Partial discharge error? 1

[TurbineGen]

We are having issues with high PD levels in one of our generators. The generator was just rewound last year. It is a 13.5 kV machine. The machine has been tested while in service twice and both times the readings were nearly the same.

We have crawled through the unit several times and find no evidence of excessive PD. In crawling through, we noticed the coaxial cable from one of the couplers is in contact with the bus ring. We are guessing that the coaxial cable from the coupler which is in contact with the bus ring could be giving a false reading. Could that be causing a false reading? If so, what clearance should there be between the cable and the bus rings? Any other thoughts/suggestions on what we should be looking for?

 

[benlanz]

I would recommend an off-line test so that you can test each electrical component separately. This may not always work for rotating machinery because some PD activity is only active during specific loading conditions. However, our experience with cable diagnostics shows that it is necessary to prove the origin of a PD signal in order to provide reliable results.

Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants

 

[electricpete]

One thing to note. It is very common for a 13.2kv air-cooled machine (don't know if yours is aircooled) which is rewound with VPI system to come back with high partial discharge (maybe 600 millivolts) which decreases over the first year. This is apparently due to incomplete curing of the VPI resins.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[JBinCA]

Good info Electricpete.

So I guess this is one case where we can put those pesk I^2R losses to good use!