Experimental study on the dielectric characteristics of liquid nitrogen with respect to various pressures

The electrical breakdown characteristics of liquid nitrogen (LN ₂) are required to design of high voltage superconducting machines which should have high reliability. This paper deals with the investigation to verify the relationships between the electrical breakdown characteristics of LN₂ its pressure condition. The electrical breakdown characteristics of LN₂ were measured by using sphere-plane electrode systems. The diameter of sphere electrode had five different values. The gaseous nitrogen was injected into a cryostat which contained the sphere-plane electrode system to control the pressure of LN₂ from 50 to 200 kPa. The AC dielectric tests in LN₂ were repeated 10 times in each pressure condition. The AC breakdown voltage at sparkover with 50% probability was calculated by commercialized statics tools. From finite element method simulation results, the field utilization factor which means the field uniformity was calculated between sphere and plane electrode and its relations with AC breakdown voltage with 50% probability were analyzed. Finally, these results provided a suitable design parameter of electrical insulation for high voltage superconducting machines such as superconducting fault current limiters, superconducting cables, and superconducting transformers.