We have partial discharge monitoring capability for thirty motors ranging in size from 2500 hp to 9000 hp, all 13.8kv voltage. We use partial discharge bus coupler capacitors supplied by a different vendor. For those just getting familiar with the technology, my opinion is that it's not worth it to dwell on the differences among partial discharge monitoring systems from different vendors at this point, but to focus on the strengths and weaknesses of partial discharge monitoring technology as a whole (others are welcome to address differences as they see fit).
We have had the monitoring equipment installed for 8 - 10 years on these machines. We test between quarterly and semi-annually while the motor is running. We trend the 10pps cutoff level "Qm" and evaluate the phase pattern in this trends up The test is an on-line test and we find it very useful.
We found one item by partial discharge which we elected not to correct and it progressed to failure within two months (we identified the failure but plant was not prepared to remove the motor); we found one item and inspected and found severely oil-degraded endwinding jumpers; and we had one motor fail to ground that never showed any PD symptoms prior to the failure. I guess you could call the 2 out of 3 caught by partial discharge technology. Also we have been monitoring for a few years two motors that have tracking two that have surface partial discharge on the interface between semi-con and grading treatments. This seems to be a fairly benign partial discharge and we gain confidence from the fact that we can see changes in behavior.
We don't use on-line partial discharge on our 4kv motors as it is generally believed that detectable partial discharge only occurs right before failure at this voltage level. There is apparently much less warning time because at this lower voltage level, partial discharge does not occur until the insulation is almost totally breached. Therefore to be useful, you would probably need continuous monitoring at this voltage level. And you would have to be prepared to respond on a moments notice to these indications. In contrast, pd on 13.2 KV machines can be trended and sometimes be effectively be used for long-term motor refurbishment/ rewind planning and prioritzation on the scale of years, depending on the severity of the symptoms.