The power factor test is more comprehensive if I had to limit myself to just one, but why would we do that?
Power factor test set = $$$
Insulation resistance test set = $
Hiring a competent testing contractor to show up and do the tests usually means you get BOTH tests anyway. They are part of a pretty standard battery of tests on oil-filled transformers above a certain size.
I am familiar with insulation resistance testing (meggar test) when commissioning a transformer, but I've never heard of an insulation power factor test. Can someone please give me a brief explaination?
It also called dissipation factor and test the real component of the leakage current through the insulation. Ideally, when insulation is in a good shape, the leakage current is purely capacitive. However, e.g. when water presents the leakage current will have resistive component.
I agree with PF/DF and tan delta being good tests, megger wont tell you much on a transformer like this. There have also been some big strides in online Partial Discharge testing of oil filled transformers.
