Monitoring partial discharges on 4-kV motor windings

Partial discharges are a symptom of most of the stator winding insulation deterioration mechanisms found in motors and generators rated 6 kV or more. Thus, partial discharge testing has been used for over 40 years as a means of determining the maintenance requirements of such high-voltage stators. As the benefits of such testing have become more widely known, there has been an interest to apply partial discharge testing to 4 kV motor and generator windings. This paper discusses the theoretical reasons which may limit testing of 4 kV stators. The results of an extensive series of offline partial discharge tests are presented which indicate that partial discharges indeed do occur. The results of a series of online partial discharge tests conducted on about ten 4 kV motors over the past two years show that partial discharges may only occur for a short time prior to failure with some types of failure mechanisms. Thus, to prevent in-service failures, continuous monitoring is necessary. An inexpensive continuous partial discharge monitor is described. It seems that partial discharge monitoring will only be cost effective for very critical process motors and generators where in-service failures may have large financial consequences.