Xantrex GT3 question [RE-wrenches]

Thu Mar 24 10:44:10 PST 2005

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John Wiles asked me to post the message below.

Inverter Backfeed Into The PV Array

Some inverters installed in the US in the past have had the
capability, under normal operation, to backfeed utility currents into
the PV array when there are faults in array wiring (line-to-line, and
ground), module failures, and even at night.   Because of this
potential to backfeed currents into the PV array wiring, the normal
PV design procedure is to have an overcurrent device at the inverter
input that is rated at 1.56 Isc.

In the current UL 1741 standard for PV inverters there is an
"abnormal" conditions test that evaluates inverter for back feeding
from the grid into the array when components in the inverter fail.
There is currently no test to evaluate inverter back feeding under
"normal" operation of the inverter, but such a test is under
consideration for the next revision of the standard,

At this time, the only way an installer can be assured that an
inverter cannot backfeed into the PV array is to have written
assurances from the inverter manufacturer that such an event is not
possible.   Some inspectors may not accept this manufacturer's
statement.  In that case, the unfused PV disconnect is replaced with
a fused disconnect.

If the inverter is not capable of back feeding currents into the
grid, then 1, 2 and sometimes more strings may be paralleled without
overcurrent devices protecting each string of PV modules.  The number
of strings that can be paralleled depends on the Isc of each module
and the the value of the maximum series protective fuse.  See
Appendix J of the new 2005 PV/NEC: Suggested Practices Document for a
discussion of this requirement and the equations necessary to
determine when overcurrent devices are required.  Go here to down
load the 143 page PDF :  http://www.nmsu.edu/~tdi/roswell-8opt.pdf
Our web site (http://www.NMSU.Edu/~tdi) has additional information.
To see the very detailed "Perspectives on PV" areticles written for
the electrical inspectors and the somewhat simpler "Code Corner"
articles written for Home Power Magazine, go to "Photovoltaics" and
then "Codes and Standards".

>Mark,
>
>Thanks for your input.  Is there an accepted protocol now for testing
>inverter backfeed-to-the-array potential?  Otherwise, how would you
>definitively know that the inverter cannot backfeed to the array?  I'm not
>trying to be a pain here.  It's just that I've heard this from a number of
>inverter manufacturers without knowing exactly how they've come to this
>determination.  And what about your PV series 3-phase inverters?  Is there
a
>backfeed potential to the DC side with those units?
>
>thanks,
>marco
>ProVision Technologies
>
>
>
>Hi Marco,
>
>Like with other string inverters no fusing is required on the dc side of
>the GT 3.0 inverter if you are only installing 1 or 2 strings. The
>maximum fault current through a short on one string cannot exceed the
>short circuit current rating of a parallel connected string, so as it
>says in 690.0 "the short-circuit currents from all sources do not exceed
>the ampacity of the conductors". The note in the manual is mainly there
>in case someone is installing 3 strings, in which case a combiner with
>fusing may be required.
>
>The GT is fully isolated, and can not backfeed to the array. In addition
>under any typical fault condition it will shut down and disconnect from
>the AC line.
>
>Hope this helps!
>
>All the best,
>
>Mark
>
>
>Mark Edmunds
>Xantrex Technology Inc.
>
>To my esteemed Xantrex rep and all of you who have installed the their GT3,
>
>Is it required, that is, are you all installing an overcurrent protection
>device (fuse or breaker) between the one or two series strings feeding into
>the inverter?  In drawing after drawing in the installation manual, it
shows
>nothing between the array and the inverter, fuse, breaker, disco or
>otherwise.  And yet on page 3-6, one finds: "Important: Depending upon
>installation and local codes, fusing and/or a combiner box may be
required."
>Again, assuming no more than two series strings, can one get by without a
>fuse/breaker?  Which begs the fundamental question: can this inverter
>backfeed into the DC side?
>
>Marco Mangelsdorf
>President
>ProVision Technologies, Inc.
>69 Railroad Avenue, Suite A-7
>Hilo, Hawaii 96720 USA
>(808) 969-3281, fax 934-7462
>www.provisiontechnologies.com
>
>
>
>
>
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--
The Inspector checklist is attached.  You may download the Draft 2005
version of the "PV Power Systems and the National Electrical Code:
Suggested Practices" Manual from this web site:
http://www.nmsu.edu/~tdi/roswell-8opt.pdf
This manual will not be finalized or printed for several months.  Our
web site (URL below) has additional information. To see the
"Perspectives on PV" and the "Code Corner" articles, go to
"Photovoltaics" and then "Codes and Standards".

If I can provide further information, please do not hesitate to call,
e-mail or fax me.

John

John C. Wiles,  Program Manager
Southwest Technology Development Institute, New Mexico State University
505-646-6105      505-646-3841 (FAX)   http://www.NMSU.Edu/~tdi
SWTDI/NMSU
Corner Research Drive and Sam Steel Way
Box 30001/MSC 3 SOLAR
Las Cruces, NM 88003-8001

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