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 SF₆ gas could not be directly applied to cryogenic equipment due to liquefaction of SF₆ in 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, SF₆ and CF₄ have been investigated. In particular, we noted the insulation characteristics of CF ₄ gas whose liquefication temperature is much higher than SF ₆ gas. Thus, in order to investigate the possibility of substituting CF₄ gas for SF₆ gas 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 SF₆ gas should be evaluated due to freezing effects. On the other hand, CF₄ gas 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.