*Iris Power Engineering, 2-1 Westside Drive, Toronto, ON, Canada M9C 1B2, gstone@irispower.com ADVANCES IN INTERPRETING PARTIAL DISCHARGE TEST RESULTS FROM MOTOR AND GENERATOR STATOR WINDINGS G.C. STONE* V. WARREN H.G. SEDDING W. McDERMID Iris Power Engineering Kinectrics Manitoba Hydro (Canada) On-line partial discharge (PD) testing has been used for decades to help maintenance personnel detect stator winding insulation problems in motors and generators. Specifically, the test can often find loose, delaminated, overheated, and contaminated windings, well before these problems lead to failure. As a result, on-line PD testing has become an important tool for planning machine maintenance. The test has also found use in determining the effectiveness of any manufacturing or repair work. With the advent of electrical noise separation technology developed in the 1970s and 1980s, reliable on-line PD testing and basic interpretation was made possible by plant engineering staff with moderate training. The result has been the widespread application of the so-called PDA and TGA tests on machines throughout the world. Over 50% of utility generators rated over 20 MW in the USA and Canada now use this technology. This widespread application has enabled the collection over the past ten years of a database with nearly 30,000 test results. The results of a statistical analysis of the PD results in this database shows that voltage class, machine cooling method, and type of partial discharge detector all are critical to interpreting test results, whereas machine power rating, insulation type and age of the winding are less important. Critical values of PD, which indicate significant risk of severe winding problems, are presented for different types of air-cooled and hydrogen-cooled machines. Thus Key Words: partial discharge, electrical insulation, stator winding, condition assessment 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. However, the best assessment of the winding condition is still obtained from the trend in PD on a single machine over time. A doubling of the PD magnitudes every 6 months, under similar operating conditions is a consequence of a rapid rate of insulation deterioration. This paper shows that the PD is affected by machine operating conditions. Thus to obtain valid trends in PD data, the testing should be performed under similar operating and environmental condition 1. INTRODUCTION Partial discharges are both a cause and a symptom of many types of stator winding insulation system deterioration mechanisms in motors and generators rated 3300 V and above. Thus it is not surprising that the on-line measurement of partial discharges has been used to assess the need for maintenance in stator windings since 1951 [1]. More particularly, on-line PD measurement has been able to determine if the electrical insulation is deteriorating due to loose coils in the slots resulting in insulation abrasion; thermal deterioration or load cycling leading to insulation delamination; and electrical tracking caused by partly conductive contamination of the endwindings [1-7]. On-line PD testing is also able to determine if manufacturing or installation problems such as poor impregnation with epoxy, or coils being too close together in the endwinding, are severe enough to shorten the winding life. 21, rue d'Artois, F-75008 Paris http://www.cigre.org Session 2002 © CIGRÉ 11-202