Measuring partial discharge on operating motors with VS-PWM drives

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.