Advancements in interpreting partial discharge test results to assess stator winding condition

On-line partial discharge (PD) testing has been used for decades to help maintenance personnel detect motor stator winding insulation problems. Specifically, the test can find loose, overheated, and contaminated windings, well before these problems lead to failure. As a result, on-line PID testing has become an important tool for planning motor maintenance. The test has also found use in determining the effectiveness of any repair work. The widespread application of on-line PID testing has enabled the collection over the past ten years of a database with over 43,000 test results. This paper presents the statistical analysis of the PID results in this database and shows that voltage class and types of partial discharge sensor all are critical to interpreting test results, whereas insulation type and age of the winding are less important. Critical values of PID, which indicate significant risk of severe winding problems, are presented for motors. Thus comparison of test results to the database allows a rough assessment of the stator winding insulation condition, without having to wait a few years for a trend to establish itself.