Phase resolved analysis of partial discharges in liquid nitrogen under AC voltages

Liquid Nitrogen (LN2) has been used as a coolant as well as a part of the insulation system in High Temperature Superconducting power apparatus. In recent decades, there has been scientific research into the pre-breakdown phenomena in this liquid. The focus of previous studies is on streamer behavior and their governing mechanisms. However, research into the characterization of partial discharge patterns in a composite insulation of liquid nitrogen and solid dielectrics has not been widely reported. This paper presents a characterization of partial discharges in LN2 under AC voltages by the means of phase resolved analysis. The results suggest that discharges in a LN2/solid insulation system occur primarily in the first and third quadrant of the applied voltage waveform due to charge storage effects. Partial Discharge (PD) pulses were also observed before the zero crossing of the waveform under higher electric stress. The conditions under which density change streamers cross a LN2 gap and reach the solid insulation barrier are also discussed. Different PD patterns were observed for different solid dielectric barriers indicating the effects of the solid/liquid interface on the behavior of discharges in point-plane composite systems at cryogenic temperature.