[RE-wrenches] how would you measure a partial short circuit

[Bill Brooks]

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 4th 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.

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at 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.






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