Stator winding failures: contamination, surface discharge, tracking

Stator winding failures are generally attributed to groundwall or turn insulation failures. The failure mechanism involves the gradual development of a weakness in the insulation. This weakness is usually caused by a trapped void that is continuously enlarging due to ever-increasing corona or partial discharge activity. The failure mode is from the inside out. Recent studies of failures in actual machines, and of coils on voltage endurance testing, indicate that failures can occur from the outside inward. Surface contamination leads to intense surface discharge and tracking. This surface discharge can lead to rapid winding failure following the inward motion of the corona activity. This paper looks at tracking as one failure mechanism and describes a series of tests on standard insulation materials aimed at comparing the antitracking capability of the materials. The tests show that combinations of insulation materials can reduce the antitracking capacity of a rather robust insulation system and predispose it to failure.