Polarisation Index and isolation on a generator 1

[mottt]

this morning I test a generator 6 kV, 3,3 Mw, this generator have been cleaned and varnished, but the values is no so good on my opinion....
Phase to earth---- with 1000 V 10 Gigaohm
Phases to earth ---with 2500 V 9 Gigaohms
Phases to earth--- with 5000 V 3 Gigaohms
Phase to phase---- with 1000 V 30 Gigaohms
Phase to phase---- with 2500 V 25 Gigaohms
Phase to phase---- with 5000 V 10 Gigaohms

The measures on the diferents phases are very close between them...
Then I test polarisation index, measuring isolation on 1 minute and isolation after 10 minutes, and them divide the ten minute value between the one minute value....
phase to earth was with 5000 V, 3 gigaohms at 1 minute and 4 gigaohms at ten, and the polarisation index was:1,33
this is a value too low, i had read that a good isolation on a winding is IP 2... or higher....
the same for the phase to phase isolation....
I hope someone of you, will know more about this concern than me....
thank in advance

 

[Yuma]

Your IR values are good, and that is usually considered enough to start up the machine. However, the low IP values show that your generator has some moisture or contamination problem.

There are some other tests that would help you to tell whether it is moisture or contamination: capacitance ratio, absorption index and leakage current ratio.

Just in case it is a moisture problem, it wouldn't hurt to apply some heat as other posters have suggested, eg, short-circuit... or maybe once the generator is reinstalled, turn on its heating resistors for some hours before starting up.

The other possibility would be contamination. In your case one could suspect it could be an oil problem, but normally oil gives capacitance ratio > 10 and _good_IP_values_, so I think oil is not your problem.

One point about varnishing: one has to be really sure that the windings are clean before varnishing. You say you are, and I believe you. But depending on what cleaning liquids you use, some of them tend to wash out dirt and move it somewhere deeper in the windings. If the windings insulation is cracked, dirt may get into the cracks and if varnish is then applied, dirt will stay there forever.