Yusuke Kikuchi

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.

This study demonstrates a high-slew-rate 5-kV pulse generator for electrical insulation tests. Electrical equipment, such as electrical actuators and traction drive motors, are exposed to severe electrical stress because recent switching inverters have high-frequency outputs with high supply voltages using wide-bandgap power devices. For an advanced electrical insulation test, a high-voltage pulse generator is required with a high slew rate; however, such generators suffer from large switching noise, followed by measurement noise, such as ground voltage fluctuations and radiation noise, hindering the detection of partial discharge (PD) phenomena. In this study, we propose a 5-kV pulse generator based on series-connected 1700-V silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs). Four 1700-V SiC MOSFETs are connected in series as a 5-kV SiC switching module, constituting a half-bridge configuration for the pulse generator. The obtained switching waveforms exhibit fast rise times of 48 ns under 5 kV and 6.2 ns under 400 V with a low voltage overshoot and ringing owing to superior device characteristics and reduced parasitic inductances. Because of the low switching noise, we detect a clear PD signal with a 1500-V pulse when using the fabricated pulse generator for a PD test of a twisted pair. The proposed pulse generator uses a hard switching configuration such that the pulse generator can vary the pulse width from 150 ns to DC and increase the switching pulse cycle beyond 1 MHz by changing the control signals of the SiC MOSFETs.