Sunnyboy ground fault mystery [RE-wrenches]

[John Berdner]

Kirk:
 
You are correct in your trouble shooting approach.
Measure Voc with the GFDI fuse removed, e.g. 323 Vdc
Measure Pv+ to Ground (PE terminal, pole, or Chassis), e.g. +133 Vdc
Measure PV - to ground , e.g.  -190 Vdc
With 6 modules the Voc per module should be 323/6 = 53.88 Vdc
Divide PV- to Ground by the Voc per module, e.g. -190/53.88 = 3.53
modules above the PV - terminal. 
Lets call this module #4 using the convention of counting up from PV-.
Alternatively, divide PV+ to Ground by the Voc per module, e.g.
+133/53.88 = 2.46 modules below the PV + terminal.
Since this is not really close to a "whole module" I suspect the
problem is not in the inter-module series wiring.
Because it close to a half module I wonder if the modules have a dual
voltage configuration jumper that could be shorting to ground.
Another option could be a bypass diode somehow shorting to ground.
You can check this by disconnecting module #4 and measuring it's Voc
and then PV+ to ground and PV- to ground when it is isolated from the
array.
I would expect something like 53.88 Vdc, 24.8 Vdc and 28.6 Vdc
respectively if it is a problem inherent to the module.
 
It is also possible that the ground fault is in the inverter itself.
Happens very rarely but we have seen this in a couple cases for
different weird reasons.
Because you said you isolated the inverter it does not sound like it in
this case but here is how to check it.
Open the dc disconnect and the >>VERY CAREFULLY<< lifting the PV + and
PV - from the inverter.
Remember you have a possible ground fault so you have to proceed as if
EVERYTHING is at lethal potential to ground and you are the ground path
waiting to happen.
Close the dc disconnect and measure Voc, PV+ to earth, and PV- to
earth, i.e. repeat the tests above.
If it is an array problem, you will see essentially the same result as
you did the first time
If it is an inverter problem then you won't see any stable potential
from the PV circuit conductors to earth.
Instead the voltage will be inconsistent and will likely fall as the
resistance of your DVM discharges the array's inherent capacitance.
 
Hope this helps.  Please let us know what you find.
 
John Berdner
 

>>> kirk at vtsolar.com 2/25/2004 11:11:07 AM >>>
Hey folks,

I have a system comprising of -6- old 60volt Mobil RA-180 ribbon
silicon
modules wired to a SunnyBoy 2500U inverter. The modules are mounted on
a
metal top-of-pole rack. The inverter detected a ground fault upon
start-up.
The open ciruit array voltage is 323 vdc. I measured the voltage from
the
pole to array pos. at +133 vdc. The measurement from pole to array
neg.
is -190 vdc. So it would appear that perhaps there is a short from the
3rd
module internally to the rack and pole. Its looks like the array
voltage is
"center-tapped" to ground somehow. However I isolated every module and
did
not find a short. I also isolated and bugged out inverter and DC
disconnect
wiring and couldn't find a short either. Any ideas on how to solve
this
mystery would be greatly appreciated.

thanks, Kirk

Vermont Solar Engineering
Kirk Herander
North American Board of Certified Energy Practitioners
Certified Solar PV Installer(TM)
Xantrex Certified Dealer Charter Member
NYSERDA-eligible installer
Vermont Solar and Wind Partner

PO Box 697
Burlington, VT. 05402
www.vermontsolar.com 
www.backeastsolar.com 
800.286.1252 - 802.863.1202
fax 802.863.7908







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