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Breakdown Characteristics of Liquefied ${\rm SF}_{6}$ and ${\rm CF}_{4}$ Gases in Liquid Nitrogen for High Voltage Bushings in a Cryogenic Environment

IEEE Transactions on Applied Superconductivity (Impact Factor: 1.2). 01/2011; 21(3):1430-1433. DOI: 10.1109/TASC.2010.2090638

ABSTRACT Highvoltagecryogenicinsulationissuesneedtobead- dressed in order to promote the commercialization of high temper- ature superconducting (HTS) equipment. One of the critical com- ponents for superconducting devices is the bushing whose role is to safely supply high current to the device. Due to a steep tem- peraturegradient,commercialbushingswhichhavebeeninsulated with gas could not be directly applied to cryogenic equipment due to liquefaction of in the cryogenic environment; there- fore, alternative suitable structure and insulation methods should be developed. As a fundamental step in the development of the op- timum bushings for HTS devices, the breakdown characteristics of liquid nitrogen mixed with liquefied insulating gases such as , and have been investigated. In particular, we noted the insulation characteristics of gas whose liquefication tempera- ture is much higher than gas. Thus, in order to investigate the possibility of substituting gas for 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 insu- lating gases have a much higher breakdown voltage compared to pure liquid nitrogen. Especially in a cryogenic environment, the usage of gas should be evaluated due to freezing effects. On the other hand, gas has shown excellent insulation properties even in a cryogenic environment and could be utilized as an insu- lation gas for high voltage bushings of HTS electrical equipment.

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