[RE-wrenches] Capacitive Discharge Test Results

[August Goers]

Hi Corey,



I’ve done a lot of ground fault troubleshooting over the years with
isolated and non-isolated inverters and I have often seen the type of
bleedoff that you describe below for the ungrounded conductors. But, it
really should start zeroing out to under five volts within about 10
seconds. I’m not being terribly accurate with my numbers here – it seems to
vary for each project. If it hangs at 50 V for a while you might indeed
have a “leakage” issue such as moisture conducting through an abraded
conductor or splice. That’s where the insulation tester comes in handy.



I have a Fluke 1503 insulation tester which has worked fine for my
purposes.



Chris Mason’s post about the low impedance meter is interesting. I have a
few meters including a Fluke 116 which apparently has a low impedance (loZ)
setting. If you have a meter with a low impedance setting maybe you could
try that?



Best,



August



Luminalt



*From:* re-wrenches-bounces at lists.re-wrenches.org [mailto:
re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of *Chris Mason
*Sent:* Friday, March 28, 2014 1:30 PM
*To:* RE-wrenches
*Subject:* Re: [RE-wrenches] Capacitive Discharge Test Results



If you are using a high resistance meter, you may be experiencing phantom
voltages. Here's a Fluke paper on it.

http://support.fluke.com/find-sales/Download/Asset/2105317_A_w.pdf



You might also discuss with the manufacturer putting a 1M ohm resistor from
the center of the strings to the grounded roof to bleed off any stray
voltage. It would only bleed 1mA at 1000V so it would not affect
performance. Usually this voltage is apparent when the strings have not
connected to the load.



On Fri, Mar 28, 2014 at 3:41 PM, Corey Shalanski <
cshalanski at joule-energy.com> wrote:

Fellow Wrenches,



We recently commissioned a 220kW rooftop PV system and are having some
inverter malfunction issues. The system details are as follows:

- Inverters: Power One Aurora Trio-27.6 (x7)

- Modules: Stion STN-130 (x1690)

- Array: Strings of 14-16 modules each, 8 strings per combiner, 1 combiner
per MPPT input



It has been less than two months since we commissioned the system, and we
have already seen five of the inverters malfunction. While troubleshooting
these issues the inverter manufacturer had me measure positive-to-ground
and negative-to-ground voltage readings. At the inverter level I measured
instantaneous voltages of 100-200V before seeing a bleedoff; at the string
level the voltages were 10-20V before bleedoff.



The inverter manufacturer is concerned with the magnitude (~200V) of this
instantaneous voltage and also the delay in bleeding off to zero - after
about five seconds the voltage level was still approximately 50V (but still
dropping). The manufacturer is claiming that this residual voltage should
drop to zero much more rapidly AND the presence of such a delayed bleedoff
indicates significant "leakage" coming from the array.They are further
claiming that this leakage is responsible for the damage to the inverters.
They suggested using a megger to test insulation resistance throughout the
installation.



My questions to the group:

- Is there any truth in the inverter manufacturer's claims?

- Do my field measurements seem out of range from what others are seeing
with ungrounded arrays?



Thanks for any information you can provide.

--

Corey Shalanski

Joule Energy

New Orleans, LA

ᐧ


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-- 
Chris Mason

President, Comet Systems Ltd

www.cometenergysystems.com

Cell: 264.235.5670

Skype: netconcepts
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