CECAccreditation_DesignGuidelinesFeb2013

7/27/2019 CECAccreditation_DesignGuidelinesFeb2013

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GRID-CONNECTEDSOLAR PV SYSTEMS

NO 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 omissionand 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 of18

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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Theobjectivesoftheseguidelinesareto:

improvethesafety,performanceandreliabilityofsolarphotovoltaicpowersystemsinstalled inthefield

encourageindustrybestpracticeforalldesignandinstallationworkinvolvingsolar

photovoltaicpowersystems

provideanetworkofcompetentsolarphotovoltaicpowersystemsdesignersandinstallers

toincreasetheuptakeofsolarphotovoltaicpowersystems,bygivingcustomersincreased

confidenceinthedesignandinstallationwork.

Theperformanceofareliableinstallationthatfulfilscustomerexpectationsrequiresboth

carefuldesignandcorrectinstallationpractice.Compliancewithrelevantstatehealthandsafetyregulationsisalsonecessary.

ThisdocumentusesthesameterminologyasoutlinedinAS/NZS5033.Twoimportantdefinitionsare:

2.1.1 Wherethewordshallisused,thisindicatesthatastatementismandatory.

2.1.2 Wherethewordshouldisused,thisindicatesthatastatementisarecommendation.

1 GENERAL

2 DEFINITIONS

NOTE:Theseguidelinesalonedonotconstituteafullydefinitivesetofrulesand aretobereadinconjunctionwithallrelevantAustralianstandards.Where theseguidelineshaveadditionalrequirementsabovethatstatedinthe Australianstandardsthentheseguidelinesshouldbefollowed.


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Accreditedinstallersshallcomplywiththefollowingstandardswhereapplicable:

3.1.1 Thegrid-interactiveinvertershallbetestedinaccordancewiththeAS4777(parts2and3) andlistedontheCleanEnergyCouncilsapprovedinverterlist.

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-InstallationAS/NZS5033 InstallationofPhotovoltaic

(PV)ArraysAS/NZS1768 LightningProtection

AS/NZS4509.2 Stand-alonePowerSystems-Design

AS/NZS3008 Selectionofcables

AS1170.2 WindLoads

4.1 ExtraLowVoltage(ELV)

4.1.1 Allextralowvoltagewiringshouldbeperformedbyacompetentperson,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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Thedesignerisrequiredtoprovidethefollowingdocumentationtotheinstaller:

Alistofequipmentsupplied.

Alistofactionstobetakenintheeventofanearthfaultalarm.

Theshutdownandisolationprocedureforemergencyandmaintenance.

AbasicconnectiondiagramthatincludestheelectricalratingsofthePVarray,andtheratingsof

allovercurrentdevicesandswitchesasinstalled.

Site-specificsystemperformanceestimate.

Recommendedmaintenanceforthesystem.

Maintenanceprocedureandtimetable.

Systemdesignersmustcomplywiththefollowingresponsibilities.

Providefullspecificationsofthesystemincludingquantity,makeandmodelnumberofthesolar modulesandinverter.

Provideasitespecificfullsystemdesignincludingallshadingissues,orientationandtilt,alongwith thesystemssite-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,isselectedthenthedesignershalldeterminethesystemsenergyoutputoveroneyear(knownastheenergyyield).

7 SITE-SPECIFICINFORMATION

8 ENERGYYIELD

Therearemanycommercialtoolsavailabletoassistincalculatingenergyyield,forexamplePV-GC,SunEye,PVSyst,SolarPathfinder,etc.Someofthesemakeanallowanceforshading.

Itisrecommendedtouseoneofthesetoolsonthesitevisittoprovideaccurateestimatesofenergyyield.


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8 ENERGYYIELD

NOTE:Theaboveformulaforenergyyieldcouldberearrangedtodetermine

thesizeofthearray,ifthesystemistobedesignedtoprovidea predeterminedamountofenergyperyear,forexamplewhena customerwantsasystemthatmeetstheirtotalannualenergyusage.

8.1 ENERGYYIELDFORMULA

Theaverageyearlyenergyyieldcanbeestimatedasfollows:

Esys=Parray_STCxmanxdirtxtempxHtiltxpv_invx invx inv_sb

where:

8.1.1 Manufacturersoutputtolerance

TheoutputofaPVmoduleisspecifiedinwatts,withamanufacturingtoleranceand isbasedonacelltemperatureof25C(STC). Example: A160Wmodulehasamanufacturerstoleranceof3%.Theworstcaseadjusted 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

man = de-ratingfactorformanufacturingtolerance,dimensionless(refertosection4.2.1)

dirt = de-ratingfactorfordirt,dimensionless(refertosection4.2.2)

temp = temperaturede-ratingfactor,dimensionless(refertosection4.2.3)

Htilt = Yearly(monthly)irradiationvalue(kWh/m2)fortheselectedsite

(allowingfortilt,orientation)

pv_inv = efficiencyofthesubsystem(cables)betweenthePVarrayandtheinverter

inv = efficiencyoftheinverterdimensionless

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 =thedaytimeambienttemperatureinC.

Solarmoduleseachhavedifferenttemperaturecoefficients.Thesetypicallyrangefrom +0.2%/Cto-0.5%/Cdependantonmoduletechnology.(Refertothemanufacturers datasheetforexactvalues). Thede-ratingofthearrayduetotemperaturewillbedependentonthetypeofmodule

installedandtheaverageambientmaximumtemperatureforthelocation.

8 ENERGYYIELD

Itisrecommendedthatthefollowingtemperaturerise(Tr)appliesfordifferentarrayframes:

paralleltoroof(150mmstandoff):+30C top-of-polemount,freestandingframeandframeonroofwithtiltangle ofabout+20degreestoslopeofroof:+25C.


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Example continued:

Assumetheaverageambienttemperatureis25C(Ta.day)andthemodule ispolycrystallineandframeisparalleltoroofbutlessthan150mmoffroof.

Theaveragedailyeffectivecelltemperatureis:

Tcell.eff=Ta.day+35=25+35=60C Intheaboveformulatheabsolutevalueofthetemperaturecoefficient()is applied,thisis-0.5%/Candcelltemperatureatstandardtestconditionsis

25C(Tstc) Thereforetheeffectivede-ratingfactorduetotemperatureis: 1+[-0.5%X(6025)]=1-17.5%=0.825 Thetemperaturede-ratingbecomes82.5%of147.4Wor121.6W.

8.1.4 Solarirradiationdata

Solarirradiationdataisavailablefromvarioussources,suchastheAustralianSolar RadiationDataHandbookortheMeteorologicalBureau.Theunitsusedareoften

MJ/m/day.ToconverttokWh/m/day(PSH)divideby3.6.

8 ENERGYYIELD

8.1.3a Temperaturede-ratingformula Thetemperaturede-ratingfactoriscalculatedasfollows:

temp =1+((Tcell.eff-Tstc)) where:

temp =temperaturede-ratingfactor,dimensionless =valueofpowertemperaturecoefficientperdegreesC(seeabove)

Tcell.eff =averagedailycelltemperature,indegreesC Tstc =celltemperatureatstandardtestconditions,indegreesC.

NOTE:Themanufacturersspecifiedvalueofpowertemperaturecoefficient isappliedincludethe-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=

EsysParray_STC

Tocomparetheperformanceofsystemsindifferentregions,shadinglossmustbeeliminated fromthecalculationofenergyyieldforthesitesbeingcompared.

8.3 PERFORMANCERATIO

Theperformanceratio(PR)isusedtoassesstheinstallationquality.Theperformanceratio iscalculatedasfollows:

PR=

EsysEideal

where:

Esys =actualyearlyenergyyieldfromthesystem

Eideal =theidealenergyoutputofthearray.

ThePVarraysidealenergyyieldEidealcandeterminedtwoways.

Method1:

Eideal=Parray_STCxHtilt where:

Htilt =yearlyaveragedailyirradiation,inkWh/m2forthespecifiedtiltangle

Parray_STC=ratedoutputpowerofthearrayunderstandardtestconditions,inwatts

Method2:

Eideal=Hpvxpv where:

Hpv =actualirradiationthatfallsonthearraysurfacearea

pv =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)Invertermanufacturersmaximumallowablearraysizespec(Watts)

2100 1900 2100 1900

d)InvertermanufacturersnominalACpower

rating(Watts)

1700 1700 1200 1200

Ismanufacturersmaxallowablearraysizespecgreaterthanarraypeakpower(c>a)?

YES NO YES NO

IsinverternominalACpowergreaterthan75%ofproposedarraypeakpower(d>b)

YES YES NO NO

ProposedarraypeakpowerInvertersizingacceptable

YES NO NO NO

Worked example:

Usingthedesignguidelinesithasbeendeterminedthatanarrayof6080Wpeak(32X190Wpanels) willmeetthecustomersneeds(offsetenergyusage,fitonavailableroofspace,meets customersbudget).

Thereforeweneedtocalculatewhat75%ofthearrayis: 6080WX0.75=4560W.

ThismeanstheinverterforthisarraycannothaveanominalACpoweroutputoflessthan 4560Wandthemanufacturerallowsanarrayof6080Wpeaktobeconnectedtoit.

Wherethemaximumallowablearraysizespecificationisnotspecifiedbytheinvertermanufacturer thedesignershallmatchthearraytotheinverterallowingforthede-ratingofthearray (seesection8.1.1to8.1.4).

9.4 ARRAYPEAKPOWERINVERTERSIZING

InordertofacilitatetheefficientdesignofPVsystemstheinverternominalACpoweroutput

cannotbelessthan75%ofthearraypeakpoweranditshallnotbeoutsidetheinverter manufacturersmaximumallowablearraysizespecifications.

Example of a 2kW array and 4 inverters with different specification

9 INVERTERSELECTION

NOTE:Theinvertermanufacturersspecificationshallbeadheredto.


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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(basedonambientof35C).(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 ambienttemperatureof35Cis0.965. Basedonfiguresof:

0.97formanufacturer 0.95fordirt

0.965fortemperature(basedonambientof35C. 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+[ x (Tcell_eff-TSTC)] where:

Ftemp=temperaturede-ratingfactor,dimensionless =powertemperatureco-efficientperC(typically0.005forcrystallinecells)

Tcell_eff=averagedailycelltemperature,inC(seesectionontemperatureeffectonmodules) TSTC =celltemperatureatstandardtestconditions,measuredinC.

Themaximumpowerpointvoltageandopencircuitvoltageareaffectedbytemperatureandthe temperatureco-efficientasa%istypicallyverysimilartothepowercoefficient. The-maximum-effectivecelltemperatureC

Tcell_eff=Tave_amb+Tr where:

Tcell_eff =theeffectivecelltemperatureinCTave_amb=thedaytimeambienttemperatureinCTr =thetemperaturerisedependentonarrayframetypeinC

(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 invertersDCoperatingvoltagesandmaximumvoltage(ifdifferent),theminimumandmaximum daytimetemperaturesforthatspecificsitearerequired.

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 temperatureof70C,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:

N

min_per_string=

Vinv_min(V)Vmin_mpp_inv(V)

where:

Vinv_min =theminimuminverterinputvoltage

Vmin_mpp_inv =theeffectiveminimumMPPvoltageofamoduleattheinverterat maximumeffectivecelltemperature Theminimumvoltageallowedattheinverter,inthisexample,is140V. TheMPPvoltageriseswithincreasesinirradiance.Sincethearrayistypicallyoperatingwith irradiancelevelslessthan1kW/mthentheactualMPPvoltagewouldbereduced.

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+[v x (Tmin- TSTC)]

where:

Vmax_oc =Opencircuitvoltageatminimumcelltemperature,volts Voc_STC =OpencircuitvoltageatSTC,volts

v =voltagetemperatureVocco-efficient,-V/C Tmin =expectedmin.dailycelltemperature,C TSTC =celltemperatureSTC,C

InmanyareasofAustralia,theminimumdaytimeambienttemperaturecanbelessthan0Cwhile thereareareaswhereitneverfallsbelow20C.

9 INVERTERSELECTION

NOTE:Someinvertersprovideamaximumvoltageforoperationandahigher voltageasthemaximumallowedvoltage.Inthissituation,theMPPvoltage isusedfortheoperationwindowandtheopencircuitvoltageforthe maximumallowedvoltage.

NOTE:Itisrecommendedthatthedesignerusetheminimumtemperaturefor theareawherethesystemwillbeinstalled.


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9.8 MAXIMUMVOLTAGEWINDOW

Intheworkedexample,assumetheminimumeffectivecelltemperatureis0C: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:

N

max_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

CECAccreditation_DesignGuidelinesFeb2013

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