In-service evaluation of motor and generator stator windings using partial discharge tests

The authors describe a type of partial discharge test which can be performed by plant personnel during normal operation of the machine, thus enabling all users of large machines to plant maintenance. The key requirement of the test is to remove all the interference caused by other sparking and discharging sources in a plant. Such noise has led to false indications of stator winding deterioration in the past. The test uses either high-voltage capacitive sensors or 1000 MHz electromagnetic couplers to detect the discharge signals. The signals are processed by a specialized electronic instrument to remove noise. The remaining partial discharges are then categorized according to number, magnitude, and phase position. The test has been implemented on over 100 machines, and the results appear to be promising