Assessment of Partial Discharge Phenomenon in AC and Fast-rising Square Voltage Using Optical Detection Technique in Needle-Plane Configuration With Pressboard in Mineral Oil

The benefits offered by silicon carbide (SiC) switches in terms of repetition rate (> 100 kHz) and switching speed (up to 100 kV/µs) make them a popular choice for the converters of future grid projects. However, the impact of the fast repetitive pulses generated by these devices on the insulator discharge mechanism is yet unknown. This could be a significant obstacle in the future development of power system applications. This paper investigates the partial discharge (PD) phenomenon for pressboard/mineral oil insulation system under unipolar square voltage (USV) waveforms (dV/dt > 50 kV/µs), using a photomultiplier tube (PMT), with a high divergent field. As compared to AC, the partial discharge inception voltage (PDIV) is found to be lower under USV for tested specimens. The phase resolved partial discharge (PRPD) pattern highlights the difference in PD mechanism between AC and USV waveforms. Also, the number of PDs (NPDs) and average charge amplitude (Qavg) obtained under AC and USV waveforms are compared and discussed.