Partial discharge (PD) testing is an important tool for assessing the condition of the high voltage insulation in motor and generator stator windings. In the past several years, many motors have been powered from inverters which facilitate variable speed motor operation. The most common type of drive being used nowadays is the voltage source, pulse width modulation (VS-PWM) type. Drives rated up to 13.8 kV are becoming more common in natural gas compressor plants, as well as other petrochemical facilities. Such drives generate high voltage spikes in the kV range with rise times in the sub-microsecond range. These high voltage spikes are a form of severe electrical interference that can make the detection of partial discharge difficult, due to the overlapping frequency content in PD and in the spikes that are 1000 times higher in magnitude. Thus, PD detection on medium voltage VS-PWM systems has been a challenge, in spite of the serious stator winding insulation ageing that the impulse output such drives may cause to their driven motors.
This paper discusses the stator winding failure mechanisms which produce PD, particularly the insulation problems that VS-PWM drives can accelerate. A prototype on-line PD monitoring system suitable for motors fed by VS-PWM drives was developed to address the difficulties associated with making reliable PD measurements on such drives. Results from the application of this system on a number of such motors are presented and discussed.