AC HV test on old spare coils

[edison123]

We want to completely rewind the stator of a failed 60 MW generator stator with a original set of complete stator bars supplied with the generator 25 years back. Before rewinding with these old spare stator bars, we would like to do AC Hipot on all the bars. Can anyone give some reference standards which specify the actual test voltage levels for such old but unused stator bars ?

 

[electricpete]

Here are some thoughts. Not much helpful I'm afraid.

You are probably aware that new machines are tested to 2E+1 where E is rated line to line voltage. For in-service machines (service-aged insulation) this number is reduced in the neightborhood of 66%-75%.

If you want coils that would be as realiable as new then you should test at full voltage. If you are prepared to accept coils that are as good as a serviced-aged machine then you should test at 66%-75%.

You probably know all that. One other thing is to consider is that if these are VPI coils which have not yet been impregnated and cured then they will not yet have their full strength.

How much degradation in dielectric withstand can practically be expected? Good question. I would think if they have been stored in clean humidity-controlled environment there would be no degradation.

You probably should under any circumstance clean and dry those coils prior to hi-pot testing. Then check IR/PI before hi-pot. Also possibly there are some less destructive tests you might consider prior to the hi-pot. For example dc step voltage test while carefully watching leakage current MIGHT give you opportunity to stop the test before damage occurs. Also Doble power factor test is a good non-destructive test. Possibly corona probe and lights out tests.

 

[busbar]

e-pete sized up the situation well. For this application, use of a long-experienced repair guy, not under tight time constraints could be an excellent investment. ”Skilled Olde World Craftsmanship” would be a desirable attribute for the subject procedure.

 

[edison123]

Thanks electricpete and busbar for your time.

Isn't sad that none of the standards (IEEE, IEC, ANSI etc.) is of help in our case? I would rather have the coils fail before rewinding than in service, which could be disastrous. These standards specify that completely rewound machines should be tested at levels as applicable to new machines (2E+1). Only, they are silent about using old spare coils in the 'complete rewinding'.

 

[jbartos]

Suggestion: There might be a motor testing service that has performed such motor tests with specific experiences and data inhouse. If you do it by yourself without industry standards for these motor tests in existence, there is a chance for noticeable errors in terms of test variable limits, and test methods. It may be better to look around for a suitable motor repair/test firm first.