Experimental study on the lightning impulse dielectric strength of liquid nitrogen insulation medium according to various pressures

The lightning impulse breakdown characteristics of liquid nitrogen are necessarily considered for designing high voltage superconducting machines required high reliability. This paper investigates the dielectric capability of liquid nitrogen (LN₂) to withstand lightning impulse. To gain the dielectric capability of LN₂ cryogenic cooling system, lightning impulse was induced in sphere-plane electrode systems which have six different diameters of sphere electrode. The pressure is one of major condition to decide dielectric strength at LN₂ cryogenic cooling system. Thus, the gaseous nitrogen was injected to control pressure in the cryostat, ranging from 100 kPa to 200 kPa. Moreover, field utilization factor was calculated between sphere and plane electrode systems by a finite element method analysis. The electric field criterion of LN₂ as insulation media was calculated from correlation between the experimental results and the field utilization factor. In the future, these results can be applied as the design parameter of electrical insulation for developing high voltage superconducting machines.