Impact of High Voltage Waveform on Partial Discharge Inception in Electrical Machines

Fast-switching inverters increase the stress on the insulation system of electrical machines in an automotive power-train due to high dv/dt. This can cause partial discharges (PDs) within minor insulation faults of the stators, leading to premature breakdowns. Therefore, it is crucial to consider the effect of pulsed supply characteristics on PD inception when designing an adequate insulation system. The scope of this paper is to investigate the impact of rise time, overshoot, and oscillation frequency of the applied voltage on the repetitive partial discharge inception voltage (RPDIV), as defined in IEC/TS 61934. Considering simple insulation models, such as hairpin wire pairs and statorettes, an overshoot or oscillation of the high-voltage (HV) pulse artificially increases the RPDIV without being essential for PD inception. Aperiodic HV pulses with rise times between 0.2 mu s and 2 mu s lead to similar RPDIV values. Applying surges to a set of hairpin stators with different rise times ranging from 0.2 to 0.55 mu s shows either no significant effect on RPDIV or a reduced inception level for the shorter rise time, depending on the test object. Since the voltage distribution within the stator is more critical for shorter rise times due to higher-order harmonics, PD inception is more likely. However, the latter also depends on the location of possible faults. Therefore, an overall trend for all stators and phase combinations is absent. Similarly, when comparing different surge capacitances, deviations in the higher-order harmonics may affect the voltage stress within the stators, leading to individual RPDIV differences.