Abstract
Composite insulation system of liquid nitrogen and solid insulator, which are widely employed in high Tc superconducting equipments, can be deteriorated by partial discharge (PD) generated in highly stressed region such as a triple junction. In this study, effect of thermal bubbles, which can be generated by AC loss of the superconductor, on PD characteristics were investigated using composite insulation system. Thermal bubbles had little effects on PD characteristics because they were rejected from the high field region by gradient force. Under atmospheric pressure condition, small bubbles were generated as a result of local vaporization by PD energy injection. However, those bubbles disappeared in sub-cooled liquid nitrogen and the number of PD pulses drastically decreased. A theoretical model was proposed to estimate temperature rise due to the PD energy injection into liquid nitrogen.
