The Implications on the PD Characteristics of Unipolar versus Bipolar PWM waveforms

Speed drives fed by pulse width modulated (PWM) inverters allows a more efficient use of electric energy. The drawback is however that the stress imposed on motor winding insulation increases due to a higher frequency harmonic content compared to the classical 50 Hz sinusoidal voltages. High frequency contributions have been observed in high voltage DC (HVDC) applications as well. In particular, partial discharges (PDs) considered the major contributor to the reduction in the insulation life time. It has been shown that the usage of electrical filters decreases electrical stress imposed on the insulation. Another option is to use multi level inverters, which have been shown to reduce magnitude of the PDs. It however remains to explain how the size of these voltage steps as well as polarity influences the level of PDs. Here twisted pair test objects are exposed to PWM inverter waveform at various rise times. Both bipolar and unipolar waveforms as well as different step sizes and rise times of each voltage flank are employed to present a gradual change between bipolar PWM to a more constant DC voltage magnitude with superimposed voltage ripple. To compare the performances, measurements of both PD inception and extinction voltages were conducted for the different cases. Additionally, the total number of PDs, their average maximum amplitude per cycle and pulse repetitive PD pattern (PRPD) are presented. The decreased step size to resemble more HVDC waveform and the resulting change in PD characteristics is discussed. Important factors are the time until PDs are observed (time lag) together with observed polarity dependences.