Diagnostic information obtained from examining a large stator winding PD result database

The online partial discharge testing of stator windings has become a routine monitoring method in the majority of North American utility generators. In the past 20 years, over 30,000 test results were analyzed. The data were collected using a system that explicitly separated PD from electrical interference. Analysis of the database shows that various motor and generator operating conditions, such as voltage, load, winding temperature and atmospheric humidity were found to have a strong effect on the results. Standard statistical analysis was also conducted to calculate the normal range of PD magnitudes under a variety of sorting criteria. The following were found to have a significant effect on the expected range of the PD: voltage class of the stator winding; cooling medium (atmospheric air vs. high pressure hydrogen); and the PD detection system. The following were found to have a minimal influence on the expected range of PD, at least within the standard deviation of the results: age of winding and type of insulation system (for example asphaltic mica splittings vs. epoxy mica paper). With knowledge of this information, and the tables published in this paper, it is possible to make an informed opinion on the state of the stator winding insulation system, without waiting for a trend to be established