Tan Delta Test Of 250MW Generator

One route to the answer is to make a telephone call to the technical helpline of the generator's manufacturer. It would certainly be faster than waiting for someone on CR4 who can be bothered to type in the correct response, and safer than risking mis-evaluating CR4 responses.

I have seen similar results when doing Doble Insulation Power Factor tests (essentially the same as tan delta) on generators. If I remember correctly, the sign is not significant, and you evaluate based on the magnitudes of the test results. Do you or your client have past test results for this generator for comparison? Usually this sort of thing is specific to a particular unit, so you will see it happen repeatedly.

You might also query the tech support people at the manufacturer of your tan delta tester. They should have some experience interpreting its output and be able to tell you what causes the polarity reversal.

Don't base your evaluation of the generator on what I say, but I believe if the results (based on magnitudes) are acceptable, the stator insulation should be good. Are the tip-up values good? Did you also take radio interference voltage (RIV) measurements while testing, and if so, are they acceptable (< 1000 μV at highest test voltage)? Is there any significant change from previous tests? These are the questions you have to ask.