Photovoltaic System Commissioning and Testing

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Photovoltaic System Commissioning and TestingA Guide for PV System Technicians and Engineers

Part of

4097 PV Com & Test_CoverV3:Layout 2 28/6/12 09:41 Page 2

1 Introduction 2

1.1 Codes and Standards 3

1.2 Safety 4

2 PV System Fundamentals 5

2.1 PV Module Performance 5

2.1.1 Response to Irradiance 7

2.1.2 Response to Temperature 8

2.1.3 Rating Conditions. 9

2.1.4 PV Arrays 10

2.2 Types of PV Systems 12

3 System Documentation 14

4 Commissioning PV Systems 16

4.1 Final Installation Checkout 16

4.2 Visual Inspection 16

4.2.1 Labels and Markings 18

5 System Testing and Measurements 19

5.1 Continuity Testing 20

5.2 Polarity Testing 23

5.3 Voltage and Current Testing 24

5.3.1 Open-Circuit Voltage Testing 25

5.3.2 Short-Circuit Current Testing 25

5.4 Insulation Resistance Testing 26

5.5 System Functional Testing 29

5.5.1 Test Reports 30

6 System Performance Testing 30

6.1 Verifying Power and Energy Production 30

6.1.1 Verifying AC Power Output 31

6.1.2 Verifying AC Energy Prodution 34

6.2 Array I-V Measurements 35

6.3 Other Tests 37

6.3.1 Power Quality Analysis 37

6.3.2 Thermal Imaging 37

6.3.3 Inverter Efficiency Tests 38

6.3.4 Maximum Power Point Tracking Tests 38

6.3.5 Shading Analysis 38

7 System Maintenance 40

7.1 Testing 41

7.2 Troubleshooting 41

Contents

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1.1 Codes and Standards

The National Electrical Code (NEC), NFPA 70governs the requirements for most non-utilityelectrical installations in the U.S., including solarphotovoltaic (PV) systems. When adopted intothe building codes by states or local jurisdictions,the NEC becomes the basis for inspections andapprovals of electrical installations.

Chapters 14 of the NEC generally apply to allelectrical systems, covering installationrequirements, wiring and protection, wiringmethods and materials, and equipment forgeneral use. Article 690 of the NEC covers specialinstallation requirements for solar photovoltaicsystems, however many other articles also apply.Additionally, Article 705 covers requirements forthe interconnecting PV systems and otherdistributed generation equipment to othersources, such as the utility grid.

While the NEC does not specifically address PVsystem performance, it does establishrequirements for the overall quality and safety ofelectrical installations. Many of theserequirements can be verified through visualinspections and a review of the system designand installation documents. However, certainNEC requirements can only be validated byelectrical tests and measurements. These

include verifying the continuity of groundingsystems, verifying system voltages and currents,and measuring insulation resistance to determinewiring integrity.

Other standards also apply to the testing andcommissioning of PV systems. Compliance withthese additional codes or standards may berequired by local authorities, or requested bybuyers, lenders or underwriters to help ensure thehighest levels of safety, quality and performancefor PV installations.

The international standard IEC 62446 GridConnected PV Systems Minimum Requirementsfor System Documentation, Commissioning Tests,and Inspection defines minimum documentation,commissioning tests and inspection criteria forgrid-connected PV systems. Information derivedfrom this standard is intended to verify the safeand proper operation of PV systems, and to serveas a guide for designers, installers and servicepersonnel. This IEC standard covers specificrequirements for testing and commissioning PVsystems not addressed by the NEC and otherelectrical installation codes, such as IEC 60364:Electrical Installations for Buildings. Section 712of IEC 60364 addresses specific requirements forsolar photovoltaic (PV) power supply systems.

Compliance with the IEC 62446 standard isrequired for many PV projects in Europe andsimilar requirements are being listed inprocurement documents for new projects in theU.S. Many of the testing procedures outlined inthis standard correlate directly with NECrequirements for the verification of safety for allelectrical systems, which are addressedthroughout this document.

1 INTRODUCTION

Solar photovoltaic (PV) systems are beinginstalled in ever increasing numbers throughoutthe world, and are expected to safely and reliablyproduce electrical energy over several decades ofoperation. However, many systems are notsatisfactorily evaluated prior to being placed intoservice, and many have had little, if any,scheduled maintenance or testing over theirlifetime. Unfortunately, this often leads to unsafeand underperforming systems with reduced valueto their owners.

Any electrical system can be tested to verifyperformance and to evaluate the condition of thewiring systems and equipment. This is particularlyimportant for PV installations, which are subjectedto extreme environmental conditions anddeteriorating effects of the elements over manyyears. To help ensure the long-term safe

operation of these systems, quality PV installationand service contractors execute a thoroughcommissioning process followed by a regularperiodic testing and maintenance program.These practices can help promote safety andoptimize performance, and provide essentialinformation required to effectively troubleshoot,diagnose and remedy problems with the system.

All PV systems require testing for performanceand safety verifications. The level of testingrequired will depend on local regulators, thecustomer desires, and quality commitments ofinstallation and maintenance contractors. Thisdocument provides an overview of thecommissioning and testing process, and appliesgenerally to interactive PV systems that areinterconnected to the utility grid. It addresses theapplicable codes and standards, in addition totesting equipment, procedures, and interpretationof the results. See Figure 1.

EnergySource

EnergyConversion

PV ArrayPV Array

PowerConditioning

PowerDistribution Electrical

Load

ElectricUtilityEnergy

Storage(optional)

PV ArrayInverter

LoadCenter

Battery

NFPA 70B Recommended Practice forElectrical Equipment Maintenanceprovides guidelines on test methods andthe preventive maintenance of electricalsystems. This standard can be used inpart to help verify NEC requirementsthrough electrical system measurements

Figure 1. PV system overview

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1.2 Safety

Conducting electrical testing on any PV systemshould be performed by qualified individualshaving knowledge and experience with electricalsystems measurements, the test equipment used,the equipment or systems being tested, and anawareness of the hazards involved. Working withPV systems involves exposure to energizedcircuits with high voltages and potentially lethalcurrents, presenting electrical shock hazards.Battery systems and higher voltage installationscan also present electrical burn and arc flashhazards. When these electrical hazards arecombined other hazards such as working atheights and in difficult locations exposed to theelements, it is imperative for those installing andservicing PV systems to follow all applicable safetystandards and guidelines.

The Occupational Safety and HealthAdministration (OSHA) issues and enforcesstandards in the U.S. for worker safety and healthin the U.S. In particular, OSHA regulations coveredin CFR 29 Part 1926 Safety and HealthRegulations for Construction address a broadscope of safety hazards likely to be encountered inconstructing and maintaining PV installations. Thestandards can be freely downloaded from theOSHA website, see: www.osha.gov

OSHA regulations require that employers providea safe and healthy workplace free of hazards, andfollow the applicable OSHA standards. Employersmust provide safety training to affected employeesaddressing all probable hazards on a constructionsite. Workers are responsible for following theemployers safety and health rules and wearingand maintaining safety gear as instructed. AnOSHA 10-Hour Construction Industry Trainingprogram covers the requirements for avoiding andmitigating a number of jobsite safety hazardsaddressed in 29 CFR 1926, including electricalsafety, personal protective equipment (PPE), fallprotection systems, stairways and ladders, handand power tools, cranes and lifts, excavations,scaffolding, and others.

Best practices for preventing electrical hazardsand other common safety hazards associatedwith PV installations include the following:

Carry out a risk assessment beforeconducting any work at the site.

Working on electrical equipment and circuitsin a de-energized state using documentedlockout and tagout procedures.

Wearing the appropriate PPE, includingprotective clothing, nonconductive Class E

Qualified Person The NEC defines a qualified person asone who has skills and knowledgerelated to the construction and operationof the electrical equipment andinstallations and has received safetytraining to recognize and avoid thehazards involved. However, the NEC isnot very specific about the extent ofsafety training required relative to thelevel of hazards and types of workinvolved, presuming individuals may bequalified for certain tasks and not others.Generally, individuals installing orservicing PV installations should haveappropriate experience in working withPV systems as well as electrical systems,and safety training consistent with therequirements outlined in NFPA 70E-2009,Standard for Electrical Safety in theWorkplace.

hardhat, electrical hazard (EH) rated footprotection, and safety glasses at all times.

Using electrically insulated hand tools andproperly grounded or double-insulatedpower tools maintained in good condition.

Avoiding contact with overhead power linesand buried electrical conductors.

Using ladders with wooden or fiberglass railswhen working on or near energizedconductors.

Mitigating fall hazards and using personal fallarrest systems (PFAS) whenever working atunprotected heights of 6 feet or more.

Maintaining an orderly work site andcautious approach.

In some cases, working on energized equipmentis unavoidable, for example when makingmeasurements on PV arrays that are alwaysenergized when exposed to sunlight. Certain testequipment, such as megohmmeters andinsulation testers also produce high test voltages,and appropriate safety precautions must beobserved when using this equipment. Properelectrical insulating gloves and other applicablePPE should always be worn when working on ortesting energized circuits. The level of PPErequired depends on the voltage levels and faultcurrents for the circuits under test. Particular careshould be exercised whenever touching a PVarray or associated conductive surfaces toprotect against electrical shock, especially whenfaults are suspected.

General safety recommendations for usingelectrical test equipment include:

Follow manufacturers instructions for the

safe operation of any test instruments.

Only use test instruments for their intendedpurpose, within their established limits andratings.

Carefully inspect test equipment and leadsprior to each use.

Properly maintain test instruments andrecommended calibrations.

Plan and review all testing, safety andemergency procedures in advance.

Use appropriate personal protectiveequipment, including electrical insulatinggloves.

Work with a partner.

2 PV SYSTEM FUNDAMENTALS

Photovoltaic systems convert solar radiation intoelectrical energy suitable for powering electricalloads or other utilization equipment. A fundamentalunderstanding of PV device performance andsystem designs is essential for conductingmeaningful tests and evaluations and interpretingthe results.

2.1 PV Module Performance

Photovoltaic cells, modules and arrays producedc power when exposed to sunlight. Theirelectrical performance is represented by theircurrent-voltage (I-V) characteristic. An I-V curverepresents an infinite number of current andvoltage operating point pairs for a PV generatingdevice, at a given solar irradiance andtemperature operating condition. See Figure 2.PV modules produce voltage and current outputthat varies with solar irradiance and temperature.Key operating points along the I-V curve are

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76

with maximum power voltage (Vmp) = 35.8 V,and maximum power current (Imp) = 4.89 A. Theload resistance required to operate this moduleat maximum power is equal to Vmp Imp = 35.8V 4.89 A = 7.32 . The maximum rated dcpower is simply the product of the maximumpower current and voltage. See Figure 3.

Figure 3. The operating point on an I-Vcurve is determined by the electrical loadresistance according to Ohms Law.

In application, the operating point on the I-Vcurve is determined by the specific equipmentconnected to the output of the PV array. If theload is a battery, the battery voltage sets theoperating point on the I-V curve. If the PV array isconnected to an interactive inverter, the inverterseeks to operate the PV at its maximum powerpoint as long as the array voltage operates withinthe inverter limits. Maximum power point tracking(MPPT) refers to the process or electronicequipment used to operate PV modules or arraysat their maximum power point under varying I-Vcurve conditions. MPPT circuits are integral tointeractive inverters, some charge controllers andalso available as separate equipment or part of

PV array source circuit combiner boxes.Measurements of PV array operating points andmaximum power point tracking can be verifiedduring system testing.

2.1.1 Response to Irradiance

Changes in solar radiation have a linear andproportional effect on the current and maximumpower output of a PV module or array. See Figure4.

Figure 4. Changes in solar radiation have adirect linear and proportional effect on thecurrent and maximum power output of aPV module or array.

Therefore, doubling the solar irradiance on thesurface of the array doubles the current andmaximum power output (assuming constanttemperature). Changing irradiance has a smallereffect on voltage, mainly at lower irradiancelevels. Because voltage varies little with changingirradiance levels over 200 W/m2, PV devices arewell-suited for battery charging applications. SeeFigure 5.

rated by the manufacturer at specified testconditions and affixed on product labels. Theserated I-V parameters are the basis for sizing anddesigning the photovoltaic source and outputcircuits, and for comparing with fieldmeasurements on PV arrays.

Figure 2. I-V curves represent the electricalperformance for PV modules and arrays.

Open-circuit voltage (Voc) is the maximum dcvoltage on a given I-V curve, and is theoperating point for a PV device with noconnected load. Voc corresponds to aninfinite resistance or open-circuit condition,and zero current and zero power output.Open-circuit voltage is independent of cellarea and increases with decreasing celltemperature, and is used to determinemaximum circuit voltages for PV modulesand arrays. For crystalline silicon solar cells,the open-circuit voltage is typically on theorder of 0.5 V to 0.6 V at 25C. Typical PVmodules have between 60 and 72 series-connected cells with Voc ranging from about34 V to 44 V.

Short-circuit current (Isc) is the maximumcurrent on an I-V curve. Isc corresponds to azero resistance and short-circuit condition, atzero voltage and zero power output. Short-circuit current is directly proportional to solarirradiance, and rated values are used to sizePV circuit conductors and overcurrentdevices. Because PV modules are inherentlycurrent-limited, PV modules can be short-circuited for testing using an appropriatelyrated shorting device. Individual solar cellsmay produce Isc be up to and over in area,with Isc ratings 8 A and higher.

The maximum power point (Pmp) of a PVdevice is the operating point where theproduct of current and voltage is at itsmaximum. The maximum power point islocated on the knee of the I-V curve, andrepresents the highest efficiency operatingpoint for a PV device under the givenconditions of solar irradiance and celltemperature. Typical PV modules have ratedmaximum power of 200 W to 300 W.

The maximum power voltage (Vmp) is thecorresponding operating voltage at Pmp,and is typically about 70% to 80% of theopen-circuit voltage.

The maximum power current (Imp) is theoperating current at Pmp, and typicallyabout 90% of the short-circuit current.

The specific operating point on an I-V curve isdetermined by the electrical load according toOhms Law. Consequently, the load resistance tooperate a PV module or array at its maximumpower point is equal to the maximum powervoltage divided by the maximum power current(Vmp/Imp). For example, consider a PV module

Current vs. voltagePower vs. voltage

Isc

Imp

Pmp

Cur

rent

(A)

Pow

er (W

)

Voltage (V) Vmp Voc

Pmp = Imp x Vmp

R=0

Cur

rent

ConstantTemperature

Voltage

Decreasingresistance

Increasingresistance

R=

Load lines ofconstant resistance

Cur

rent

ConstantTemperatureVoltage

Current increases withconstant resistance

Maximum power increases with increasing irradiance

Maximum power voltage changes little with irradiance

Voc changes littlewith irradiance

1000 W/m2

750 W/m2

500 W/m2

250 W/m2


98

Figure 6. For crystalline silicon PV devices,increasing cell temperature results in adecrease in voltage and power, and a smallincrease in current.

Since PV modules produce their highest voltageat the lowest temperatures, this voltagedetermines the minimum voltage ratings requiredfor the modules and associated dc circuitcomponents [NEC 690.7]. For crystalline siliconPV modules, the maximum voltage for PV arrays

is determined by multiplying the module ratedopen-circuit voltage (Voc) by the number ofmodules in series, and by a voltage correctionfactor [NEC Table 690.7]. Where other thancrystalline silicon (thin-film) PV modules are used,or if temperature coefficients are provided withlisted installation instructions, temperaturetranslations should follow manufacturersinstructions using provided coefficients.

2.1.3 Rating Conditions

Standard Test Conditions (STC) is a universalrating condition for PV modules and arrays, andspecifies the electrical output at a solar irradiancelevel of 1000 W/m2 at AM 1.5 spectraldistribution, and 25C cell temperature. Theperformance of PV modules and arrays issometimes represented at other test conditionscloser to actual field operations, such as PVUSATest Conditions (PTC), which is based on 1000W/m solar irradiance, 45C cell temperature,1 m/s wind speed.

Figure 5. PV module current and voltageare affected differently by solar irradiance.

The short-circuit current (Isc), maximum powercurrent (Imp), and maximum power (Pmp) at onecondition of solar irradiance may be translated toestimate the value of these parameters atanother irradiance level:

Isc2 = Isc1 (E2/E1)

Pmp2 = Pmp1 (E2/E1)

Imp2 = Imp1 (E2/E1)

where

Isc1 = rated short-circuit current at irradiance E1 (A)

Isc2 = short-circuit current at new irradiance E2 (A)

E1 = rated solar irradiance (W/m2)

E2 = new solar irradiance (W/m2).

Pmp1 = rated maximum power at irradiance E1 (W)

Pmp2 = new maximum power at new irradiance E2 (W).

Imp1 = original maximum power current at irradiance E1 (A)

Imp2 = new maximum power current at new irradiance E2 (A).

Installers and commissioning specialists verifythe performance of PV systems in the field bymeasuring the solar irradiance incident on PVarrays, and correlating the measured poweroutput with specifications. For example, if it hasbeen established that the rated peak output of aPV array is 10 kW under incident radiation levelsof 1000 W/m2 at normal operating temperatures,then the output of the array should be expectedto be around 7 kW if the solar irradiance is 700W/m2, assuming constant temperature.

2.1.2 Response to Temperature

The current and voltage output of a PV moduleare temperature dependent. For crystalline siliconPV devices, increasing cell temperature results ina measureable decrease in voltage and power,and a slight increase in current. Higher celloperating temperatures also reduce cellefficiency and lifetime. The temperature effectson PV module current are an order of magnitudeless than on voltage and power, and neglectedas far as any installation or safety issues areconcerned.

Temperature coefficients relate the effects ofchanging PV cell temperature on its voltage,current and power output. For crystalline siliconPV devices, the temperature coefficient forvoltage is approximately -0.4%/C, thetemperature coefficient for short-circuit current isapproximately +0.04 %/C, and the temperaturecoefficient for maximum power is approximately -0.45 %/C. Note that the power and voltagetemperature coefficients are negative, as theseparameters decrease with increasingtemperature. See Figure 6.

Ope

n-C

ircui

t Vol

tage

(Voc

)

Sho

rt-C

ircui

t Cur

rent

(lsc

)

ConstantTemperatureIrradiance (W/m2)

Isc increases withincreasing irradiance

Voc changes little withirradiance above 200 W/m2

0 200 400 600 800 1000

Cur

rent

Voltage

Increasing temperature

reducespower output

Increasing temperature

increases current

Increasing temperaturereduces voltage

T=0C

T=25C

T=50C

Example: PV Module Voltage-Temperature Correction

Consider a crystalline silicon PV module with a rated open-circuit voltage of 44.4 V at 25C,and a voltage-temperature coefficient of 0.33% / C. What would the open-circuit voltage beat a cell temperature of 60C?

Vtrans = Vstc + [Vstc x Cv x (Tpv Tstc)]

Vtrans = 44.4V + [44.4V x 0.0033 x (60 25)C] = 39.2VC

If the same PV module operates at 10C (35C lower than the reference temperature), thetranslated voltage is:

Vtrans = 44.4V + [44.4V x 0.0033 x (10 25)C] = 49.6VC


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current output for a string of dissimilar currentoutput devices is ultimately limited to the lowestcurrent output device in the entire string, andshould be avoided. However, it is acceptable toconnect PV modules with different voltage outputin series, as long as each module has the samerated current output. See Figure 10.

Figure 10. Avoid connecting dissimilar PVdevices in series.

Series strings of PV modules are configuredelectrically in parallel by connecting the negativeterminals of each string together and the positivestrings together. Usually, an overcurrent device isrequired in each string [NEC 690.9]. For theparallel connection of strings, the string currentsadd and the resulting string voltage is theaverage of the individual string voltages. Parallelconnections of strings with different currentoutput, or from strings in different planes areacceptable, but may require different circuitsizing as required. See Figs 11 & 12.

Figure 11. PV cells or modules areconnected in parallel by connecting thenegative terminals together and thepositive terminals together at a commonpoint.

2.1.4 PV Arrays

Photovoltaic power sources are constructedfrom the series and parallel connections ofindividual PV modules to achieve an intended dccurrent and voltage output. Photovoltaic sourcecircuits typically consist of individual moduleswired in series to achieve a desired outputvoltage intended for the connected dc utilizationequipment. PV source circuits are thenconnected in parallel at combiner boxes locatedwithin the array or at inverters to build currentand power output to the desired levels. SeeFigure 7.

Figure 7. Photovoltaic power sourcesconsist of parallel-connected sourcecircuits of series-connected PV modules.

A string is a series connection of PV devices. PVcells or modules are configured electrically inseries by connecting the negative terminal of onedevice to the positive terminal of the next device,and so on. For the series connection of similar PVmodules, the voltages add and the resultingstring voltage is the sum of the individual modulevoltages. The resulting string current outputremains the same as the current output of anindividual module. See Figure 8 & Figure 9.

Figure 8. PV cells or modules areconfigured electrically in series byconnecting the negative terminal of onedevice to the positive terminal of the nextdevice, and so on.

Figure 9. Connecting similar PV devices inseries increases voltage while currentstays the same.

Connecting PV modules in series with dissimilarcurrent ratings results in loss of power, similar ineffect to partially shading an array, or havingparts of a series source circuit located onsurfaces facing different directions and receivingdifferent levels of solar irradiance. The resultant

Photovoltaic Power Source

PV Array PV Source Circuits

PVModule

PV OutputCircuits

To disconnectmeans and

DC equipment

Pos (+) Neg (-)

1(+) (-) 2(+) (-) n(+) (-)C

urre

nt (A

)

Voltage (V)

1 device 2 devicein seriesn devices

in series

Module-Level Inverters

PV arrays designed using ac modulesand micro-inverters are becomingincreasingly popular for smallerinstallations having non-homogeneousarrays. Module-level inverters canoptimize the output for arrays located indifferent planes receiving differentamounts of solar radiation, or usingdifferent types of modules, or for arrayswith partial shading. These designs haveno field-installed PV dc source or outputcircuits, but rather combine the ac outputof multiple inverters in parallel atdedicated overcurrent device. Thesedesigns are essentially feeder circuitsconsisting of ac sources, as opposed tobranch circuits supplying electricalloads.

Pos (+) Neg (-)

Pos (+) (+) Neg (-)(-)A B

Pos (+)

Neg (-)

1(+)

(-)

2(+)

(-)

n(+)

(-)

Vseries string = V1 + V2 + VnVseries string = V1 x n

Iseries string = I1 = I2 = In (for similar devices)

For similar PV devices in series:Vseries string = V1 + V2 + VnVseries string = V1 x nIseries string = I1 = I2 = In

When dissimilar PV devices areconnected in series, the voltages still add,but the current is limited by the lowestcurrent output device in series.

Not acceptable.

Vseries = VA + VBIseries = IA < IB

For PV devices in parallel:Vparallel = V1 = V2 = Vn (for similar devices)Vparallel = (V1 + V2 + Vn)/nIparallel = I1 + I2 + In


Interactive PV systems operate in parallel and areinterconnected to and synchronized with theelectric utility grid. When connected to localdistribution systems, interactive PV systemssupplement utility-supplied energy to a buildingor facility. The ac power produced by interactivesystems either supplies on-site electrical loads oris back-fed to the grid when the PV system

output is greater than the site load demand. Atnight, during cloudy weather or any other periodswhen the electrical loads are greater than the PVsystem output, the balance of power required isreceived from the electric utility. Interactive PVsystems are required to disconnect from the gridduring utility outages or disturbances for safetyreasons. Only special battery-based interactive

Figure 12. Connecting similar PV devices inparallel increases current while voltagestays the same.

Monopole PV arrays consist of two output circuitconductors, a positive and negative. Bipolar PVarrays combine two monopole arrays with agrounded center tap. Consequently, bipolararrays have a positive and negative voltagereferenced to ground. Certain inverters arespecifically designed and listed for use withbipolar arrays. See Figure 13.

Figure 13. Monopole PV arrays consist oftwo output circuit conductors; whilebipolar PV arrays combine two monopolearrays with a center tap.

Bypass diodes are connected in parallel withseries strings of cells to prevent cell overheatingwhen cells or parts of an array are shaded. SeeFigure 14. Bypass diodes are essentially electricalcheck valves that permit the flow of current in onlyone direction. When modules in series strings arepartially shaded, it may cause reverse voltageacross the shaded cells or modules. The bypassdiode shunts current around the shaded area andprevents cells overheating. Most listed PVmodules are equipped with factory installedbypass diodes. Bypass diodes may or may notbe serviceable via module junction boxes in thefield.

2.2 Types of PV Systems

Types of PV systems are classified based on theloads they are designed to operate, and theirconnections with other electrical systems andsources. The specific components neededdepend on the type of system, and its functionaland operational requirements.

Stand-alone PV systems operate independentlyof other electrical systems, and are commonlyused for remote power or backup applications,including lighting, water pumping, transportationsafety devices, communications, off-grid homesand many others. Stand-alone systems may bedesigned to power dc and/or ac electrical loads,and with a few exceptions, use batteries forenergy storage. A stand-alone system may use aPV array as the only power source, or mayadditionally use wind turbines, an engine-generator, or another auxiliary source in hybridapplications. Stand-alone PV systems are notintended to produce ac output that operates inparallel with the electric utility system or othersources. See Figure 15.

Pos (+) Neg (-)

PV Array

Pos (+) Center Tap

PV Array

Neg (-)

PV Array

Pos(+)

Neg(-)

Shadedcell

When cells are not shaded, the bypassdiode is reverse biased and does notconduct current

When a cell is shaded, thebypass diode is forward biased and conducts current

PV Source Circuits

PV Output Circuit

Source CircuitCombiner Box

Ground FaultProtection

PV FusedDisconnect

ChargeController

DCLoads

ACLoads

Inverter DCinput Circuit

InverterOutputCircuit

Inverter/Charger

AuxiliaryAC Source

Inverter FusedDisconnect

Battery FusedDisconnect

Battery

PV Array

Stand-Alone System

Cur

rent

(A)

Voltage (V)

Device 1+2 independently

Devices 1+2 in parallel

For PV devices in parallel:Vparallel = V1 = V2 = Vn (for similar devices)Vparallel = (V1 + V2 + Vn)/nIparallel = I1 + I2 + In

1312

Figure 14. Bypass diodes are connected in parallel with series strings of cells to preventcell overheating when cells or parts of an array are shaded.

Figure 15. Stand-alone PV systems operate autonomously and are designed to meetspecific electrical loads.


1514

on buildings or relative to property lines oreasements. In some cases, a shadinganalysis and performance estimates may beprovided with project proposals, and shouldalso be including with the final systemdocuments.

A single line diagram depicting the overallsystem design, including the types ofmodules, total number of modules, modulesper string and total number of strings; thetypes and number of inverters; and anyother major components. For larger projects,complete as-built electrical and mechanicaldrawings are usually required at project closeout.

The types, sizes and ratings for all balance-of-system components annotated on thesingle line diagram, or noted and provided ina separate table, including specifications forall conductors, raceways, junction boxes,source circuit combiner boxes, disconnects,overcurrent protection devices, andgrounding equipment, as applicable.

Data sheets and specifications for PVmodules, inverters and other majorcomponents, including module mountingsystems. For most inverters, installation anduser/operator manuals are available andprovide important information regarding thesafe operation and maintenance of theequipment.

Operation and maintenance informationincluding procedures for verifying propersystem operation and performance, and howto determine if there is a problem and whatto do. Procedures for isolating/disconnectingequipment and emergency shutdown

procedures should also be provided. Amaintenance plan and intervals should beprovided for all routine (scheduled) systemmaintenance, such as array cleaning asrequired. Operating and maintenanceguidelines should differentiate what tasks canbe performed by the owner or caretakers,from those that require professional servicedue to the complexity of the tasks, specialequipment needs, or safety concerns.Maintenance agreements, plans andrecordkeeping forms or sheets should alsobe provided to document maintenanceactivities over time.

Warranty details on major componentsindicating the terms and conditions, and howthe warranty process is handled and bywhom. System warranties should also beaddressed, including quality of workmanship,roof weathersealing or performancewarranties as applicable.

Copies of all commissioning test reports andverification data.

Contracting and financial details are also animportant part of system documentation, andmay be included with the technical itemsdiscussed above or under a separate file.These documents would include constructioncontracts, invoices and payments formaterials and labor, building permits,inspection certificates, interconnectionagreements, and applications and approvalsfrom incentive programs, such as rebatesand tax forms.

A variety of aids are available for installers,including software products such as theSolarCert from Seaward Solar and testingand commissioning report.

inverters can provide stand-alone power forcritical loads independent from the grid duringoutages. See Figure 16.

3 SYSTEM DOCUMENTATION

All PV installations should have adequatedocumentation providing details of the systemdesign and all components and materials used inits construction. The documentation should alsoinclude safety information, and procedures foroperating and maintaining the system. Propersystem documentation helps ensure safe andreliable system operations, and is generallyrequired for the following purposes:

Plan review and permitting process with localbuilding officials

Interconnection approval from the local utility

Installation and maintenance contractors

Owners and caretakers

Informing emergency services

PV system documentation is a permanent recordassociated with a PV installation, includingmaintenance and testing records. Thisinformation is critical for the effectivemaintenance and evaluation of the system overtime. Key components of a PV systemdocumentation package should include thefollowing:

The system DC and AC power ratings; themanufacturer, model and quantity of PVmodules, inverters, batteries, controllers andall other major components, as applicable.The dates of the system installation,commissioning and inspection should alsobe noted.

The names, postal addresses, phonenumbers and email addresses for thecustomer/owner, system designer,installation contractor and any otherresponsible parties or subcontractors.

A site layout identifying equipment locations

PV Source Circuits

Source CircuitCombiner Box

Ground FaultProtection

DC FusedDisconnect

DCLoads

Electric Utility

Inverter

AC FusedDisconnect

UtilityDisconnect

PV Array

PV OutputCircuit Inverter Input Circuit

Interactive System

Inverter Output Circuit

Integral components in manysmall string inverters < 12 kW

Figure 16. Utility-interactive PV systems operate in parallel with the electric utility gridand supplement site electrical loads.


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ensure that the system remains in a safe andproperly functioning condition. There are manyareas to evaluate with visual inspections, with thefrequency and level of detail depending on thetype and size of the system involved. Visualinspections are supplemented with otherobservations, test measurements andperformance data to fully evaluate the safety andcondition of PV systems.

Initial inspections are primarily used to identifyunfinished installation details and verifycompliance with the applicable coderequirements. Visual inspections conducted afterinstallation during periodic routine maintenancetend to look for physical damage or degradationof equipment from temperature extremes,moisture or other environmental conditions.

Prior to initial operation, all PV systems should beinspected for full compliance with the many NECrequirements. Checklists are often used to reviewand verify these requirements at the time ofinspection, for examination and approval by localauthorities. Among the key NEC requirementscovered in Article 110 Requirements for ElectricalInstallations include:

All equipment shall be properly listed,identified and labeled, suitable for theconditions of use, and be installed accordingto the listed product instructions [110.3].

All equipment shall be installed in a neat andworkmanlike manner, consistent with qualitycraftsmanship standards in the electricalconstruction industry [110.12].

All equipment shall be mechanically securedand provided with adequate ventilation orcooling as required [110.13].

All electrical terminations and connectionsshall be made using approved products andinstallation methods [110.14]. This includesconsideration of conductor and terminalmaterials, temperature ratings, and use ofspecially approved terminals for use with finestranded conductors or more than a singleconductor. Pressure connectors using a setscrew have required tightening torques, andthese values should be recorded and verifiedat commissioning.

All electrical equipment shall be marked withthe manufacturers identification andapplicable specifications and ratings[110.21].

Sufficient working spaces shall be providedabout any electrical equipment that is likelyto be serviced or maintained while energized[110.26]. Clear spaces and dedicatedspaces are also required about certainelectrical equipment, such as panelboards orswitchgear.

NEC requirements covered in Article 690 SolarPhotovoltaic Systems should also be evaluatedand verified during visual inspections. Theserequirements address the following areas:

Calculating circuit voltages and currents

Determining conductor and over currentdevice sizes and ratings

Locating disconnecting means

Wiring methods and connectors

Equipment and system grounding

Markings and labels

Connecting to other sources (also Art. 705)

Installing batteries and charge controllers

4 COMMISSIONING PV SYSTEMS

Commissioning of PV systems involves visualobservations as well as tests and measurements toverify the safe and proper operation of the system.Commissioning is performed immediately after PVinstallations are completed, prior to being operatedand put into service for the first time. A thoroughcommissioning process helps improve safety andquality control, provides verification the installationmatches the plans and code requirements, and isperforming as expected. Some of the testsconducted during commissioning may berepeated during periodic routine maintenance tohelp ensure that the system remains in asatisfactory operating condition over its lifetime.

Key steps of a PV system commissioningprocedure typically include:

Completing final installation details.

Completing visual inspections.

Verifying compliance with NEC requirements.

Conducting electrical verification tests.

Vo/c, Is/c, insulation resistance, polarity.

Verifying system functionality includingstart-up, operations, shut-down andemergency procedures.

Verifying system power output and energyproduction meet performance expectations.

Completing system documentation, includingchanges for as-built drawings.

Conducting user orientation and training onsystem operations and safety.

4.1 Final Installation Checkout

A final checkout confirms that the installation is

complete before conducting any testing andbeginning operations. Typically, the installationcontractor will perform the final checkout, prior toformal inspections by building officials. With theexception of the PV array, all circuits should bede-energized wherever possible in preparationfor system testing. A punch list can be used tohelp check off items as they are completed, andtypically includes the following items:

Verifying that all structural and electricalcomponents are properly installed andsecured.

Verifying that all components are installed ina neat and workmanlike manner, includingwire management practices.

Verifying proper connections andterminations, including terminal torquespecifications.

Verifying that all required system andequipment labels, marking and placards arecorrect and in the proper locations.

Verify that any calibrations or adjustments forinverters, charge controllers or otherequipment are properly set or programmed.

Verifying that all disconnects are open, fusesare removed and lockout/tagout proceduresare in place.

Identifying and completing any unresolveditems.

Completing site clean-up and restoring siteto original conditions.

4.2 Visual Inspection

Visual inspections of PV systems should beperformed as part of commissioning androutinely over the system lifetime to verify and


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5 SYSTEM TESTING AND MEASUREMENTS

PV systems should be thoroughly tested at thetime of commissioning and periodically over theirlifetime to ensure proper performance and safeoperation. Baseline measurements at the time ofsystem commissioning are compared to thesystem ratings and expectations for acceptance,and serve as a baseline for comparison withfuture measurements. Changes in test resultsover time are used to track system degradation,and identify problems that require attention orservice for safety or performance reasons.Circuits or components that are modified orreplaced should be retested accordingly.

There are several types of electrical testsconducted on PV systems that are used to verifyNEC requirements and system performance.Many of these tests can be conducted withcommon electrical test equipment, while somemeasurements require special meters andinstruments. In many cases, system performanceinformation is measured, recorded and displayedby PV system inverters or charge controllers, andcan be used to verify system functions andproper operation.

The following summarizes common types oftesting conducted on PV systems whatinformation it provides:

Continuity and resistance testing verifiesthe integrity of grounding and bondingsystems, conductors, connections and otherterminations.

Polarity testing verifies the correct polarityfor PV dc circuits, and proper terminationsfor dc utilization equipment.

Voltage and current testing verifies that PVarray and system operating parameters arewithin specifications.

Insulation resistance testing verifies theintegrity of wiring and equipment, and usedto detect degradation and faults to wiringinsulation.

Performance testing verifies the systempower and energy output are consistent withexpectations. These tests also requiremeasurements of array temperature andsolar irradiance.

For stand-alone or hybrid PV systemsincorporating energy storage and additionalenergy sources, the following additional testsmay be conducted:

Measurements of battery voltage, capacityand specific gravity.

Verification of charge controller set pointsand temperature compensation.

Verification of charging current and loadcontrol functions.

Verification of performance and wiringintegrity for other sources, such asgenerators.

Multi-function PV system testers are nowavailable, such as the Seaward PV150, thatconduct many of the recommended tests,including continuity and resistance, polarity,voltage and current tests, and insulationresistance tests. By combining these testfunctions into single instruments, testingpersonnel avoid having to purchase, carry andmaintain multiple meters. Multi-functional PVsystem testers simplify and speed up testing.These instruments can also store data for later

Some sources for PV system inspectionchecklists and guidelines include:

http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html

http://irecusa.org/wp-content/uploads/2010/07/PV-Field-Inspection-Guide-June-2010-F-1.pdf

http://www.jimdunlopsolar.com/vendorimages/jdsolar/PVInspectionChecklist.pdf

Many articles in the first four chapters of the NECalso apply to most PV installations, including butnot limited to:

Article 110 Requirements for ElectricalInstallations

Article 230 Services

Article 240 Overcurrent Protection

Article 250 Grounding and Bonding

Article 300 Wiring Methods

Article 310 Conductors for General Wiring

Article 314 Outlet, Device, Pull, and JunctionBoxes

Article 338 Service-Entrance Cable: TypesSE and USE

Article 344 Rigid Metal Conduit: Type RMC

Article 356 Liquidtight Flexible NonmetallicConduit: Type LFNC

Article 358 Electrical Metallic Tubing: Type EMT

Article 400 Flexible Cords and Cables

Article 408 Switchboards and Panelboards

Article 445 Generators

Article 450 Transformers

Article 480 Storage Batteries

Article 705 Interconnected Electric PowerProduction Sources

4.2.1 Labels and Markings

Numerous markings, labels and signs arerequired to identify PV systems and theircomponents, and to warn operators, servicepersonnel or emergency responders ofhazardous conditions. Manufacturer markingsand labels identify the size, type, specificationsand ratings for PV modules, inverters, controllers,combiner boxes, conductors, raceways,overcurrent devices, switchgear and all otherelectrical components. These markings areplaced on the product at the time ofmanufacture, and include listing marks from theapproval agency. Building officials may verifythese markings during inspections, and rely onthem for their approvals [110.2, 110.3, 100.21].

Additional markings and labels are required forthe overall system and certain components in PVsystems, and are to be provided and placed bythe installer. These include additional labels on dcconductors and raceways [690.4, 690.31],connectors [690.33], disconnecting means[690.14, 690.17], and at the point of utilityconnection [690.54, 705.10, 705.12]. Labels andmarkings are also required on PV modules[690.51], alternating-current modules [690.52],the PV power source [690.53], ground faultprotection equipment [690.5] and battery storagesystems [690.55]. Special labeling is alsorequired for bipolar arrays [690.7], ungroundedPV arrays [690.35], facility with either stand-alonesystems or multiple power sources [690.56] andstand-alone inverters providing a single 120-voltsupply [690.10].


with ground-fault protection [690.5], thisconnection is usually made internal to the inverterat dedicated terminals according tomanufacturers instructions.

The connection between the groundedconductor and grounding electrode system ismade through the ground-fault protection device.When a ground-fault condition is detected bycurrent flow in this conductor, the circuit isopened and the inverter displays a ground-faultindication. Special labels are required to warnthat normally grounded conductors may beungrounded and energized. Appropriate safetyprecautions should be exercised around any PVarrays when ground-faults are suspected.

Ungrounded PV arrays are permitted by theNEC, but require specially listed invertersdesigned for use with ungrounded arrays, specialcabling or protection for wiring, and requiredisconnecting means, overcurrent protection

and ground-fault protection in both ungroundedarray dc conductors [690.35]. Ungrounded PVarrays are common in Europe, and expect to seemore of these designs installed in the U.S. incoming years, due to lower inverter costs andcapability for better array fault detection.

Equipment grounding for PV systems isparticularly challenging, due the large numbers ofindividual PV modules in arrays, and their multipleconnections to metallic support structures. PVmodule manufacturers are required to providedetails of acceptable equipment groundingmethods and components in their listedinstallation instructions, according to the UL1741 standard for module safety. However,different mounting structures supplied bydifferent manufacturers often require specializedgrounding practices and hardware. Be sure tofollow module and mounting systemmanufacturers instructions for proper equipmentgrounding practices.

retrieval and processing into commissioning testreports that become part of the systemdocumentation record. See Figure 17.

Figure 17. The Seaward PV150 handheldmeter provides multiple PV array testingfunctions.

A variety of instruments are available for thePV installer, but it is important to make surethe correct instrument is selected and avoidproducts which only provide one or two tests,reducing the number of instruments neededas well as calibration requirements. TheSeaward Solar PV150 is a hand held meterthat performs a variety of tests in a safe andsimple fashion.

5.1 Continuity Testing

Continuity testing is commonly used to verifygrounding and bonding connections in electricalsystems. These tests also verify the properoperation of disconnecting means and thefunction of overcurrent protection devices likefuses and circuit breakers. Measurements ofresistance can also be used for estimatingvoltage drop in conductors, terminations andother connections and for evaluating windings inmotors and transformers.

Proper grounding of PV systems reduces the riskof electrical shock to personnel and the effects oflightning and surges on equipment. Thegrounding requirements for PV systems can bequite complex to understand and installationpractices and hardware are continually evolvingand improving. There are two basic types ofgrounding. System grounding connects acurrent-carrying conductor in an electricalsystem to ground, or earth potential. Equipmentgrounding connects non-current carrying metalparts to ground, such as PV module frames,racks, enclosures, junction boxes, conduit andother metallic components. All PV systemsrequire equipment grounding, and most alsorequire system grounding. Specific groundingrequirements for PV systems are addressed inNEC Article 690, Part V and general groundingand bonding requirements are covered in NECArticle 250. See Figure 18.

Most PV arrays in the U.S. today are installedwith a grounded dc circuit conductor, either thepositive or negative conductor, or the center tapof bipolar arrays [690.41]. This connection mustbe made at a single point on the photovoltaicoutput circuit [690.42]. For interactive inverters

Photovoltaic Array: Four modulesconnected in series shown

Grounding Electrode:8ft., copper-coated,

optionally supplementary

PV Disconnect:Fused disconnect

or breaker

Inverter:240 VAC output,with 690.5 GFPD

LockableDisconnect:

Utility required

AC Disconnect:2-pole breaker

Bond electrodes together, or connectDC GEC to AC grounding electrode

AC MainsPanel:

Distributionbreakers

To Utility:240 VAC

DC Grounding Electrode:8ft., copper-coated

Note: No overcurrent protectionneeded with some inverters

AC Grounding Electrode:8ft., copper-coated

2120

Figure 18. All PV systems require equipment grounding, and most also require systemgrounding.


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alarm when continuity is verified. Oftentimes,extra-long test leads may be required to verify thecontinuity of grounding connections in PVsystems due to the long separation distancesbetween components. For accurate readings,the resistance of the test leads is accounted foror nulled by zeroing the ohmmeter prior to anyresistance measurements. A clean surface andreliable test lead connection to the circuit undertest is also crucial. Continuity and resistancetesting should only be performed onde-energized circuits and conductors.

An instrument such as the Seaward SolarPV150 is especially designed to providecontinuity and resistance testing on PVinstallations.

5.2 Polarity Testing

As for any dc circuits, the polarity of array wiringand dc equipment is a critical concern for PVinstallations. The polarity of every source circuitand the entire PV power source must be verifiedprior to connecting to any dc utilizationequipment, such as batteries, charge controllers,inverters or electrical loads. Without required PVarray source and output circuit overcurrentprotection [690.9, 240], reversing the polarity ofan array connection to a battery can lead todisastrous results and damage PV modules orsource circuit wiring. A reversed polarity arraycan also act as a load, and discharge currentfrom a battery without some form of reversecurrent protection like a blocking diode. Someinverters have rectifiers on their dc input toprotect them from reverse polarity connections,but many do not and can be damaged byinadvertently reversing the connections.

Reversing the polarity of a PV module within agiven series source circuit can occur for hard-wired connections, although this problem hasbeen largely eliminated by factory-installed PVmodule connectors and the requirement for themto be polarized, marked and non-interchangeable [690.33]. For PV modules usingbypass diodes, reversing module polarity within aseries source circuit will force that module tooperate at reverse voltage limited by the voltagedrop across the blocking diodes, usually around0.7 V for each diode. The result is the currentfrom other modules in the series string willbypass the reversed module, and the netvoltage output for the entire string will be reducedby the voltage of the reversed module plus thevoltage drop across the bypass diodes. Bypassdiodes prevent PV modules from operating atlarge reverse voltages that can result in highpower dissipation and damage to the module.

Reversing the polarity of an individual PV sourcecircuit within a combiner box will simply cancelthe current output from other parallel-connectedstrings wired with proper polarity. For larger PVinstallations with dozens of source circuitsterminating at several combiner boxesthroughout the PV array, keeping track of eachsource circuit and its polarity can be a challenge.To help reduce confusion and promote betterwire management practices, the 2011 NECrequires all PV circuit conductors to be clearlyidentified by labeling, grouping, or color coding atall terminations, connections or splices [690.4].All source circuit fuses and module bypassdiodes should be checked if modules or stringsare ever connected in reverse polarity.

Electrical resistance and continuity testing can beused to confirm the following NEC requirements:

Verify the finish or anodizing on aluminum orother metallic components has beensatisfactorily removed or penetrated forequipment bonding connections [250.12,250.96].

Verify that all available grounding electrodesin a building are bonded together and form asingle grounding electrode system [250.50].This includes underground metallic waterpipes; building structural steel; concrete-encased electrodes; ground rings; and rod,pipe and plate electrodes, as available.

Verify the grounding electrode conductor iscontinuous and the viability of any irreversiblesplices, welds or other connections aremade using approved means wherepermitted [250.64]. Also verify theconnection of the grounding electrodeconductor to the grounding electrode.

Verify that metal raceways, enclosures,frames, fittings, and other componentsserving as equipment grounding conductorsare properly bonded together [250.86,250.96]. Circuits operating over 250 volts toground require special bonding methods toensure electrical continuity of metallicraceways, such as the use of listed bondinglock nuts and bushing [250.97].

Verify the continuity of the equipmentgrounding conductor is first-make last-breakfor any plug and receptacle equipment, andis not opened by any disconnect device[250.124].

Verify that metallic raceways and enclosures

are joined into a continuous electricalconductor [300.10]. Loss of mechanicalcontinuity, due to improper installation orinadequate support can also lead to loss ofelectrical continuity [300.11, 310.12].

Verify that metal parts of all PV moduleframes, support structures and otherequipment are bonded to the equipmentgrounding system [690.43]. Specialequipment bonding devices, listed andidentified for bonding grounding moduleframes to support structures are allowedwhere approved for specific types ofmodules and mounting structures. Refer toPV module and mounting systemmanufacturers instructions for additionalinformation.

Verify the continuity of equipment groundingconnections and bonding jumpers when PVmodules or inverters are removed from acircuit for service [690.48]. Similarrequirements also apply to maintaining thecontinuity of equipment groundingconductors to metallic boxes containingreceptacles, luminaires and other devicesthat may be removed for service [250.148].

Verify that continuity between the groundedPV array source circuit and output circuitconductors remain bonded to the groundingelectrode conductor whenever an inverter orother equipment is removed for service[690.49].

Continuity tests can be conducted using aconventional ohmmeter capable of measuringlow resistances. Most digital multimeters alsoinclude continuity test features that produce andindication of pass or fail, such as an audible


25

Many PV inverters have integral data monitoringthat measures many system parameters on thedc and ac sides of the inverter, including dcvoltage, current and power. Usually, sourcecircuit measurements need to be made atcombiner boxes located throughout the arrayfield in larger systems, where the source circuitscan be isolated and identified for testing.

5.3.1 Open-Circuit Voltage Testing

Prior to closing the PV array dc disconnects, theopen-circuit voltage (Voc) for each PV arraysource circuit should be tested and comparedwith expectations. This test can also be used toverify proper polarity. These tests simply verifycorrect installation, and are not intended to verifyperformance. Open-circuit voltage tests require asuitable voltmeter capable of reading AC and DCvoltages of 600 V to 1000 V.

Most PV systems have identical stringsconsisting of the same number of seriesconnected modules. In these cases, similaropen-circuit voltage readings should beexpected under the same testing conditions,typically within 5% of each other. Lower thanexpected voltage can be due to improper arraywiring, failed modules or shorted bypass diodes.The string open-circuit voltage measurementsshould also be verified to operate within the dcvoltage limits for inverters, charge controllers orany other dc utilization equipment.

If average cell temperatures are also measured,the open-circuit voltage measurements can beeasily translated to a reference temperaturecondition for comparison with systemspecifications. Based on a standard -0.4%/Cvoltage-temperature coefficient for crystalline

silicon PV modules, the corresponding voltagechange should be 2.5% for every 10C change inmodule temperature.

5.3.2 Short-Circuit Current Testing

Short-circuit current tests are conducted on PVstring source circuits to verify proper readingsand that the circuits are clear from major faults.Similar to the open-circuit voltage tests, thesetests are only intended to verify proper systemoperation, not performance. Suitable testequipment, capable of safely short-circuitinghigh-voltage dc circuits is required. Most digitalmulti-meters can measure dc current up to 10 A,but require a suitable shorting device to safelymeasure the current. Clamp-on ammeters arealso available for dc current measurements, andrequire and external shorting device as well.Some circuit breakers and disconnect switchedmay have appropriate DC current and voltageload-break ratings for shorting array sourcecircuits. The ideal solution is a tester such as theSeaward PV150.

Short-circuit current is directly proportional to thesolar irradiance incident on the array.Consequently, these tests must be done quickly,under steady clear sky conditions at as close tothe same irradiance level as possible. Make surethe array is not shaded or particularly soiled priorto testing. Short-circuit current readings takenunder steady conditions should typically bewithin 5% or one another for acceptance. Ifirradiance measurements are also madesimultaneously with short-circuit currentmeasurements, the results can easily betranslated to a reference irradiance condition, forexample 1000 W/m2, to compare withspecifications.

Polarity is verified by measuring the voltage onenergized circuits, prior to closing disconnectsand operating the system for the first time. Mostdigital multimeters can be used for this purpose,as well as specialized PV array testing equipmentcapable of measuring dc voltage over theappropriate range. Electronic meters typicallyindicate the polarity of dc voltage measurementswith a plus (+) or minus (-) symbol on the display,according to the connections of the positive andnegative test leads. Analog voltmeters are notused for polarity testing, as the metermovements on these instruments can bedamaged by connecting them in reverse polarity.

Polarity should be verified on the following dccomponents and circuits PV system:

Photovoltaic modules

Photovoltaic source circuits

Photovoltaic output circuits

Disconnecting means

Battery and charge controller circuits

Inverter input terminations

Electrical loads

5.3 Voltage and Current Testing

Basic voltage and current tests are conducted onboth dc and ac circuits in PV systems to verifythese parameters are within acceptable limitsprior to closing disconnects and beginningsystem operations. Detailed voltage and currenttests may be conducted on PV source circuitsand arrays for performance verification purposes.Testing for performance verification requiresadditional measurements of solar irradiance andarray temperatures, and translation of test resultsto a reference test condition.

Prior to operations, voltage should be verified forthe following circuits and sources in PV systems:

Verify ac voltage and correct phasing atutility supply, inverter ac terminals anddisconnects, and electrical generators asapplicable.

Verify dc voltage and correct polarity for PVarray source and output circuits and at dcdisconnects.

Verify dc voltage and correct polarity forbattery, charger and charge controllersubsystems.

RPE

VSO

Auto

NULL

24

Measured to ensure correct installation and operation of each PV string. Measured valuesshould be compared with expected values. For systems with multiple identical strings, values should be within 5% of other PV strings in array.

Figure 19. PV String open circuit voltage


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To maintain safety and avoid damagingcomponents during testing, the followingguidelines and precautions apply to insulationtesting of PV system circuits and associatedequipment:

Always use insulated rubber gloves withleather protectors when conductinginsulation tests. Make sure test area andequipment are protected from access byunauthorized persons.

Isolate circuits for testing by openingdisconnects, and verify that circuits are de-energized using LOTO procedures prior toconnecting insulation tester. The exception isfor PV dc source circuits which are alwaysenergized when exposed to light.

Grounded test lead should always be thefirst to make, and last to break any circuitmeasurement. Never use insulation testers inan explosive environment or aroundcombustible materials.

Never use insulation testers on circuits withany electronic equipment, including inverters,charge controllers, or instrumentation, orsurge suppression equipment, as theapplication of high test voltages can damagethis equipment.

Never conduct insulation tests on batteriesor other energy storage systems.

Always ensure that circuits are properlydischarged before and after insulation tests,either through the test equipment orexternally with a load resistor. This isparticularly important for PV modules, motorwindings and parallel wiring methods withcapacitive characteristics.

Test Equipment

Insulation resistance testing is commonly measuredwith a megohmmeter. A megohmmeter is a specialtype of ohmmeter that measures very high values ofresistance by applying high test voltages betweenconductors and measuring the leakage current.Megohmmeters may be powered by an externalsource, internal batteries or by a hand crank. Testvoltages range from 50 Vdc to 5000 Vdcdepending on the instrument and its purpose.

Handheld insulation multimeters often combineinsulation test functions with voltage, resistanceand continuity measurements. This testequipment is lightweight and easy to use, and canaccommodate a variety of electrical systemtesting requirements. Specialized multi-functioninsulation multimeters are available that arepurposely designed for testing PV arrays. SeeFigure 20.

Figure 20. The Seaward PV150 PhotovoltaicTester measures and stores data forverifying PV electrical system integrity andperformance.

The Seaward Solar PV150 performs a shortcircuit test at the touch of a button andavoids situations where installers areexposed to the possibility of arcing orexposure to high voltages in themeasurement process.

5.4 Insulation Resistance Testing

Insulation resistance tests are used to verify anddemonstrate the integrity of electrical wiringsystems and equipment, as required by the NEC[110.7]. These tests can be used to assessdegradation or damage to wiring insulation andto locate faults within PV arrays and other systemcircuits. Insulation tests are an important elementof commissioning, acceptance testing andpreventative maintenance for PV systems.

Electrical conductors are usually insulated withan outer covering to protect them from cominginto contact with personnel, equipment or otherconductors. Where conductors are exposed attermination points or on busbars, the terminalspacing and air gaps provide insulatingproperties. Compared to conductors that thathave very low resistance on the order of a fewohms or less, insulators have very high resistancemeasured in megohms (M).

The quality of conductor insulation can bedetermined by measuring its resistance.Insulation resistance is determined by applying aconstant test voltage to a conductor andmeasuring the current flow between theconductor and ground, or between other de-energized system conductors. This is analogousto pressure testing a plumbing system for waterleaks. As wiring insulation degrades, itsresistance decreases and the small current flow

through the insulation increases. The higher itsresistance value, the better the conductorinsulating quality. Excessive leakage current canpresent an electrical shock hazard, damageequipment and degrade system performance.

Damage to wiring insulation can result fromimproper installation, or any number ofenvironmental factors, including from impacts orvibration, from animals or insects, ordeterioration from temperature or sunlight, orother adverse conditions. Due to the degradationof PV modules and wiring systems over manyyears of direct exposure to the elements, olderPV arrays will naturally have lower insulationresistance than when they were new. Insulationresistance tests are recommended on a 3-yearmaintenance interval, or more frequently ascircumstances dictate.

Insulation resistance is measured between theconductors under test and ground, or betweenother conductors. Insulation resistance andleakage current testing are used to verify theintegrity of the following PV system circuits andcomponents:

Verifying integrity of PV modules and arraysource and output circuit conductors.

Measuring the leakage current throughovercurrent and disconnect devices,including fuses, switches and circuitbreakers.

Measuring the insulation resistance for anyequipment with windings, such as motors,generators, transformers and inductors.

Testing the grounding electrode resistance.


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For systems not bonded to ground, the testsshould be carried out between array cables andground, and also between array cables andframe.

The IEC 62446 standard recommends 0.5 Mas an acceptable minimum insulation resistancevalue for PV arrays operating at less than 120 V.For system voltages higher than 120 V, 1 M orhigher is considered acceptable. Testsconducted during system commissioning maybe used as a baseline for which latermeasurements can be compared to evaluatewiring condition and degradation.

ASTM E2047, Standard Test Method for WetInsulation Integrity Testing of Photovoltaic Arraysprovides guidelines on test procedures andinterpretation of results for wet insulationresistance testing of PV arrays. This standardcan be used in conjunction with the IEC 62446standard for conducting insulation resistancetesting on PV arrays.

5.5 System Functional Testing

System functional testing verifies proper systemoperation, including start-up, shut-down andnominal operating conditions. These testsconfirm that system operating parameters arewithin expected and nominal limits, but are notintended to verify system ratings in accordancewith specifications or warranty provisions.Additional detailed testing, using additionalmeasurements and normalizing data are requiredto verify performance with system ratings.

Knowledge of the specific equipment used andthe product installation and operationinstructions are crucial to verifying their safe and

proper operation during functional tests. Mostinverters and charge controllers provide someindication of performance and operating status,such as power output or energy production, aswell as fault or error indications such as out oflimit parameters or array ground faults. Thisinformation is extremely helpful in verifying propersystem operation, and may reduce the need tomake hand measurements on energized circuits.

System functional tests ensure that the systemstarts and operates properly, and can be safelydisconnected. Among the types of functionaltests conducted on PV systems include:

Verifying the proper operation ofdisconnecting means and componentconnection and disconnection sequences.

Verify that interactive inverters and acmodules de-energize their output to utilitygrid upon loss of grid voltage [690.61]. Thisis a safety requirement to prevent interactiveinverters from operating as an islandedelectrical system without voltage orfrequency control, and preventing them backfeeding de-energized electrical systems.These functions are performed internally byall utility-interactive inverters listed accordingto the IEEE 1547 and UL1741 standards.

Verify that interactive inverters automaticallyreconnect to their output to the grid oncethe voltage has been restored for at least 5minutes [690.61].

Verify that battery-based interactive invertersdisconnect ac loads from the utility sourcewhen operating in stand-alone mode[690.61].

Testing Considerations

Insulation resistance varies with the type ofinsulation and is inversely proportional to thevolume of insulation tested. For example, aninsulated conductor 500 feet long would typicallyhave one one-fifth the insulation resistance as thesame insulated conductor of only 100 feet inlength. Insulation resistance also decreases withage as the insulation degrades, and is alsoaffected by temperature and moisture. Insulationresistance is inversely proportional totemperature; as temperature decreases by 10C,insulation resistance effectively doubles. Forexample, a conductor having an insulationresistance of 4 M at 20C would have aresistance of 1 M at 40C. Consequently, it isimportant to measure and evaluate insulationresistance readings at constant temperaturesand record this information with test results. Toavoid condensation and moisture effects, dryinsulation resistance tests should only beconducted when the conductor temperature isabove the dew point.

Test Procedures

Insulation resistance testing is conducted byapplying high voltages to conductors andequipment, and determining the resistance bymeasuring the leakage current. All circuits mustbe isolated from others for testing anddischarged before and after testing. Groundingor bonding connections are left connected. Anysurge suppression equipment must be removedfrom the circuits. The test leads must make areliable connection with the circuit under test,and may require filing or grinding some coatedmetal components.

The test voltage depends on the circuits orequipment tested. It is usually higher than theoperating voltages for the circuits or equipmentunder test, but not higher than the voltage ratingsof the equipment or conductor. Higher testvoltages may be used for proof testing ofequipment by manufacturers than for field tests.Note that the maximum system voltage rating formost PV modules in the U.S market is 600 V. TheIEC 62446 standard recommends a test voltageof 250 V for PV arrays with maximum voltage lessthan 120 V, a test voltage of 500V for PV arrays120 V to 500 V, and a test voltage of 1000 V forarrays greater than 500 V (ensure that modulesare rated for 1000 V for this test).

Insulation resistance and other electrical tests onPV arrays are generally measured at sourcecircuit combiner boxes, where the individual arraycircuits can be accessed. The tests can beconducted dry, or a wetting agent can besprayed on portions of an array to better pinpointfault locations. Insulation tests may be performedusing either of the following two methods:

1. Between the positive dc conductor andground, and between the negative dcconductor and ground.

2. Between the shorted positive and negativedc conductors and ground. Requires anappropriate shorting device rated for circuitcurrent and voltage.

The grounding connection is made to metallicmodule frames or support structures, thebuilding grounding electrode systems, or directlyto earth. Some modules and supports structuremay not have a metallic support structure orframes.


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daily energy consumption can be used by thesystem owner/operators to control their loads andmanage the available energy. Measurements ofdaily minimum daily battery voltage and the netamp-hours delivered to and withdrawn from abattery can also be used to assess whether thebattery is receiving enough charge. Deficitcharging will usually be indicated by decliningminimum battery voltages. The performance ofelectrical loads can be verified by measuring theircurrent or power consumption, and if theyfunction as intended.

6.1.1 Verifying AC Power Output

The ac power output for interactive systems isrelated to the rated maximum dc power output ofthe array adjusted by a number of deratingfactors. The factors include several types of acand dc system losses and the inverter powerconversion efficiency, which in combination resultin ac power output normally 70 to 85 percent ofthe PV array dc rating at Standard Test Conditions(STC). The actual ac output of an interactive PVsystem at any moment can be compared withexpectations, using the basic translation formulasfor solar radiation and temperature.

Accurate simultaneous measurements of moduletemperature and inplane solar radiation arerequired if power measurements are to becompared with reference data at STC. This can beachieved using an instrument such as the SolarSurvey 100 and 200R.

The typical procedure for verifying the expected acpower output for interactive PV systems beginswith the nameplate dc ratings for the PV array andapplying applicable derating factors. The array dcrating is usually taken as the sum of all individual

module nameplate maximum power ratings (Pmp)at STC. The product of the derating factors and dcrating give the estimated system ac power output.Translations for temperature and solar radiationprovide an estimate for actual operatingconditions.

Following are the primary derating factors used toestimate the ac power output of PV systemsunder given operating conditions.

Nameplate Ratings: This derating factoraccounts for the accuracy of themanufacturers power rating and guaranteedperformance. Manufacturing tolerances resultin most PV modules producing less than theirrated power at STC, and the UL 1703module listing standard only requires modulepower to be within 10% of the modulerating. Additional design qualification testscovered in IEC 61215 may establish powertolerance within 3 percent of nameplateratings. A derate factor of 0.95 representsthat the array power output at STC is 5% lessthan the manufacturers nameplate rating.

Inverter and Transformer: The deratingfactor for the inverter and transformer is theircombined efficiency in converting dc powerto ac power. Modern interactive invertershave weighted efficiencies on the order of95%. The efficiencies of transformersseparate from inverters need to beconsidered additionally. This factor shouldalso include the array maximum power pointtracking efficiency for the inverter, which isgenerally around 98% to 100%.

Verify the proper grid voltage and frequencyto operate inverters, including evaluatingvoltage drop between the inverter ac outputand point of connection to the grid.

5.5.1 Test Reports

Measurements and test results for PV systemsshould be clearly summarized in a test report thatincludes the following information:

System information.

Visual inspection record and observations.

Identification of circuits tested, testsperformed, and record of measurements.

Interpretation and summary of results,identifying special maintenance needs orcorrective actions.

Signatures of responsible person(s) anddate(s) of tests.

An example test report format is presented in theannex of the IEC 62446 standard.

Seaward Solar provides a wide range ofreports which are especially designed tomeet the requirements of IEC 62446,simplifying the record keeping process.SolarCert Elements is an inexpensivesoftware product, especially designed toproduce test reports of PV systems and canbe used in conjunction with the PV150SolarlinkTM Test Kit.

6 SYSTEM PERFORMANCE TESTING

PV systems are designed to produce a specifiedelectrical output under certain operatingconditions. Performance testing verifies thesystem power output and energy production are

as expected, based on component and systemratings and the given operating conditions.Performance data can also help identify problemsthat require service or maintenance, and may alsobe used for determining system financialrevenues and warranty provisions.

6.1 Verifying Power and Energy Production

Power and energy production are thefundamental metrics for PV system performance.The methods and procedures to verify PV systemperformance are based on the principles of PVdevice operation, and accounting for the effectsof temperature and solar radiation. Manyparameters may be monitored automatically anddisplayed by meters, inverters, controllers orother power processing equipment in the system.

For interactive PV systems, the key indicators forsystem performance are ac power output (kW)and energy production (kWh). Measurement andverification of ac power output can be done atany time when the system is operating understeady sunlight conditions near peak outputlevels. Energy production is measured over longerperiods of months and years to compare withexpectations and the available solar resource.

Performance verification for stand-alone PVsystems is inherently more complex, involvingmore measurements and multiple interactionsbetween energy storage, charge control andsystem loads that affect PV system production.Battery health is the key to stand-alone PV systemperformance, and battery failure is often theindicator of many other system problems. Usuallystand-alone PV systems are designed to producea specified amount of energy on an average dailybasis to meet system loads. Measurements of


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Example: Verifying AC Output for InteractivePV Systems

The following worksheet can be used to estimatethe expected ac power output for interactive PVsystems without energy storage. This procedureis only valid for interactive systems using flat-plate crystalline silicon PV arrays (no specialbifacial modules or concentrating modules). Theentire PV array must be oriented in the same

direction and unshaded. The inverter must beoperating the array at its maximum power point,and not exceeding inverter ratings withinprescribed voltage limits. Measurements of solarradiation, temperature and power output mustbe done simultaneously. For best results,measurements should be within 2 hours of solarnoon with incident solar radiation levels 800W/m2 or higher and under clear sky conditions.

Module Mismatch: The derating factor forPV module mismatch accounts formanufacturing tolerances that yield PVmodules with slightly different current-voltagecharacteristics. Consequently, whenconnected together electrically, usually inseries, the modules do not operate at theirrespective peak efficiencies. A derating factorof 0.98 represents a loss of 2% due tomismatch, and is a reasonable assumption.PV systems using ac modules or micro-inverters would not have any modulemismatch, because PV modules areindividually connected to maximum powerpoint tracking inverters.

DC Wiring: This derating factor accounts forlosses in the dc circuits from the PV array tothe inverter. It includes voltage drops inwiring, connections, fuses, diodes, switchesor any other components, as applicable.These losses can be determined in the field,by measurements of the current and voltagedrops through the entire dc circuit. Losses indc circuit conductors can be easilycalculated from Chapter 9 Table 8 in theNEC. Total DC wiring and connection losseswill usually be on the order of 2% for mostPV systems, resulting in a derating factor of0.98. Systems using ac modules or micro-inverters have insignificant DC wiring andconnection losses, and ac modules are ratedfor ac, not dc specifications.

AC Wiring: This derating factor accounts forlosses in the ac circuits from the inverter tothe point of utility interconnection, or wherethe system output is measured. It includesvoltage drops in wiring, fuses anddisconnects, and is usually on the order of1% or less for most PV systems, resulting ina derating factor of 0.99.

Soiling: This factor accounts for theaccumulation of dirt, snow and other foreignmatter on the surfaces of PV modules thatreduces the amount of solar radiationreceived. PV arrays located in arid, dustyareas with infrequent rain can experiencesoiling losses 20% a year or higher.

Routine maintenance and array cleaning asrequired can largely eliminate soiling losses,and a default derating factor of 1.0 may beused for clean arrays.

Age: The derate factor for age accounts forlosses in performance over time due primarilyto degradation of the PV modules. The lossin performance is typically 0.5% to 1% peryear for most crystalline silicon PV arrays.

INTERACTIVE PV SYSTEM PERFORMANCE WORKSHEETEstimating and Verifying System AC Power Output

PV Array DC Power Rating at STC - 1000 W/m2, 25 C (kW) .............................................10

Derating Factors

Nameplate Ratings ...........................................................................................................0.95

Inverter and Transformer ...................................................................................................0.95

Module Mismatch .............................................................................................................0.98

DC Wiring .........................................................................................................................0.98

AC Wiring .........................................................................................................................0.99

Soiling ...............................................................................................................................1.00

Age ...................................................................................................................................1.00

Combined Derating Factors ...................................................................................................0.86

Estimated System AC Power Output at STC - 1000 W/m2, 25 C (kW) ...............................8.6

Temperature Adjustments

Array Power-Temperature Coefficient (%/C) ....................................................................-0.5

Measured Array Operating Temperature (C) ...................................................................60

Estimated System AC Power Output at 1000 W/m2 and Operating Temperature (kW) ........7.1

Solar Radiation Adjustments

Measured Solar Irradiance in Plane of Array (W/m2) ......................................................850

Estimated System AC Power Output at Operating Temperature and Solar Irradiance (kW) ..6.0

Example


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6.2 Array I-V Measurements

To evaluate the performance of PV modules andarrays for rating pu

Documents

Photovoltaic System Commissioning and Testing