[RE-wrenches] Capacitive Discharge Test Results

Corey Shalanski cshalanski at joule-energy.com
Fri Mar 28 12:41:58 PDT 2014


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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