Evaluation of the Reliability of Insulating Gases for the Bushing in Cryogenic Environment

Conventional high-voltage bushings, which adopt SF
₆
gas as an insulation media, cannot be directly applied in cryogenic environment due to the liquefaction of SF
₆
gas. In our previous study, it was shown that SF
₆
gas could not be used as an insulation gas in cryogenic environment, but instead, CF
₄
gas has shown excellent insulation properties with little liquefaction phenomena, and have a possibility to be applied for the insulation gas for high-voltage bushings in cryogenic environment. However, in order to apply CF
₄
gas for cryogenic bushing, it should be determined whether CF
₄
gas can maintain its insulation characteristics in long-term operation. In order to reveal the long-term insulation properties of various gases, which have a possibility to be applied in high-voltage bushings in cryogenic environment, the long-term breakdown characteristics of CF
₄
gas, SF
₆
gas, N
₂
gas, and dry-air were compared over time in a cryogenic environment. For an experiment, a high-vacuum cryostat and a cryo-cooler were prepared to prevent the evaporation of liquid nitrogen for long-term duration. An fiberglass reinforced plastics bushing was installed on the cryostat to apply high voltage up to 60 kV. A rod to plane electrode was set in a cryostat to determine the breakdown voltage of insulating gases. The lightning impulse and ac overvoltage tests have been performed and the test results were analyzed. From the test, the reduction of breakdown voltage of CF
₄
gas were observed as time passed by, and it was more severe in SF
₆
. However, stable breakdown characteristic have been shown with N
₂
gas. Therefore, increasing the pressure of N
₂
would be a viable option for increasing the breakdown voltage in a cryogenic environment.