Breakdown Characteristics of Liquefied SF₆ and CF₄ Gases in Liquid Nitrogen for High Voltage Bushings in a Cryogenic Environment

High voltage cryogenic insulation issues need to be addressed in order to promote the commercialization of high temperature superconducting (HTS) equipment. One of the critical components for superconducting devices is the bushing whose role is to safely supply high current to the device. Due to a steep temperature gradient, commercial bushings which have been insulated with SF6gas could not be directly applied to cryogenic equipment due to liquefaction of SF6in the cryogenic environment; therefore, alternative suitable structure and insulation methods should be developed. As a fundamental step in the development of the optimum bushings for HTS devices, the breakdown characteristics of liquid nitrogen mixed with liquefied insulating gases such as N2, SF6and CF4have been investigated. In particular, we noted the insulation characteristics of CF4gas whose liquefication temperature is much higher than SF6gas. Thus, in order to investigate the possibility of substituting CF4gas for SF6gas for the bushings of HTS electrical equipment, impulse tests, AC withstanding voltage tests, and partial discharge (PD) tests have been performed. As a result of these tests, it was shown that mixtures of liquefied insulating gases have a much higher breakdown voltage compared to pure liquid nitrogen. Especially in a cryogenic environment, the usage of SF6gas should be evaluated due to freezing effects. On the other hand, CF4gas has shown excellent insulation properties even in a cryogenic environment and could be utilized as an insulation gas for high voltage bushings of HTS electrical equipment.