Analysis of the Composite Dielectric Characteristics of Nitrogen for a Superconducting Coil System

Abstract

A high-temperature superconducting (HTS) coil system often operates in an environment cooled with liquid nitrogen (LN2). However, it could be surrounded in part by gaseous nitrogen (GN2) as a result of film boiling due to thermal quenching. This may negatively affect the dielectric reliability in a composite dielectric system relative to that of a liquid dielectric system because the intensity of the electric field increases at the gaseous dielectric layer. The increase in the electric field intensity with the gaseous dielectric layer may degrade the dielectric reliability of the HTS coil system. This study investigates the dielectric characteristics of a composite insulation system with GN2 and LN2 to design an HTS coil system with high reliability in a quench state. The dielectric experiments use a sphere-to-plane electrode system made of copper, and three types of voltages (AC, DC, and lightning impulse) are applied to the sphere electrode with a grounded plane electrode. The electrical breakdown voltage of the composite dielectric system is measured and analyzed with respect to the height of the gaseous nitrogen layer. Finally, the dielectric characteristics of the composite dielectric system corresponding to a given height of the gaseous nitrogen layer are compared with those of GN2 and LN2 dielectric systems.