Analysis on the dielectric characteristics of a composite insulation system composed of LN2 and GN(2)

A liquid nitrogen (LN2) is usually used to be a coolant and insulant for a HTS coil system. HTS wires for a superconducting apparatus may be surrounded by gaseous nitrogen (GN(2)) due to film boiling generated by a quench or voids occurred by electrical breakdown. The increased maximum electric field intensity at GN(2) may result in the degradation of dielectric strength of a HTS coil system. In this paper, a study on the dielectric characteristics of a composite insulation system composed of LN2 and GN(2) is performed. A sphere-to-plane electrode system made with stainless steel is used to perform the experiments under AC and lightning impulse voltage condition. A sphere electrode is surrounded by GN(2) and a plane electrode is immersed into LN2 to conduct dielectric experiments with a composite insulation system. The dielectric experiments are performed according to the level of LN2 from the plane electrode to a sphere electrode. It is found that the dielectric characteristics of a composite insulation system are dependent on the level of LN2 and the field utilization factor of an electrode system.