
η inv = efficiencyoftheinverterdimensionless
η inv_sb = efficiencyofthesubsystem(cables)betweentheinverterandtheswitchboard.
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8.1.3 De-ratingduetotemperature
Asaminimum,inaccordancewithAS4059.2,theaveragetemperatureofthecellwithin thePVmodulecanbeestimatedbythefollowingformula:
Tcell.eff =Ta.day+25oC
where:
Tcell.eff =averagedailyeffectivecelltemperature,indegreesC
Ta.day =daytimeaverageambienttemperature(forthemonthofinterest),indegreesC.
Arrayframesinstand-alonepowersystemsaretypicallytiltedathigheranglesand themoduleshavegoodairflow.Withrooftopgrid-connectedsystems,highertemperatures havebeenobserved.
Forgrid-connectsystemstheeffectivecelltemperatureisdeterminedbythe followingformula:
Tcell.eff =Ta.day+Tr
where:
Tr =effectivetemperatureriseforspecifictypeofinstallation
Ta.day =thedaytimeambienttemperaturein°C.
Solarmoduleseachhavedifferenttemperaturecoefficients.Thesetypicallyrangefrom +0.2%/°Cto-0.5%/°Cdependantonmoduletechnology.(Refertothemanufacturer’s datasheetforexactvalues). Thede-ratingofthearrayduetotemperaturewillbedependentonthetypeofmodule installedandtheaverageambientmaximumtemperatureforthelocation.
8 ENERGYYIELD
Itisrecommendedthatthefollowingtemperaturerise(Tr)appliesfordifferentarrayframes: → paralleltoroof(<150mmstandoff):+35°C → rack-typemount(>150mmstandoff):+30°C → top-of-polemount,freestandingframeandframeonroofwithtiltangle ofabout+20degreestoslopeofroof:+25°C.
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Example continued:
Assumetheaverageambienttemperatureis25°C(Ta.day)andthemodule ispolycrystallineandframeisparalleltoroofbutlessthan150mmoffroof.
Theaveragedailyeffectivecelltemperatureis:
Tcell.eff=Ta.day+35=25+35=60°C Intheaboveformulatheabsolutevalueofthetemperaturecoefficient(γ)is applied,thisis-0.5%/°Candcelltemperatureatstandardtestconditionsis
25°C(Tstc)
Thereforetheeffectivede-ratingfactorduetotemperatureis: 1+[-0.5%X(60–25)]=1-17.5%=0.825 Thetemperaturede-ratingbecomes82.5%of147.4Wor121.6W.
8.1.4 Solarirradiationdata
Solarirradiationdataisavailablefromvarioussources,suchastheAustralianSolar RadiationDataHandbookortheMeteorologicalBureau.Theunitsusedareoften MJ/m²/day.ToconverttokWh/m²/day(PSH)divideby3.6.
8 ENERGYYIELD 8.1.3a Temperaturede-ratingformula Thetemperaturede-ratingfactoriscalculatedasfollows:
ƒtemp =1+(γ×(Tcell.eff-Tstc)) where:
ƒtemp =temperaturede-ratingfactor,dimensionless
γ =valueofpowertemperaturecoefficientperdegreesC(seeabove)
Tcell.eff =averagedailycelltemperature,indegreesC
Tstc =celltemperatureatstandardtestconditions,indegreesC.
NOTE:Themanufacturer’sspecifiedvalueofpowertemperaturecoefficient isapplied–includethe-vesignasshownonthedatasheet.Theformula determineswhetherthetemperaturefactorisgreaterorlessthan1due toactualeffectivetemperatureofthecell.
NOTE:Grid-connectedsolarPVsystemsaretypicallymountedontheroofof thehouseorbuilding.Theroofmightnotbefacingtruenorthoratthe optimumtiltangle.ThePSHfigurefortherooforientation(azimuth) andpitch(tiltangle)shallbeusedwhenundertakingthedesign.
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8.1.5 Effectoforientationandtilt
Whentheroofisnotorientatedtruenorthand/ornotattheoptimuminclination, theoutputfromthearraywillbelessthanthemaximumpossible.
Tablesareavailabletodownloadfromsolaraccreditation.com.authatcontain thefollowinginformation:
• averagedailytotalirradiationforvariousorientationsandinclinationanglesfor eachmonthoftheyear,representedasapercentageofthetotaldailyirradiation fallingonahorizontalsurface.
Thetablesprovidethedesignerwithinformationontheexpectedaveragedailytotal irradiationforvariousorientationsandinclinationanglesforeachmonthoftheyearas apercentageofthetotaldailyirradiationfallingonahorizontalsurface(whenthearray islocatedflatonahorizontalsurface).Thetablesalsoprovidethedesignerwith informationtocalculatetheenergyyieldoftheaveragedailyperformanceestimate inkWhforeachmonthofsolargeneration.
Tablesareavailabletodownloadfrom solaraccreditation.com.auforthefollowing majorcities:
→ Hobart → Melbourne → Canberra → Sydney → Brisbane → Cairns → Adelaide → AliceSprings → Darwin → Perth
8.1.6 DCcableloss ItisrecommendedthatthemaximumvoltagedropbetweenthePVarrayandtheinverter isnogreaterthan3%. Example continued: Thede-ratedmoduleof121.6Wwouldbede-ratedbyafurther3%duetoDCcableloss: 121.6Wx0.97=118W.
8.1.7 Inverterefficiency Thisisobtainedfromtheinverterspecifications. Example continued (using an inverter efficiency specification of 90%): Thede-ratedmoduleof118Wwouldbede-ratedbyafurther10%duetoinverterefficiency: 118Wx0.90=106.2W.
8.1.8 ACCableloss Itisrecommendedthatthevoltagedropbetweentheinverterandthemainswitchboard notgreaterthan1%
Example continued: Thede-ratedmoduleof106.2Wwouldbede-ratedbyafurther1%duetoACcableloss: 106.2Wx0.99=105.1W.
8 ENERGYYIELD
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8 ENERGYYIELD 8.2 SPECIFICENERGYYIELD
ThespecificenergyyieldisexpressedinkWhperkWpandiscalculatedasfollows:
SY=
Esys
Parray_STC
Tocomparetheperformanceofsystemsindifferentregions,shadinglossmustbeeliminated fromthecalculationofenergyyieldforthesitesbeingcompared.
8.3 PERFORMANCERATIO
Theperformanceratio(PR)isusedtoassesstheinstallationquality.Theperformanceratio iscalculatedasfollows:
PR=
Esys
Eideal
where:
Esys =actualyearlyenergyyieldfromthesystem
Eideal =theidealenergyoutputofthearray.
ThePVarraysidealenergyyieldEidealcandeterminedtwoways.
Method1:
Eideal=Parray_STCxHtilt
where:
Htilt =yearlyaveragedailyirradiation,inkWh/m2forthespecifiedtiltangle
Parray_STC=ratedoutputpowerofthearrayunderstandardtestconditions,inwatts
Method2:
Eideal=Hpvx ηpv
where:
Hpv =actualirradiationthatfallsonthearraysurfacearea
ηpv =efficiencyofthePVmodules
and
Hpv=HtiltxApv
where:
Htilt =yearlyaveragedailyirradiation,inkWh/m2forthespecifiedtiltangle
Apv = totalareaofthePVarray.
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Theselectionoftheinverterfortheinstallationwilldependon:
→theenergyoutputofthearray
→thematchingoftheallowableinverterstringconfigurationswiththesizeofthearrayinkW andthesizeoftheindividualmoduleswithinthatarray
→whetherthesystemwillhaveonecentralinverterormultiple(smaller)inverters.
9.1 MULTIPLEINVERTERS
9.1.1 Ifthearrayisspreadoveranumberofroovesthathavedifferentorientationsand/ortilt anglesthenthemaximumpowerpointsandoutputcurrentswillvary.Ifeconomic,installing aseparateinverterforeachsectionofthearraywhichhasthesameorientationandangle willmaximisetheoutputthetotalarray. Thiscouldalsobeachievedbyusinganinverterwithmultiplemaximumpowerpoint trackers(MPPTs).
9.1.2 Multipleinvertersallowaportionofthesystemtocontinuetooperateevenif oneinverterfails.
9.1.3 Multipleinvertersallowthesystemtobemodular,sothatincreasingthesysteminvolves addingapredeterminednumberofmoduleswithoneinverter.
9.1.4 Multipleinvertersbetterbalancephasesinaccordancewithlocalutilityrequirements.
Thepotentialdisadvantageofmultipleinvertersisthatingeneral,thecostofanumberofinverters withlowerpowerratingsisgenerallymoreexpensive.
9.2 INVERTERSIZING
Inverterscurrentlyavailablearetypicallyratedfor:
→maximumDCinputpoweri.e.thesizeofthearrayinpeakwatts →maximumDCinputcurrent →maximumspecifiedoutputpoweri.e.theACpowertheycanprovidetothegrid.
9.3 ARRAYPEAKPOWER
Thepeakpowerofthearrayiscalculatedusingthefollowingformula:
ArrayPeakPower=Numberofmodulesinthearrayxtheratedmaximumpower(Pmp) oftheselectedmoduleatSTC.
9 INVERTERSELECTION
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SYSTEM 1 SYSTEM 2 SYSTEM 3 SYSTEM 4
a)Proposedarraypeakpower(eg10x200W) 2000 2000 2000 2000
b)75%ofproposedarraypeakpower(Watts) 1500 1500 1500 1500
c)Invertermanufacturers’maximumallowablearraysizespec(Watts)
2100 1900 2100 1900
d)Invertermanufacturers’nominalACpowerrating(Watts)
1700 1700 1200 1200
Ismanufacturers’maxallowablearraysizespecgreaterthanarraypeakpower(c>a)?
YES NO YES NO
IsinverternominalACpowergreaterthan75%ofproposedarraypeakpower(d>b)
YES YES NO NO
Proposedarraypeakpower–Invertersizingacceptable
YES NO NO NO
Worked example:
Usingthedesignguidelinesithasbeendeterminedthatanarrayof6080Wpeak(32X190Wpanels) willmeetthecustomer’sneeds(offsetenergyusage,fitonavailableroofspace,meets customer’sbudget).
Thereforeweneedtocalculatewhat75%ofthearrayis: 6080WX0.75=4560W.
ThismeanstheinverterforthisarraycannothaveanominalACpoweroutputoflessthan 4560Wandthemanufacturerallowsanarrayof6080Wpeaktobeconnectedtoit.
Wherethemaximumallowablearraysizespecificationisnotspecifiedbytheinvertermanufacturer thedesignershallmatchthearraytotheinverterallowingforthede-ratingofthearray (seesection8.1.1to8.1.4).
9.4 ARRAYPEAKPOWER–INVERTERSIZING
InordertofacilitatetheefficientdesignofPVsystemstheinverternominalACpoweroutput cannotbelessthan75%ofthearraypeakpoweranditshallnotbeoutsidetheinverter manufacturer’smaximumallowablearraysizespecifications.
Example of a 2kW array and 4 inverters with different specification
9 INVERTERSELECTION
NOTE:Theinvertermanufacturer’sspecificationshallbeadheredto.
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9.5 ARRAYDE-RATINGFORMULA
Inthesectiononde-ratingmoduleperformance,thetypicalPVarrayoutputinwattsisde-rated
dueto:
→manufacturerstoleranceofthemodules →dirtandtemperature.
9.5.1 Inverterwithcrystallinemodules
Basedonfiguresof:
→0.97formanufacture
→0.95fordirt
→0.825fortemperature(basedonambientof35°C).(Refertosection8.1.3)
Thede-ratingofthearrayis: 0.97x0.95x0.825=0.76
Asaresultofthistypeofde-ratingbeingexperiencedinthefield,theinvertercan berated76%ofthepeakpowerofthearray.
9.5.2 Inverterwiththinfilmmodules
Thetemperatureeffectonthinfilmmodulesislessthanthatoncrystallinemodules. Assumingthetemperaturecoefficientisonly0.1%thenthetemperaturede-ratingat ambienttemperatureof35°Cis0.965. Basedonfiguresof:
→0.97formanufacturer →0.95fordirt →0.965fortemperature(basedonambientof35°C. Thede-ratingofthearrayis:0.97x0.95x0.965=0.889
Asaresultofthistypeofde-ratingbeingexperiencedinthefield,theinvertercaneasily berated89%ofthepeakpowerofthearray.
Example: Assumethearraycomprises16ofthe160Wpcrystallinemodulesthenthearraypeak power=16x160=2.56kW.TheinvertershouldhaveamaximumDCinputratingofat leastof2.56kWandanominalACpoweroutputratingof1.92kW(2.56kWX75%).
IfthemanufacturerdoesnotprovideDCinputspecificationsthenfollowingthe aboveguidelines. Thisarraycanbeconnectedtoaninverterwithanoutputratingof: 0.76x2.56kW=1.95kW(forcrystallinemodules)
Ifthinfilmmodulesareusedthentheinvertercouldhaveanoutputratingof: 0.889x2.56kW=2.27kW
9 INVERTERSELECTION
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9.6 MATCHINGINVERTER/ARRAYVOLTAGE Theoutputpowerofasolarmoduleisaffectedbythetemperatureofthesolarcells.Incrystalline PVmodulesthiseffectcanbeasmuchas-0.5%forevery1degreevariationintemperature. (NOTE:forotherPVcelltechnologiesthemanufacturersdatamustbeused).
Thetemperaturede-ratingfactorfortheoutputpoweris:
Ftemp=1+[γ x (Tcell_eff-TSTC)] where:
Ftemp=temperaturede-ratingfactor,dimensionless
γ =powertemperatureco-efficientper°C(typically0.005forcrystallinecells)
Tcell_eff=averagedailycelltemperature,in°C(seesectionontemperatureeffectonmodules)
TSTC =celltemperatureatstandardtestconditions,measuredin°C.
Themaximumpowerpointvoltageandopencircuitvoltageareaffectedbytemperatureandthe temperatureco-efficientasa%istypicallyverysimilartothepowercoefficient. The-maximum-effectivecelltemperature°C
Tcell_eff=Tave_amb+Tr where:
Tcell_eff =theeffectivecelltemperaturein°C
Tave_amb=thedaytimeambienttemperaturein°C
Tr =thetemperaturerisedependentonarrayframetypein°C (refertosectionontemperaturede-ratingofsolararrayfortypicalvalues)
Theformulaonpage13canalsobeappliedasthede-ratingfactorforopencircuitvoltageand maximumpowerpointvoltage.Withtheoddexception,grid-interactiveinvertersincludemaximum powerpointtrackers(MPPTs).
Manyoftheinvertersavailablewillhaveavoltageoperatingwindow.Ifthesolararrayvoltage isoutsidethiswindowtheneithertheinverterwillnotoperateortheoutputpowerofthesystemwill begreatlyreduced.Minimumandmaximuminputvoltageswillbespecifiedbythemanufacturer. Themaximumvoltageisthevoltagewhereabovethistheinvertercouldbedamaged.Some inverterswillnominateavoltagewindowwheretheywilloperateandthenamaximumvoltage, higherthanthemaximumoperatingvoltageofthewindow,whichisthevoltagewheretheinverter couldbedamaged.
Forthebestperformanceofthesystemtheoutputvoltageofthesolararrayshouldbematchedto theoperatingvoltagesoftheinverter.Tominimisetheriskofdamagetotheinverter,themaximum voltageoftheinvertershallneverbereached.
Asstatedearlier,theoutputvoltageofamoduleiseffectedbycelltemperaturechangesinasimilar wayastheoutputpower.
ThePVmodulemanufacturerswillprovideavoltagetemperatureco-efficient.Itisgenerally specifiedinV/°C(ormV/°C)butitcanbeexpressedasa%/°C.
Todesignsystemswheretheoutputvoltagesofthearraydonotfalloutsidetherangeofthe inverter’sDCoperatingvoltagesandmaximumvoltage(ifdifferent),theminimumandmaximum daytimetemperaturesforthatspecificsitearerequired.
9 INVERTERSELECTION
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9.7 MINIMUMVOLTAGEWINDOW Whenthetemperatureisatamaximumthenthemaximumpowerpointvoltage(Vmp)ofthearray shouldnotfallbelowtheminimumoperatingvoltageoftheinverter.Theactualvoltageattheinput oftheinverterisnotjusttheVmpofthearray,thevoltagedropintheDCcablingmustalsobe includedwhendeterminingtheactualinverterinputvoltage.
Thetemperaturede-ratingfactorcanbeadaptedtodeterminethemaximumpowerpointvoltage ataspecifiedtemperature.
Vmp_cell.eff= Vmp-STC+[γv x (Tcell_eff-TSTC)]
Where:
Vmp_cell.eff =Maximumpowerpointvoltageateffectivecelltemperature,involts
Vmp-STC =MaximumpowerpointvoltageatSTC,involts
γv =Voltagetemperature(Vmp)coefficientinvoltsper°C
Tcell_eff =celltemperatureatspecifiedambienttemperature,measuredin°C
TSTC =celltemperatureatSTC,measuredin°C
Tomaximisetheperformanceofthearray,theminimumarrayvoltageshouldneverfallbelowthe minimumvoltageoperatingwindowoftheinverter.Thenumberofmodulesinthestringshouldbe selectedsothatthemaximumpowervoltageofthearrayforthehighesttemperatureexpectedis abovetheminimumvoltageoperatingwindowoftheinverter.
SincethedaytimeambienttemperatureinsomeareasofAustraliacanreachorexceed35°Citis recommendedthatmaximumeffectivecelltemperatureof70°Cisused.
9 INVERTERSELECTION
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9.7 MINIMUMVOLTAGEWINDOW Worked example: Assumethattheminimumvoltagewindowforaninverteris140V.Themoduleselectedhasarated
MPPvoltageof35.4Vandavoltage(Vmp)co-efficientof-0.177V/°C
UsingequationforVmp_cell.effabove,theminimumMPPvoltageatamaximumeffectivecell temperatureof70°C,thetemperaturede-ratingis:
Vmin_mpp= 35.4+[-0.177 x (70-25)] = 27.4V
Ifweassumeamaximumvoltagedropinthecablesof3%thenthevoltageattheinverterforeach modulewouldbe
0.97x27.4=26.6V
ThisistheeffectiveminimumMPPvoltageinputattheinverterforeachmoduleinthearray,
Vmin_mpp_inv
Theminimumnumberofmodulesinthestringcanbedeterminedbythefollowingequation:
Nmin_per_string=
Vinv_min(V) Vmin_mpp_inv(V)
where:
Vinv_min =theminimuminverterinputvoltage
Vmin_mpp_inv =theeffectiveminimumMPPvoltageofamoduleattheinverterat maximumeffectivecelltemperature Theminimumvoltageallowedattheinverter,inthisexample,is140V. TheMPPvoltageriseswithincreasesinirradiance.Sincethearrayistypicallyoperatingwith irradiancelevelslessthan1kW/m²thentheactualMPPvoltagewouldbereduced.
Intheworkedexampleabove,aminimuminvertervoltageof1.1x140V=154Vshouldbeused. Theminimumnumberofmodulesinastringis:
Nmin_per_string=154/26.6=5.8roundedupto6modules.
9 INVERTERSELECTION
NOTE:Theexactvariationisdependentonthequalityofthesolarcellsoit isrecommendedthatasafetymarginof10%isused
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9.8 MAXIMUMVOLTAGEWINDOW Atthecoldestdaytimetemperaturetheopencircuitvoltageofthearrayshallneverbegreaterthan themaximumallowedinputvoltagefortheinverter.Theopencircuitvoltage(Voc)isusedbecause thisisgreaterthantheMPPvoltageanditistheappliedvoltagewhenthesystemisfirstconnected (priortotheinverterstartingtooperateandconnectingtothegrid).
Inearlymorning,atfirstlight,thecelltemperaturewillbeveryclosetotheambienttemperature becausethesunhasnothadtimetoheatupthemodule.Therefore,thelowestdaytimetemperature fortheareawherethesystemisinstalledshallbeusedtodeterminethemaximumVoc. Thisisdeterminedbythefollowingequation:
Vmax_oc= Voc_STC+[γv x (Tmin- TSTC)]
where:
Vmax_oc =Opencircuitvoltageatminimumcelltemperature,volts
Voc_STC =OpencircuitvoltageatSTC,volts
γv =voltagetemperatureVocco-efficient,-V/°C
Tmin =expectedmin.dailycelltemperature,°C
TSTC =celltemperatureSTC,°C
InmanyareasofAustralia,theminimumdaytimeambienttemperaturecanbelessthan0°Cwhile thereareareaswhereitneverfallsbelow20°C.
9 INVERTERSELECTION
NOTE:Someinvertersprovideamaximumvoltageforoperationandahigher voltageasthemaximumallowedvoltage.Inthissituation,theMPPvoltage isusedfortheoperationwindowandtheopencircuitvoltageforthe maximumallowedvoltage.
NOTE:Itisrecommendedthatthedesignerusetheminimumtemperaturefor theareawherethesystemwillbeinstalled.
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9.8 MAXIMUMVOLTAGEWINDOW Intheworkedexample,assumetheminimumeffectivecelltemperatureis0°C:
Voc_STCis43.2V andthemaximumopencircuitvoltage-atminimumeffectivetemperatureis:
Voc_max =43.2+(-0.16×(0-25)) =43.26+(-0.16x-25) =43.2+4 =47.2V
Forourexample,assumingthemaximumvoltageallowedbytheinverteris400V(Vinv_max)
Themaximumnumberofmodulesinthestring,Nmax_per_string,isdeterminedby thefollowingequation:
Nmax_per_string=
Vinv_max(V) Voc_max(V)
=400/47.2=8.47roundeddownto8modules
Intheexamplepresented,thePVstringmustconsistofbetween6-8modulesonly. Intheworkedexample,forsizingtheinverter16moduleswererequired.
Thereforewecouldhavetwoparallelstringsof8modules.
9.9 INVERTERDCINPUTCURRENT
EnsurethatthetotalshortcircuitcurrentofthearraydoesnotexceedthemaxDCinputcurrent specificationoftheinverter.
9.10 EFFECTSOFSHADOWS
Intownsandcitieswheregrid-connectedPVsystemswillbedominant,theroofofthehouseor buildingwillnotalwaysbefreeofshadowsduringpartsoftheday.Careshouldbetakenwhen selectingthenumberofmodulesinastringbecauseshadingcouldresultinthemaximumpower pointvoltageathightemperaturesbeingbelowtheminimumoperatingvoltageoftheinverter.
9 INVERTERSELECTION