Abstract: On-line partial discharge (PD) testing is widely used as a means to assess the condition of the stator winding insulation system in operating motors and generators. Over the past 15 years over 60,000 test results have been collected on thousands of motors and generators with one test method. This data has been statistically analyzed and in over 200 machines, correlated with the known condition of the high voltage insulation as determined by visual inspection of the insulation. As a result, it is now possible to objectively determine if an insulation problem exists by a single on-line measurement. The paper briefly reviews the test method and the PD levels associated with
significant insulation deterioration. The statistical analysis has also produced some surprising results. It seems there is little difference in the PD level distribution between old asphaltic mica insulation, and modern
epoxy mica insulation. Of even greater interest, it seems that stator windings made in the past 10 years by 4 of the world’s largest 9 manufacturers have higher PD than those made before 1995. This may be due to the higher design stresses used in modern windings.
