Repetitive Partial Discharge Phenomena on Electrical Motor Coil Windings Under High-Repetition Nanosecond Pulsed Voltages Driven by SiC MOSFET Inverter

This paper presents new findings on partial discharge (PD) phenomena under high-repetition nanosecond pulsed voltages, essential for the insulation design of electrical motors fed by silicon carbide (SiC) inverters with high-switching frequencies. Here, a twisted pair sample simulating the turn-to-turn insulation of a random-wound motor coil winding was subjected to high-repetition nanosecond pulsed voltages generated by a SiC inverter-based pulse generator. In this experiment, the rise time at a pulsed voltage of 1.5 kV is 23 ns, the pulse width is 500 ns, and the pulsed repetition frequency (PRF) ranges from 60 pps to 1 Mpps. In this PRF range, the partial discharge inception voltage (PDIV) was found to be almost constant. Meanwhile, the repetitive partial discharge inception voltage (RPDIV) significantly reduced when the PRF exceeded several tens of kpps. Under high PRF conditions, the high-density ions produced by the preceding PD remain in the gap space between enameled wires during the pulse-off period and contribute significantly to the initiation of subsequent PD. The resulting increase in the probability of PD occurrence reduces the RPDIV. Unlike previous reports, this study clarified the difference in the frequency characteristics between PDIV and RPDIV, providing valuable reference data for the insulation design of SiC inverter-fed motors. Based on the obtained results, we propose revising the IEC60034-18-41 standard, which verifies a PD-free system, to incorporate the effect of the pulsed voltage waveform of SiC inverters.