Sunnyboy ground fault mystery [RE-wrenches]

[Kirk Herander, VSE]

Upon very close inspection, -2- of these -6- old modules are shorted to
their respective frame. This is why I don't warranty work on used equipment.
Anyway there will be a happy ending, at least for me, since I've convinced
the customer he should sell these panels as a handyman's special and buy new
ones.

thanks for everyone's good advice here.
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

----- Original Message -----
From: "Bill Brooks" <billb at endecon.com>
To: <RE-wrenches at topica.com>
Sent: Thursday, February 26, 2004 12:10 PM
Subject: RE: Sunnyboy ground fault mystery [RE-wrenches]


> John and Kirk,
>
> That module has two series strings of 54 cells that are then paralleled in
> the junction box. Each series string of 54 cells is divided into 3 strings
> of 18 cells, each with their own bypass diode. It appears, if the fault is
> not in the inverter, that the fault is in middle string of one of the sets
> of 54 cells. Not knowing exactly how the j-box is laid out, I can't say
how
> to separate those two strings and test them individually. If one string is
> grounded, it could be permanently bypassed with a wire, but it raises
> serious doubt about that module in general. If you have a warranty, claim
> it.
>
> Bill.
>
>
> -----Original Message-----
> From: John Berdner [mailto:jberdner at sma-america.com]
> Sent: Wednesday, February 25, 2004 5:09 PM
> To: RE-wrenches at topica.com
> Subject: Re: Sunnyboy ground fault mystery [RE-wrenches]
>
>
> 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
>

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