a safety concern for larger photovoltic systems

7/30/2019 A Safety Concern for Larger Photovoltic Systems

1/18

Solar America Board for Codes and Standards

www.solarabcs.org

The Ground-Fault Protection

BLIND SPOT:

A Safety Concern for Larger

Photovoltaic Systems

in the United States

A Solar ABCs White Paper

January 2012

Prepared for:

Solar America Board forCodes and Standards

Bill Brookswww.brooksolar.com


2/18


3/18

Solar America Board for

Codes and Standards White Paper

The Ground-Fault ProtectionBLIND SPOT:

A Safety Concern forLarger Photovoltaic Systems

in the United States

Prepared for:Solar America Board for Codes and Standards

Bill Brooks

www.brooksolar.com

January 2012


4/18

The Ground-Fault Protection BLIND SPOT: A Safety Concern for Larger Photovoltaic Systemsii

Disclaimer

This report was prepared as an account o work sponsored by an agency o the United States government. Neither the

United States government nor any agency thereo, nor any o their employees, makes any warranty, express or implied,

or assumes any legal liability or responsibility or the accuracy, completeness, or useulness o any inormation,

apparatus, product, or process disclosed, or represents that its use would not inringe privately owned rights.

Reerence herein to any specic commercial product, process, or service by trade name, trademark, manuacturer, or

otherwise does not necessarily constitute or imply its endorsement, recommendation, or avoring by the United States

government or any agency thereo. The views and opinions o authors expressed herein do not necessarily state or

refect those o the United States government or any agency thereo.

Download a copy of the report:

www.solarabcs.org/blindspot


5/18iiSolar America Board forCodesand Standards White Paper

EXECUTIVE SUMMARY

This document addresses an important saety issue in the design o many U.S. photovoltaic (PV) systems. This

saety issueundetected aults in grounded PV array conductorscame to light during investigations into two

well-publicized PV system res. The rst occurred on April 5, 2008, in Bakerseld, Caliornia, and the second

occurred on April 16, 2011, in Mount Holly, North Carolina.

The investigations into these res expose a blind spot in ground-ault protection in larger PV systems and

provide an opportunity to explore the saety implications o inadequate ground-ault protection in a public orum.As investigators develop an understanding o the root causes o the Bakerseld and Mount Holly res, they will

also develop a better understanding o the complex nature o aults and ault currents in PV arrays, which will

benet all stakeholders.

This paper provides a basic explanation o the cause o these res, ollowed by an outline o a limited research

plan designed to develop solutions. It also includes preliminary re mitigation strategies and equipment retrot

recommendations to reduce re danger in new and retrot applications based on results o the re investigations

to date.

The preliminary mitigation strategies and equipment retrot recommendations listed in this white paper include:

proper installation techniques with close attention to wire management,

annual preventative maintenance to identiy and resolve progressive system damage,

introduction to the use o data acquisition to monitor the operation o all PV systems at a level sucient to

determine i unscheduled maintenance is required, and

additional ground-ault and PV array isolation sensing devices that can be incorporated into the data system

to alert operators to potential problems so that maintenance personnel can be dispatched well in advance o

damage that could lead to a re.

The Solar America Board or Codes and Standards (Solar ABCs) is leading a broad industry- and stakeholder-

based working group to research this problem and develop eective mitigation strategies. Until the working

group research is completed, it is not known which inverters and which GFDI schemes are most susceptible to

the blind spot phenomenon, but larger inverters with the blind spot represent a potential re hazard that must be

addressed. Early results rom large PV systems retrotted with protective devices indicate that these devices may

eliminate the blind spot without requiring redesign o the system.

The types o protective devices that have currently been retrotted to existing inverters or evaluation include:

dierential current sensors (also known as residual current detectors or RCDs) installed on eeders entering

the array combiners on each system, and

insulation resistance monitors that measure the resistance to ground on a PV array while it is not operating.

In addition, other protective equipment may help mitigate re danger in new systems, including:

contactor combiners, which constitute an additional saety step beyond the dierential current sensors and

insulation monitors included in new inverters; arc ault detectors, which are required by the 2011 National Electrical Code; and

module-level controls, which can shut the power o rom each module.

Based on the limited investigations to date, it is recommended that PV systems with damaged conductors be

identied and repaired as soon as practical. Once the Solar ABCs research project is complete, a nal report

will be published to inorm all potentially aected PV system owners o all ndings and recommendations

necessary or them to make inormed decisions about whether or not to implement retrot or other proposed

modications.


6/18

The Ground-Fault Protection BLIND SPOT: A Safety Concern for Larger Photovoltaic Systemsiv

AUTHOR BIOGRAPHY

Bill Brooks

Bill Brooks has worked with utility-interconnected photovoltaic (PV) systems since the late 1980s. He is a

consultant to the PV industry on a variety o perormance, troubleshooting, and training topics. During the past

11 years, his training workshops have helped thousands o local inspectors, electricians, and installers understand

and properly install PV systems.

His eld troubleshooting skills have been valuable in determining where problems occur so that training can

ocus on the issues o greatest need. Mr. Brooks has written several important technical manuals or the industry

that are now widely used in Caliornia and beyond. His experience includes work on technical committees or

the National Electrical Code, Article 690, and IEEE utility interconnection standards or PV systems. In 2008, the

Solar Energy Industries Association appointed him to Code Making Panel 4 o the National Electrical Code.

Mr. Brooks holds bachelor and master o science degrees in mechanical engineering rom North Carolina State

University and is a registered proessional engineer in both North Carolina and Caliornia.

Brooks Engineering Website: www.brooksolar.com

Solar America Board for Codes and Standards

The Solar America Board or Codes and Standards (Solar ABCs) is a collaborative eort among experts to ormally

gather and prioritize input rom the broad spectrum o solar photovoltaic stakeholders including policy makers,

manuacturers, installers, and consumers resulting in coordinated recommendations to codes and standards

making bodies or existing and new solar technologies. The U.S. Department o Energy unds Solar ABCs as part

o its commitment to acilitate widespread adoption o sae, reliable, and cost-eective solar technologies.

Solar America Board for Codes and Standards website:

www.solarabcs.org


7/18vSolar America Board forCodesand Standards White Paper

Table of Contents

1 Introduction............................................................................................................................1

2 The ProblemIneective Ground-Fault Protection .................................................................2

2.1 The Bakerseld and Mount Holly Fires ...............................................................................22.2 A Common Thread .............................................................................................................2

2.3 The Sequence o Events Leading to the Fires ......................................................................3

3 Codes and Standards Requirements .......................................................................................7

3.1 National Electrical Code .....................................................................................................7

3.2 Ground-Fault Protection Device Test Standard ....................................................................7

3.3 The Blind Spot....................................................................................................................7

4 The Root Cause ......................................................................................................................9

5 Solar ABCs Ground-Fault Blind Spot Research Project ...........................................................10

6 Possible Mitigation Strategies to Prevent Fires in Large PV Systems ...................................... 11

6.1 Installation Inspections ....................................................................................................11

6.2 Annual Preventative Maintenance ....................................................................................12

6.3 PV System Monitoring ......................................................................................................12

6.4 Possible Retrots or Existing PV Systems ........................................................................14

6.5 Equipment to Consider or New PV Systems ....................................................................15

6.6 Implementation o Mitigation Strategies ...........................................................................16


8/18

The Ground-Fault Protection BLIND SPOT: A Safety Concern for Larger Photovoltaic Systemsvi

1. IntroductionThis document addresses an important saety issue in the design o many U.S. photovoltaic (PV) systems. This

saety issueundetected aults in grounded PV array conductorscame to light during investigations into two

well-publicized PV system res. The rst occurred on April 5, 2008, in Bakerseld, Caliornia, and the second

occurred on April 16, 2011, in Mount Holly, North Carolina.

There have been other similar res, but they are not public knowledge and investigation results are not available

or open discussion. The res at Bakerseld and Mount Holly, thereore, provide an opportunity to explore thesaety implications o suspected inadequate ground-ault protection in a public orum.

Solar America Board or Codes and Standards (Solar ABCs) has initiated a short-term research project with broad

participation rom PV industry, owners, and the enorcement community to better understand the blind spot

problem and develop recommendations to address any saety concerns. Ultimately, however, the individual PV

system owner will have to evaluate the risk and decide which recommended corrective actions to implement.

The intent o this white paper is not to blame PV component and system installers, designers, or manuacturers.

As investigators develop an understanding o the root causes o these res, they also develop a better

understanding o the complex nature o aults and ault currents in PV arrays, a valuable benet to all

stakeholders.

This paper provides a basic explanation o the current understanding o the cause o these res, ollowed by an

outline o a limited research plan designed to propose solutions. Based on results o this research, Solar ABCs will

recommend strategies or mitigating uture res in existing and new installations. Eective ground-ault detection

is a high priority or ensuring the saety o PV systems in the United States.

2. The ProblemIneffective Ground-Fault Protection

2.1 The Bakersfield and Mount Holly Fires

Pete Jackson, an electrical specialist with the city o Bakerseld, documented the Bakerseld re (Jackson, 2009)

shortly ater the event. The Jackson report provides basic insight into the cause o the re, but lacks emphasison key actors that contributed to the re. The author o this white paper wrote a ollow-on article or SolarPro

magazine (Brooks, 2011) that expands on Jacksons report and emphasizes the larger saety concerns that the

Bakerseld Fire exposed. Based on an understanding o the source o the Bakerseld re, the article predicted

that similar res would occur in the uture.

The April 16, 2011, re in Mount Holly, North Carolina, was such a recurrence. This re was documented in a

report or Duke Energy by this author, which has seen limited but open distribution within the utility community.

Many o the key details o that report are presented in this white paper.

2.2 A Common Thread

Both reerence res show evidence o signicant arcing at one location in the PV array, although the re ignitedin a completely dierent section o the array. That is, in both res there was signicant damage in a seemingly

unrelated portion o the array away rom the initial sources o ignition. Jackson connects the coincidence o

these aults in his report, but he questions the likelihood o a repeat event. The SolarPro article showed that the

problem was ultimately in the blind spot o ground-ault protection equipment and thereore an apparent general

concern to the PV industry in the United States.

As in the Bakerseld Fire, the Mount Holly re caused signicant damage in two dierent locations at the same

time. At Mount Holly, the aults existed inside two dierent combiner boxes at the same time. The likelihood o

these two aults initiating simultaneously in dierent locations is extremely small. Thus, it is likely that one ault

existed or some period o time prior to the initiation o the second one. This is the undamental insight that led

to an understanding o the root cause o the res.


9/181Solar America Board forCodesand Standards White Paper

In summary, it appears that at some time beore the ignition o the res, one ault occurred that was allowed to

continue undetected, apparently due to an ineective ground-ault detection scheme.

The National Electrical Code (NEC) and Underwriters Laboratories (UL) 1741 require that a ground-ault detector

and interrupter (GFDI) be part o every PV system. Traditional saety requirements are based upon a single ault

scenario. However, a problem may arise with a ground ault occurring in the grounded PV array conductor (the

array positive or negative conductor that is bonded to earth ground, typically in the inverter). I this ault does

not trip the GFDI, a current path may be created which inhibits an inverter rom interrupting uture ground aults

when they occur. Within this white paper, ground aults on the grounded conductor o insucient magnitude to

be detected by existing ground-ault equipment are said to all within a detection blind spot. The blind spot is a

potential condition that can occur in any conductor o the grounded pole o an array.

Recent system reviews by the Solar ABCs team and others have ound that aults on grounded conductors in

a PV array can be extremely dicult to detect with methods typically used in currently available ground-ault

protection equipment. This is because the conductor is already grounded, so additional grounds on the conductor

have little voltage dierential to drive the current necessary to blow the protective ground-ault use. In this

undetected condition, ground aults on the grounded conductor may exist indenitely in a PV array, establishing

a new normal condition that renders the ground-ault protection device unable to interrupt a second ground-

ault current. In this condition, the clearing o the ground-ault use caused by an ungrounded conductor ault

may drive the re event by applying 500 volts to the rst initially grounded conductor ault location.

2.3 The Sequence of Events Leading to the Fires

Based on the authors investigation, a similar sequence o events preceded the res in Bakerseld and Mount

Holly. In both cases, the evidence suggests that an undetected ault existed in the grounded conductor o the

array. The line diagram in Figure 1 shows a ground-aulted string conductor similar to the likely rst ault in the

Bakerseld re. The line diagram in Figure 2 shows a ground-aulted subarray conductor similar to the likely rst

ault in the Mount Holly re. How long these assumed aults existed is unknown.

F15

F2

F1

22 STRING SUBARRAY

WITH 159-AMP MAX CIRCUIT CURRENT

AND 558 VOLT MAX SYSTEM VOLTAGE

AT 800 W/m2

6.2-AMPS

136 AMPS

133-AMPS

INVERTER DC INPUT

3.1 AMPS

3.1-AMPS

6.2-AMPS

0-AMPS

Stage 1- Faulted Grounded String Conductor

136 AMPS

136 AMPS

136 AMPS

136 AMPS

136 AMPS

136 AMPS

5A

FUSE

300A FUSE

300A FUSE

300A FUSE

300A FUSE

FAULT

CURRENT OF

3.1 AMPS NOT

ENOUGH TO

CLEAR FUSE

String Conductor

Grounding Conductor

FAULTED CONDUCTOR

INSULATION DAMAGE IN STRING CONDUCTOR

UNWANTED GROUND FAULT CURRENT 3.1AMPS

COMBINER BOX 8CB3 22 STRING

SUBARRAY

CB2 22 STRING

SUBARRAY

CB1 22 STRING

SUBARRAY

CB3 22 STRING

SUBARRAY

CB2 22 STRING

SUBARRAY

CB1 22 STRING

SUBARRAY

Figure 1: Ground-Faulted String Conductor, the Assumed First FaultBakersfeld Fire


10/18

The Ground-Fault Protection BLIND SPOT: A Safety Concern for Larger Photovoltaic Systems2

F15

F2

F1

22 STRING SUBARRAY

WITH 159-AMP MAX CIRCUIT CURRENT

AND 558 VOLT MAX SYSTEM VOLTAGE

AT 800 W/m2

6.2-AMPS

136 AMPS

134-AMPS

INVERTER DC INPUT

136 AMPS2-AMPS

6.2-AMPS

0-AMPS

Stage 1- Faulted Grounded Subarray Conductor

136 AMPS

136 AMPS

136 AMPS

136 AMPS

136 AMPS

136 AMPS

5A Fuse

300A Fuse

300A Fuse

300A Fuse

300A Fuse

FAULT

CURRENT OF

2 AMPS NOT

ENOUGH TOCLEAR FUSE

String Conductor

Grounding Conductor

FAULTED CONDUCTOR

INSULATION DAMAGE IN SUBARRAY CONDUCTORUNWANTED GROUND

FAULT CURRENT 2 AMPS

Combiner Box 8CB3 22 STRING

SUBARRAY

CB2 22 STRING

SUBARRAY

CB1 22 STRING

SUBARRAY

CB3 22 STRING

SUBARRAY

CB2 22 STRING

SUBARRAY

CB1 22 STRING

SUBARRAY

Figure 2: Ground-Faulted Subarray Conductor, the Assumed First FaultMount Holly Fire

The evidence suggests that sometime ater the initiation o an undetected ault in the grounded conductor, a

second ault occurred in the ungrounded conductor. Ordinarily, when a ault is detected on the ungrounded

conductor, the ground-ault detection circuit opens the ground-ault use, thus opening the aulted circuit and

interrupting all current fow.

As the gures illustrate, when there is an existing ault in the grounded conductor, blowing the ground-ault

use can no longer interrupt all ault current. Because a return path has already been established in the aultedgrounded conductor, the array becomes short circuited between the aulted grounded conductor and the second

ungrounded conductor ault. The ground-ault protector in the inverter is not in this circuit and is unable to stop

the fow o current.

Figure 3 shows the second ault in a re similar to the Bakerseld event. In this scenario, the short circuit current

is orced through the small 10 American wire gauge (AWG) grounded string conductor, causing a conductor

insulation re. Figure 4 shows the second ault in a re similar to the Mount Holly event, in which the array short

circuit current is orced through the poor inadvertent grounding connection, causing an arcing re.

A 500-kilowatt (kW) PV array can have more than 1,300 amps o available short circuit current, which can cause

enormous damage during this short circuit condition. I the two connection points were intentionally made with

terminations able to handle the current, there would be little i any hazard. Unortunately, these connections are

made with only incidental contact that can evolve dramatically into intense arcs with operating temperatures o

more than 2,000C. These arcs can jump gaps between metal parts up to six inches apart.


11/18


12/18


3. Codes and Standards Requirements

3.1 National Electrical Code

The NEC, in article 690.5 (A), requires the ollowing:

(A) Ground-Fault Detection and Interruption. The ground-ault protection device or system shall be capableo detecting a ground-ault current, interrupting the fow o ault current, and providing an indication othe ault.

The requirement in the NEC and UL 1741 standard that covers PV inverters addresses only ground-ault current ingrounded PV systems. The NEC requirements do not address ault locations or ault current trip limits. Experience

in the United States has ound that although ault current is undesirable, all U.S. PV arrays have small current

fows to ground, generally reerred to as leakage current. These current fows increase in larger PV arrays and

with certain PV module technologies and designs.

3.2 Ground-Fault Protection Device Test Standard

The present version o UL 1741 requires inverters to detect ground aults based on the magnitude o the ault

current measured by the GFDI device. The trip limits set in the standard are based on the size o the system, and

are derived by extrapolating the maximum allowable leakage current rom a single PV module to the leakage

rom a large array eld. The trip current limits are set conservatively to prevent nuisance tripping, particularly or

modules that only marginally meet the leakage requirements. The allowable trip limits in UL 1741 range rom 1amp or inverters rated up to 25 kW to 5 amps or inverters greater than 250 kW.

3.3 The Blind Spot

I a ground ault occurs with current less than the trip threshold established by the UL 1741 standard, it cannot

be detected or seen by the GFDI device. Hence, in this context, the device is said to have a blind spot (more

ormally known as a lower limit o detection). It is axiomatic that bigger PV systems, which have higher trip

thresholds than smaller systems, must also have a larger blind spot than smaller systems.

This may explain why our rst observations o this problem occurred in larger PV systems. With research,

we may nd that scenarios that lead to res can also develop in smaller PV systems with smaller detection

thresholds, possibly coupled with reduced short circuit current designs. A key area o ocus or uture Solar ABCs

research will be understanding minimum system sizes that have a blind spot and identiying the appropriate aultcurrent detection settings that enable the system to operate saely.

4. The Root Cause

A large re oten consumes the evidence o its origins. However, what evidence remained rom the two res

under discussion here suggests similar root causes.

The evidence suggests that the ignition o the Bakerseld re resulted rom two aults occurring in two

signicantly dierent wire sizes in the PV array. The rst ault was in a small grounded conductor. Failure to

detect this rst ault created the potential or the aulted grounded conductor to become part o a circuit or

which it was not designed.

Physical evidence rom the Bakerseld re suggests that about 1,000 amps fowed through the aulted grounded

conductor, a small unprotected 10 AWG wire. The wire insulation burned and ignited other uel in the immediate

vicinity. Because NEC installation requirements do not allow overcurrent protection devices in the grounded

conductors o grounded PV arrays [NEC Article 690.13], ull array current fowed unimpeded through the ground

ault in the string-level wiring o the Bakerseld system.

The same scenario is suggested in the Mount Holly re, but here the evidence indicates that both aults occurred

in large combiner box eeder conductors. This was unexpected, because conventional wisdom assumes that a

ault in a grounded large conductor would blow the ground-ault use and shut down the inverter. However, ull



review o the Mount Holly re suggests that even large grounded eeder conductors can be aulted and remain

in the blind spot o the ground-ault protection device. Two months beore the Mount Holly re, maintenance

inspection personnel photographed conductor insulation damage in two sections o the array that were later

severely damaged in the re. Although the inspectors were concerned that this damage would cause the ground-

ault protection system in the inverter to shut the system down, they did not ully understand the threat o re

because so ew res have been reported.

5. Solar ABCs Ground-Fault Blind Spot Research Project

Solar ABCs has initiated a short-term research project to better understand the blind spot problem and developrecommendations to guide the PV industry as it addresses any saety concerns. Ultimately, the PV system owner

must evaluate the risk and implement corrective actions. This evaluation requires a solid basis o data in order to

make inormed decisions.

The Solar ABCs research project has the ollowing objectives:

Determine the conditions uner which existing ground-ault protection is inadequate. For example, conrmthe identication o systems at risk with eld tests by metering systems with no aults, introducing aault, and observing whether the inverter detects the ault. Factors to examine include:

o array size, age, topology, and possibly climate (what is the minimum system size that will developthe blind spot problem, or example.);

o dierent PV technologies, including crystalline and thin-lm and their respective leakage charac-teristics; and

o protection schemes used by inverters that are either eective or ineective at detecting the intro-duced ault.

Develop a mitigation proposal that can be implemented through changes to the NECand UL standards.

o The mitigation proposal currently under consideration is a combination o a morning check anddetection o ault current by measuring dierential current. The research project will determinevalues or insulation resistance and dierential current.

The Solar ABCs will publish a nal report detailing the research results and possible new system design guidelines

to improve the saety o uture PV systems.

6. Possible Mitigation Strategies to Prevent Fires in LargePV Systems

Owners o large PV systems should consider the ollowing re mitigation strategies, which, although not

comprehensive, are based on an analysis o the probable causes o recent res. As noted earlier, renement o

these and additional mitigation means may be identied when the Solar ABCs team completes its testing and

analysis.

Field experience suggests several steps in the design, installation, and maintenance o a PV system that must beollowed to prevent PV system res. These steps include:

using proper installation techniques with close attention to wire management, such as taking care toavoid abrading insulation and using correctly-sized wire;

perorming annual preventative maintenance to identiy and resolve progressive system damage due tothermal cycling and mechanical vibration;

using detailed data acquisition to monitor PV system operations and determine i unscheduled mainte-nance is required; and


14/18


installing additional ground-ault and PV array isolation sensing devices that can be incorporated into thedata system to alert operators about potential problems so that maintenance personnel can be dispatchedin advance o a ailure that could lead to a re.

6.1 Installation Inspections

The rst step in preventing or mitigating ground aults must be taken prior to energizing the array or the rst

time. Conduct a detailed review o all installation-related issues and develop a punch list to address concerns,

including wire management, grounding, and equipment installation for the entire system. Once the punch list has been

resolved, the commissioning procedure should include:

insulation resistance tests on all eld-installed conductors, including modules and module wiring;

open-circuit voltage and polarity tests on all string and eeder circuits;

operational current readings on all series strings and eeders; and

thermography o all inverters, disconnects, and combiner boxes at 50% load or higher as well as ther-mography o the array to scan or hot spots not caused by shading or other normal temporary conditions.

6.2 Annual Preventative MaintenanceNext, develop a maintenance schedule that establishes consistent inspection, documentation, and maintenance

procedures to identiy and correct problems beore they result in a re. The act that a maintenance inspection

prior to the Mount Holly re identied conductor damage that was later recognized as a potential cause o the

subsequent re reveals the value o this type o inspection. This case also highlights the need to adequately train

maintenance personnel so that they recognize the visual and testing indicators that a re is possible.

Maintenance procedures should include:

visual inspection o all equipment and eld connections in equipment or signs o damage or degrada-tion;

visual inspection o all accessible electrical junction boxes and raceways to see i conductors are damagedand in need o repair or replacement;

visual inspection o string conductors to identiy any physical damage that is in need o repair and addi-tional protection to prevent progressive damage;

operating voltage and current tests at dened conditions o irradiance and module temperature to com-pare output o strings;

insulation resistance testing o modules, string wiring, and photovoltaic output circuits in the array (sometimes reerred to as a megger test); and

thermography o all inverters, disconnects, and combiner boxes at 50% load or higher, as well as ther-mography o the array to scan or hot spots not caused by shading or other normal temporary conditions.

6.3 PV System Monitoring

Another step that owners o large PV systems can take to mitigate the risk o undetected ground aults is to install

and use automated perormance monitoring instrumentation that recognizes ground aults and other equipment

ailures as soon as they occur. Perormance monitoring and regular data review can be key elements in the sae

operation o any power plant. Careul attention to system and subsystem data trends enables early recognitiono component ailures, and helps to establish eective maintenance schedules that ensure reliable long-term

operation. The ollowing is a description o a generic monitoring program based on best practices identied by

the author. It is oered as a model and not a solution to all monitoring requirements.

Monitoring systems should include one-second data averaged, recorded, and accurately time-stamped at a

minimum o every 15 minutes, and preerably every ve minutes. With some averaged values, a minimum and

maximum value or each time period can provide insight into the range o values that the average represents.



Averages and minimum/maximum values should be recorded or:

plane o array irradiance,

ambient temperature and wind speed,

alternating current (AC) voltage on each inverter,

AC power output on each inverter, and

ground ault current on each inverter.

Items requiring averaged data include:

direct current (DC) voltage on each inverter,

DC current on each inverter, and

morning array insulation resistance (recorded once a day).

Optional items include:

string currents,

ground ault current on each PV power source circuit,

array temperature (back o representative module),

global horizontal irradiance, and wind direction.

One important use o this data is to establish the daily perormance index (PI) based on actual versus expected

perormance. PI is a daily value that is calculated based on irradiance, temperature, wind speed, and system

conguration.

A response schedule should be established when the data shows:

an inverter o-line,

perormance o less than 90% o expected PI,

dierential current above the prescribed maximum threshold, or

ground resistance less than the prescribed minimum threshold.

6.4 Possible Retrofits for Existing PV Systems

Finally, the Solar ABCs research will help identiy which inverter designs have GFDI schemes susceptible to the

blind spot phenomenon and suggest retrots that may be able to eliminate the blind spot without requiring

redesign o the system. Because o the larger power involved, it is expected that larger inverters with the blind

spot represent a potential re hazard that should be addressed. The ollowing protective devices and system

retrot modications may provide additional protection rom grounded conductor ground aults and can be

tailored or the leakage current o the specic PV arrays.

6.4.1.1 Differential Current Measurement

The simplest pieces o equipment to add to existing systems are dierential current sensors (also known as

residual current detectors or RCDs) installed on eeders entering the array combiners on each system. Array

conductors can be bundled and run through the current transducers o the RCD, but to the extent possible, it is

better to use individual sensors installed on the individual subarray conductors to achieve the highest resolution

o dierential current. Some commercial residual current monitors can monitor up to 12 current sensors.

Employing multiple sensors on a large PV array increases the sensitivity o ground-ault current measurements

by making measurements on smaller array segments. Breaking the array into smaller segments allows use o

lower trip current levels and signicantly reduces the blind spot in large arrays associated with the use o large

ground-ault use sizes.


16/18


6.4.1.2 Insulation Resistance Monitor

A second important piece o equipment needed to eliminate the blind spot is an existing product used to

measure the resistance to ground on a PV array. This measurement should be taken every morning prior to the

operation o a grounded PV inverter, and should be taken while the array is in open circuit condition. Beore

determining array resistance to ground, the operator may need to open a contactor to isolate the grounded

conductor rom ground during this measurement. Some inverters have a grounding contactor option. Without

such an option on a DC grounded PV array, a separate device that operates independently o the inverter would

be necessary.

Once the test has been completed, the array insulation monitor device either will report that the system is ree to

start normally or the day or it will signal that the ground resistance is below the minimum allowable value and

prevent the inverter rom starting. In the latter case, the inverter will remain ofine, providing indication o a ault

and reducing the possibility o a re.

6.5 Equipment to Consider for New PV SystemsIn addition to the equipment recommended for retrotting existing systems, new system designers should consider the

following equipment to improve safety.

6.5 Ungrounded Inverters

The basis or the problem outlined in this document is the act that the array conductors are solidly grounded,

thus making it dicult to detect insulation damage in the grounded conductors. Systems in much o the world

do not have conductors solidly grounded on the DC side o the inverter. These ungrounded systems employ

more sensitive methods to detect ground aults. Recently, the inverter test standard, UL1741, has been updated

to include requirements or transormerless and other ungrounded inverters. Until that standard was updated,

ew inverter manuacturers were willing to bring ungrounded inverters into the United States. With that technical

issue resolved, the door is open or many ungrounded inverter designs to be certied and installed in U.S.

systems. Ungrounded inverters (and systems) are not susceptible to the blind spot problem.

6.5.1.1 Contactor Combiners

Dierential current sensors and insulation monitors are included in new inverters, but using contactor combiners

can provide additional saety. These combiners can de-energize the eeder circuits rom the combiner boxes tothe array com biners when the AC power is disconnected. As noted earlier, the control circuit or the contactor

combiners can be wired through a relay at the inverter controlled by the dierential current sensors and the

insulation monitor.

Products are also available that provide string-level dierential current measurements, and individually isolate a

aulted string rom the rest o the array. Combined with monitoring, this can provide immediate notication that

a ault exists.

6.5.1.2 Arc Fault Detectors

Although the three measures discussed above will go ar to eliminate the causes o most res, they cannot

address series arc aults. Series arc aults occur when an electrical connection comes apart under load. These arcaults are particularly problematic because the arc condition is stable and remains until the conductor erodes to

the point that it burns away. Unortunately, i combustible material is present, as it was in both the Bakerseld

and Mount Holly res, the re will continue burning ater the arc extinguishes.

The probability o an arcing ault, though less likely than the aults that caused the Bakerseld and Mount Holly

res, remains high enough to justiy installing arc ault detectors. The 2011 National Electrical Code requires arc

ault detectors, which are expected to be available in 2012. These units may include contactor combiners or

string-level control and eature ground-ault detection unctions.



6.5.1.4 Module-Level Controls

Below the string level, the only way to reduce the voltage within the PV array to levels too small to initiate arcing

is or electronics to be installed at the PV module. These electronics, oten reerred to as module-level control,

or smart modules, can be designed to shut o the power rom each individual PV module, thus removing the

source o ault current and the voltage that could shock a technician or reghter. Recently introduced products

claim to have these eatures, but testing standards are not yet available to certiy that a product can perorm

this saety unction reliably. As adequate certication test procedures are developed, these products should be

considered or new installations and even or retrotting existing systems.

6.6 Implementation of Mitigation Strategies

The Solar ABCs team is evaluating the mitigation approaches described in this report to rene its

recommendations or PV system owners. Implementing these strategies will take time, and all the strategies may

not be necessary or every PV system. Some o the equipment is not yet on the market, and implementation will

require a phased approach.

PV systems with damaged conductors must be identied and repaired as soon as practical. Once the conductor

damage is addressed, the dierential current monitors can be installed and incorporated into the inverter

controls. Ater these initial steps, owners can add the other measures over time to urther enhance saety.


18/18

REFERENCES

Brooks, B. (Bill). (2011). The Bakerseld re. SolarPro magazine, February/March 2011, page 62.

http://solarproessional.com/home/. [Access requires ree subscription.]

Jackson, P. (Pete). (2009). Bakerseld re report. Investigators report, April 29, 2009.

http://npa.typepad.com/les/target-re-report-09apr29.pd.

a safety concern for larger photovoltic systems

Documents