Analysis on the dielectric characteristics of various insulation gases for developing a sub-cooled liquid nitrogen cooling system

A sub-cooled liquid nitrogen cooling system is known as a most promising method to develop large scale superconducting apparatuses such as superconducting fault current limiters and superconducting cables [1]. Gaseous helium (GHe), gaseous nitrogen (GN2) and sulfur hexafluoride (SFe) are commonly used for designing an high voltage applied superconducting device as an injection gaseous medium [2, 3]. In this paper, the analysis on the dielectric characteristics of GHe, GN2 and SFe are conducted by designing and manufacturing sphere-to-plane electrode systems. The AC withstand voltage experiments on the various gaseous insulation media are carried out and the results are analyzed by using finite element method (FEM) considering field utilization factors (I). It is found that the electric field intensity at sparkover (Emax) of insulation media exponentially decreases according to I increases. Also, the empirical expressions of the functional relations between Emax and I of insulation media are deduced by dielectric experiments and computational analyses. It is expected that the electrical insulation design of applied superconducting devices could be performed by using the deduced empirical formulae without dielectric experiments.