PD properties when varying the smoothness of synthesized waveforms

The increased use of power electronic components in power systems makes it important to understand how rapidly rising voltages affect insulation systems. One vital aspect of this challenge is the measurement of partial discharges, PDs, which are considered as being a sign of weakness and can affect the life of insulation considerably. In this paper an approach is presented to measure PDs in a dielectrically insulated cavity when exposed to pulse width modulated (PWM) voltage shapes with different degree of smoothness. This is a continuation of our earlier investigations on the different behavior of PDs where voltages characterized by different rise times were applied. The present investigation shows that the PD amplitude decreases significantly already at a moderate level of PWM voltage smoothness to a magnitude that is about the same as for sinusoidal voltage shape. For the phase resolved PD (PRPD) pattern to become similar to the normal AC pattern it is required that the remains from PWM steps are lower than the extinction voltage. This work elucidates how PDs are affected by synthesized waveforms and limits for a sufficient smoothing level are found, which is of importance when designing insulation systems exposed to fast transients.