Experimental Study on the Lightning Impulse Dielectric Characteristics of Sub-Cooled Liquid Nitrogen for a High Voltage Superconducting Fault Current Limiter

Sub-cooled liquid nitrogen (LN₂) condition is a very useful method to increase the current rating capacity of YBCO tapes. Thus, the lightning impulse dielectric characteristics of sub-cooled LN₂ condition should be investigated for designing high voltage superconducting machines. The sphere-plane electrode systems were used to simulate the internal structure of high voltage superconducting machines such as superconducting fault current limiters. The lightning impulse dielectric tests on saturated and sub-cooled LN₂ condition were carried out by using the sphere-plane electrode systems. In addition, a quasi-uniform electric field distribution between the sphere-plane electrodes was calculated by the finite element method (FEM) simulation tool. The field utilization factor was calculated by the FEM simulation results. The lightning impulse breakdown voltage of various geometrical configurations between the sphere-plane electrodes in sub-cooled LN₂ conditions was compared with that of in saturated LN₂ condition. Finally, the lightning impulse breakdown voltage with 50% probability and its relations with the utilization factor have been described in this paper.