[RE-wrenches] Capacitive Discharge Test Results

[Chris Mason]

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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