How useful are diagnostic tests on rotating machine insulation?

A critical review of commonly used diagnostic tests for the stator winding insulation of rotating machines is made from the perspective of one largely hydro utility. Relatively low voltage measurements of insulation resistance and polarization index are shown to be important indicators of winding dryness for all thermoplastic insulation systems as well as for those polyester systems which are prone to tape separation. The DC ramp test has proven to be very useful for nondestructively detecting cracks and fissures in both thermoplastic and polyester insulation systems. The TVA probe test measures both the internal PD (partial discharge) and the slot discharge associated with individual connected coils/bars. These data frequently correlate with online PD tests and/or other tests and inspections. The CEA PDA (PD analyzer) with permanent couplers has proven to be an important addition to available diagnostic test methods. Frequent monitoring allows PD-related problems to be detected before much damage has occurred, while extending the period between outages for other more costly tests and inspections