Nick,
It does sound like a ground fault, which leaves the question: why
doesn't the SB5000 see enough current to trip? Maybe it is something
else. I'd disconnect PV+ and PV- and then measure the string voltage.
Even with a ground fault it should be within normal range; if not there
is something besides a ground fault. Then measure PV+ to ground and PV-
to ground. These two numbers should add up to Voc. The ratio PV-/Voc
times the number of modules will tell you about how many modules are
between the fault and where the array is normally grounded. Caution:
normally grounded conductors, in fact all the conductors, may be
energized in reference to ground. Use appropriate personal protective
gear.
Kent Osterberg
Blue Mountain Solar
Nick Vida wrote:
Hi wrenches.
I had an interesting case today that I have been thinking about since
it happened. I dont have all the details because the homeownter had to
leave before I was done with the service call, but here is what I do
know. SB5000 with 2 strings of 15 modules. String one was 437 vdc, and
string 2 was 247vdc. I decided to look for a ground fault, and I turned
off the inverter. I measured for ground faults and found no evidence of
anything wrong in that regard. As the homeowner was out of time and
wanted me to go, I re-landed the wires in the dc disco, and when I
replaced the fuses, I noticed a minor arc when one of the fuses was
going it. Although I had no time to measure it, I think that there
might be a partial short circuit, possibly with one complete string and
some panels of the other string, and the remaining panels on the second
string. My mind was racing with how I could have measured for a partial
short circuit without seeing it as an arch. I can imagine such a thing
on a bigger system being a dramatic electrical event.
needless to say, I cant wait to get back there and go through the
signal path properly and find out what the electrician did. And of
course i realized that replacing the fuses in such a questionable
situation was probably not the best idea.
Do any of you have any thoughts on how one would measure for such a
thing?
nick vida
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