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Wh ar vrcrrt prtctivdvics (fss r circit brakrs)rqird i dirct crrt circits btw

th PV mdls ad rid-dirct ivrtrs?

Beore answering this questiondirectly, it is important tonote that properly rated uses andcircuit breakers are equivalent inthis application and are collectivelyreerred to as overcurrent protectivedevices (OCPDs). his is true eventhough the required label on the backo certiied and listed PV moduleslists the use rating.

In most electrical systems, the NECrequires that every ungrounded circuitconductor be protected rom overcur-rents that might damage that conduc-tor. OCPDs provide this unction. Allcommercial scale systems with centralinverters require an OCPD or eachseries string o modules, but some

smaller grid-direct systems may notneed OCPDs in the dc circuits betweenthe inverter and PV modules.

In UL Standard 1703, Underwrit-ers Laboratories has established thatmodules must have an externalseriesOCPD i there are externalsources ocurrent that can damage the inter-nalmodule conductors. In the caseo an external or internal ault, themodule can be damaged i reversecurrents are present in excess o the

value o the modules maximum seriesuse rating. However, i there are nosources o external currents thatexceed this marked value, then noOCPD is needed to protect the inter-nal module wiring.

PV modules are current-limiteddevices, and their worst-case, continu-ous outputs or code calculations are1.25 times the rated short-circuit cur-rent (Isc). An exception to NECArticle690.9(A) allows conductors, typically

rated at 1.56 times Isc, to be used with

no OCPD where there are no sourceso external currents that might dam-age that conductor.

Backfeed considerations.Many o the smaller grid-direct invert-ers (below approximately 10 kW) aredesigned so that they cannot backeedcurrents rom the utility into arrayaults. Established, abnormal opera-tion tests or backeed do not rule out

backeed during normal operation othe inverter. Currently there are nonormal operation tests in UL Standard1741 to validate the lack o backeed-ing rom the utility. As a result, themanuacturer should provide writtencertifcation that the inverter cannotbackeed rom the utility into an arrayault. Larger central, transormerlessand bipolar inverters may requireadditional certifcation veriying that

they cannot backeed.

I the inverter can backeed util-ity currents into the dc PV wiring,the NECrequires that an OCPD beinstalled in series with the output oall individual module strings to pro-tect the cables and the modules romreverse currents rom any backedac currents through an inverter. Inlarge systems, used combining boxesare typically mounted at the array.In these systems, assuming that

the inverter is a potential source oovercurrents, an OCPD may also beneeded at the inverter input. hisOCPD will have a minimum ratingbased on the number o stringsconnected in parallel on that circuitand the short-circuit current o eachstring. It will be sized to allow maxi-mum orward currents rom the array(all strings o modules) to pass throughwithout interruption while keeping the

overcurrent device C o n T I n u e D o n P A g e 2 4

Q&A

Series String OCPD Requirements forGrid-Direct Inverter Applications

Central inverter combiners These UL-listed SMA SCCB-28-420 combiners

provide an OCPD for each series string of modulesa requirement for projects

utilizing central inverters.

Courtesysma-america.com

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rom operating at more than 80% oits rated value.

Guidelines for string inverters.he ollowing guidelines and exam-ples relate only to grid-direct PV sys-tems using inverters certiied by themanuacturer to not allow backedcurrents. he goal is to determinehow many strings o modules can beconnected in parallel and still meetNECand UL requirements beorean OCPD is needed on each string.Where required, one OCPD willprotect all modules and conductorsin a single series string o modulesper Section 690.9(E).

Additionally, the NECrequiresin Article 110.3(B) that the manu-acturers instructions and labels beollowed. Tese materials list themaximum value or the OCPD. Lesservalues can be used, according toArticle 690.8(A) and (B), as long asthey meet the NECrequirement o1.56 times the module Isc in order toprotect the conductor associated

with the module or string o modules.Note that in these examples we arenot determining the ratingo anyrequired OCPD. We are merely makingsome calculations to indicate whetheror not an OCPD is needed on eachstring o modules.

Example 1: single series string.

In a one-string system, no using isrequired since there are no externalsources o overcurrents. An unuseddc PV disconnect can be used on this

type o system as long as the modulesmaximum series use rating is at least1.56 Isc, and the connecting cables arerated at 1.56 Isc or higher.

Example 2: two strings in

parallel. In this example, each stringo modules can generate a maximumo 1.25 Isc. I a ault occurs in onestring, the electrical potential o thesecond, unaulted string can resultin a current o 1.25 Isc in the aultedstrings circuit. However, we know that

the modules in the aulted string can

withstand currents up to at least 1.56Isc or higher (i their marked seriesuse rating is higher), and the conduc-tors have an ampacity o at least 1.56Isc or greater. Tereore, with only twostrings o modules, no currents existin the PV array that can damage themodules or the wiring, and no OCPDsare required.

Example 3: three strings in

parallel. In this case, a ault in onestring could be subjected to currentsrom the two unaulted strings. Eacho these unaulted strings could deliverup to 1.25 Isc under worst-case condi-tions or a total o 2 1.25 Isc = 2.5 Isc.Suppose that the module manuac-turer had rated the value o the maxi-

mum series use at exactly 1.56 Isc,and the wiring was sized at exactly1.56 Isc. Te currents rom the twounaulted strings at 2.5 Isc would begreater than the series use rating othe module and ampacity o the con-ductors, and the modules could bedamaged. OCPDs in all three stringsat a minimum value o 1.56 Isc wouldbe required.

However, modules rarely havea marked maximum use value o

exactly 1.56 Isc. ypically moduleswill pass the UL reverse-current testsat a higher current such as 15 A. Forexample, assume the modules have theollowing characteristics: an Isc o 5 A,a marked maximum series use valueo 15 A and interconnecting conduc-tors rated at 15 A ater the appropriatederatings or conditions o use havebeen applied. In a system with threeseries strings o this module, the twounaulted strings could deliver 2 5

1.25 = 12.5 A. Since the cumulative

current is less thanthe 15 A rating o

the conductors andis also less thanthe modules 15 Amaximum seriesuse requirement,OCPDs are notrequired. Te

actual conductor ampacity would nothave to be 15 A, but it would have to beat least 12.5 amps ater deratings orconditions o use.

In a inal variation on Example3, assume that the module intercon-necting conductors are still rated at15 A, as is the modules series userating, but the module has an Isc o8 A. wo unaulted strings could eedup to 2 8 1.25 = 20 A to the aultedstring. he resulting 20 A exceedsboth the conductor ampacity and theability o the module to withstandreverse currents. o protect againstthese currents, OCPDs are requiredin each string o modules. he OCPDmust be at least 1.56 Isc (1.56 8 =

12.48 A) and not greater than 15 A.In this case a 15 A OCPD wouldnormally be used.

Summary. As can be seen romthese examples, when more than twostrings o modules are connected inparallel, a calculation is necessaryto determine i an OCPD is requiredin each string. When three strings omodules are connected in parallelwithout uses, the conductor ampac-

ity may have to be greater than thenormal 1.56 Isc.

Most utility-interactive PV sys-tems with only one or two strings oPV modules will not require OCPDsin direct current circuits between thePV modules and the inverter. Systemswith three or more strings in parallelrequire a simple calculation to deter-mine the OCPD requirements.

John Wiles / Institute for Energy and

the Environment / Las Cruces, NM /

nmsu.edu/~tdi/

SSystems with three or more strings

in parallel require a simple calculation to

determine the OCPD requirements.

Q&A