Partial discharge inception characteristics under butt gap condition in liquid nitrogen/ PPLP® composite insulation system for high temperature superconducting cable

The partial discharge (PD) inception characteristics are studied in liquid nitrogen (LN₂)/polypropylene laminated paper (PPLP^®) composite insulation system for high temperature superconducting (HTS) cable. Experimental results revealed that the magnitude of the initial PD increased as the PD inception electric field strength was increased, because the injected energy increased. Initial PD was generated at the first and third quadrant of applied AC voltage phase. The probability of initial PD at the positive and negative voltage phase was almost the same. The reason is because liquid nitrogen is a nonpolar molecule and we used symmetric electrode configuration with uniform electric field distribution. Finally, it was pointed out that PD inception electric field strength (PDIE) depended on the volume of the butt gap because of the increasing probability of weak points of electrical insulation, and PDIE linearly decreased with increasing stressed volume of the butt gap in the log-log scale.