[RE-wrenches] Capacitive Discharge Test Results

[Erika Weliczko]

Given the recent discussions about transformerless inverters, troubleshooting, misc voltages

Do we need some guidance from inverter manufacturers and others to evaluate BEFORE all this equipment is in?
Electrical tests for metal roofs, etc?


Erika Weliczko
REpower Solutions
cell: 216.402.4458

On 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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