On 7/1/2011 10:04 AM, Bill Brooks wrote:
Nick,
If your voltage is low and there is no ground fault, either positive
and negative are connected mid string, bringing the voltage down, or a
bunch of modules never got connected in series. In either case, the
Voc difference between the two strings will create a significant
current flow at open circuit. That is because the two strings are no
longer at open circuit, they are connected together so they have a
circuit. You will see significant current flow in those two strings
even though they are not connect to anything else. This is a function
of the IV curve. The higher voltage string is generating power in the
first quadrant (the one everyone sees on spec sheets), and the lower
voltage string is operating in the 4^th quadrant---the quadrant nobody
wants to see. This is the region where you take the IV curve from
where it crosses the voltage access at Voc and proceed into the
negative current, positive voltage region. Therefore the power
generated by the higher voltage string is being absorbed in the lower
voltage string. An IV curve tracer would do a nice job of showing both
of these curves and then the composite curve is going to look very
strange.
Bill.
That's what I was going to say, Bill...
Only having two strings means that the series fuse rating will not be
exceeded
and those fuses won't blow. So that won't hurt any modules if that's
all it is.
Maybe there is some junction box that is shorting out more than one
module ? Maybe the
modules that are shorted (if that's what it is), are in one row ?
boB
*From:*re-wrenches-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@lists.re-wrenches.org] *On Behalf Of *Nick
Vida
*Sent:* Friday, July 01, 2011 7:03 AM
*To:* wrenches
*Subject:* [RE-wrenches] how would you measure a partial short circuit
Hi Bob,
thanks for thinking about my question!
I landed the fuses and put the fuses back in (not it- typo) on my sma
disconnect, and there was an arc across the fuse holder. Seems like it
could have been closing a short circuit on the bus once the parallel
connection was made.
--------
Hi Bill,
Although I am not entirely sure there is no ground fault,
I took the conductors off the terminals so they were in free air, and
measured pos to neg, pos to ground, neg to ground on both strings, and
all I saw was the taper towards 0 from about 20 vdc, and no steady
voltage anywhere but pos to negative. That makes me think the voltage
is in a short circuit somewhere.
I cant yet make any conclusions about the wiring mistake because I had
no access to the pull box where the different parts of the strings
were connected to each other and to the home run. I do think there
might be a short circuit between strings because when the fuse went
back in there was an arc on the fuse holder as if there was a short
circuit instead of a simple parallel connection. I guess it might have
had to do with the 2 different voltages, but there is voltage
'missing' somewhere.
Thanks for thinking about my question.
------------
Bob,
re: inverter capacitors charging, I havent seen that much of an arc
with SMA disconnects, and the switch was in the off position, so I
dont think there was any signal path to any real electronics. I always
pull my fuses and do not replace them until everything is landed and
hot and the voltages look proper when I install, and this arc was way
out of range of normal.
re: backfeeding, I just suspected that too while thinking about your
responses, and yes the 2 voltages were different by about 200 volts.
Thanks for the thoughts Bob.
_______________________________________________
List sponsored by Home Power magazine
List Address: RE-wrenches@lists.re-wrenches.org
Options & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
