Composite insulation system of liquid nitrogen (LN_2) and solid insulator, which are widely employed in high-temperature super-conducting 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 LN_2 or gaseous nitrogen (GN_2) was measured. The longer half bandwidth and the shorter half bandwidth on the PD current waveform were typically observed in LN_2. After passing low-pass filter, the PD pattern in LN_2 was clearly separated into two clusters. The half bandwidth, being longer in the two clusters, also agreed with that in GN_2 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 LN_2. And a frequency analysis of PD current waveform also leads to the appropriate separation between the PD in LN_2 and that in bubbles.
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