ADVANCES IN INTERPRETING PARTIAL DISCHARGE TEST RESULTS FROM MOTOR AND GENERATOR STATOR WINDINGS

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. Thu