SMA Ground Fault troubleshooting advice. [RE-wrenches]

Wed Nov 21 13:55:42 PST 2007

Dave/Wrenches,

John's description is pretty right on. I'd add the following.  

Check the GFDI fuse to make sure it's actually bad. 

Go when there is good sun and time to spend chasing it down.  Particularly
true if it is an "intermittent" or moisture-induced fault. I've seen more
than a couple of cases during winter where chasing a fault is frustrating
because it's not there when you are. Being prepared to cover the array for
the duration of the actual fix is a nice idea, but don't expect to find a
sneaky fault without power.

Carry a DC clamp ammeter to check current flow in conductors before
"opening" anything. Once you're on the roof, you may or may not be able to
get a voltage probe on a conductor without breaking a connection. Knowing
whether or not current is flowing in that conductor before opening the
connection is crucial for safety.

Be safe and good luck,

Matt Lafferty

-----Original Message-----
From: John Berdner [mailto:jberdner at sma-america.com] 
Sent: Wednesday, November 21, 2007 12:18 PM
To: RE-wrenches at topica.com
Subject: RE: SMA Ground Fault troubleshooting advice. [RE-wrenches]

Dave/Wrenches:

Locating ground faults in the array is fairly straight forward albeit a
little dangerous.
A DVM and some spare fuses (just in case) is all you really need.
Be very careful when you are doing all of this because you will be working
with live wires hanging in space and PV negative is no longer necessarily at
ground potential.  If the equipment grounding on the array is not up to
snuff it is also possible to see hazardous voltages on the module frames or
structure so measure everything and be really careful.  Treat everything as
hot and potentially lethal until your meter says otherwise. Remember each
time you disconnect something the whole situation can change so measure
again to be sure.  Also, depending where the ground fault is and the order
in which you pull the wires out of the inverter (or disconnect) it is
possible to see double the array Voc.  This can be a very bad thing since
the spacing on all components was designed for 600 Vdc max and you could be
at double that.  A 960 Vdc arc at  5 to 8 Amps is pretty ugly.

Start by opening the DC disconnect and removing the GFDI fuse in the
inverter.
Measure the array open circuit voltage form the PV plus in the dc disconnect
to the PV negative in the inverter.
Lets say it is 480 Vdc.
Now measure PV positive to ground and then the PV negative to ground.
An array without a ground fault should give you roughly equal values of
around 240 Vdc but the measurement should decay down towards zero.  What you
are seeing is a discharge of the capacitive coupling between the array and
ground.

In an array with a ground fault you will measure a fixed voltage to ground
on both the PV positive and PV negative.
The sum of the PV positive to ground and PV negative to ground measurements
should equal the open circuit voltage.
The difference in this case is that the voltage will be fixed and will not
decay towards zero.
The voltages you measure actually tell you where to start looking for the
ground fault.

For example: with 12 modules in series and an open circuit voltage of 480,
each module is equal to 40 Vdc.
If you measure PV positive to ground and get 0 volts then you should see a
PV negative to ground voltage of - 480 Vdc.  This tell you the ground fault
is in the PV positive output conductor.
Alternatively, if you were to see PV positive to ground at 400 Vdc and PV
negative to ground at -80 Vdc this tells you the ground fault is between the
2nd and 3rd modules "up" from the PV negative.

To figure out which string the ground fault is in, isolate each string
(remove the wires from the disconnect/inverter) and repeat the PV positive
to ground and PV negative to ground measurements.  On one string the
measurement will float around and the other will give you a fixed
measurement to ground.  The fault is in the string that has the fixed
measurement to ground.  My suggestion is to pull both positive and negative
wires for string 1 and measure.  If you are lucky you picked the right
string the first time out and you don't need to pull the second string at
all. 
It is unlikely but also possible to see a ground fault in the "middle"
of a module.  In this case you might see 380 Vdc from PV positive to ground
and -100 Vdc from negative to ground.  This would indicate a fault in the
middle of the third module "up" from the PV negative. 
Normally this means the module itself has a ground fault due to a nick in
the backskin, fault in the edge seal, etc. You can verify this by isolating
(unplugging) the suspect module then measuring PV positive to ground and PV
negative to ground for that isolated module. 

Lastly, In some very rare case the ground fault will actually be in the
string combiner, dc disconnect, or (heaven forbid) in the inverter itself.
You can verify this if the array voltage measurements show a ground fault
when connected to the combiner/disconnect/inverter and then both strings
float when measured in hanging in free space.  While unlikely, if you find
this situation reconnect the PV back up then disconnect one thing at a time
and measuring relative to PV positive and PV negative to ground until you
find where the fault is.

Best Regards,

John Berdner

>>> ipl at sover.net 11/21/2007 07:40:51 AM >>>

Wrenches,

I have a grid tied system with a SB3800U inverter being fed by 2 strings of
(12) Evergreen 180's (4.32kW). UniRack rail mounted on garage roof.
Good
installation all wiring neat and well supported. Only DC bond to ground is
in the inverter etc. And no external monitoring equipment.

The system has been operating very well (over 5,000 kWh over 2 months in
Northern Vermont), this is the first problem.

We are in the middle of a snowy spell and the array is covered in snow, but
I should be able to check out the array within a few days. According to the
error message on the Sunny Boy the GFDI 1 amp fuse is open. The array has
made it through lots of rain, ice and snow over the past year.

My suspicion is in the MC connector(s) themselves. We had a bad batch of MC
cables purchased from a large distributor 12 to 18 months ago. The MC
connector would pull off the wire when tested. Of course not all of them
where tested before they got into the field, so one could be on this roof.

Obviously I will check connections in the combiner, the dc disconnect etc,
could be a moisture issue. Of course it could be a squirel I guess.

Any advice on troubleshooting this ground fault would be appreciated.
Safety
wise do we need to cover the array with a dark tarp?

Dave

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