Partial breakdown characteristics in the simulated electrode system of high temperature superconducting coils

In this study, partial breakdown phenomena in a composite insulation system of GFRP and liquid nitrogen are investigated to understand the breakdown mechanism and characteristics in high temperature superconducting (HTS) coils at quench. The electrode system consists of concentric coil-to-cylindrical electrode with an insulation barrier and spacers and is immersed in liquid nitrogen. A heater is mounted inside the coil electrode to generate boiling which occurs on quenched superconducting coils. The experimental results show the following: (1) at atmospheric pressure, partial breakdown (PBD) appears through gas bubble locked between the coil and concentric barrier by the effect of the electrical gradient force and spacer, (2) at higher pressure, complete breakdown (BD) between coil and cylindrical electrode can occur without the partial discharge, and (3) voltages for initiation of PBD and BD depend on the pressure. Pressurizing the liquid nitrogen is very effective to suppress the bubble formation and PBD inception in the liquid and gas nitrogen