Identification of surface discharge based on discharge current waveform in composite insulation system of LN2 and solid insulator

Composite insulation system of liquid nitrogen (LN₂) and solid insulator, which are widely employed in high-temperature superconducting power equipments, can be deteriorated by partial discharge (PD) generated in highly stressed region such as a triple junction. To investigate the PD mechanism, an individual current waveform of PD generated around the triple junction in LN₂ or gaseous nitrogen (GN₂) was measured. The longer half bandwidth and the shorter half bandwidth on the PD current waveform were typically observed in LN₂, After passing low-pass filter, the PD pattern in LN₂ was clearly separated into two clusters. The half bandwidth, being longer in the two clusters, also agreed with that in GN₂ at 77 K. Therefore, it is suggested that the current waveform measurement with the measurement of the PD pattern brings the possibility to ascertain presence of the bubbles in LN₂. And a frequency analysis of PD current waveform also leads to the appropriate separation between the PD in LN₂ and that in bubbles.