Electrical Insulation Characteristics of LN₂/CF₄ Mixture at Cryogenic Temperatures

This is the authors' abstract. We don't add key points for this paper.

High-temperature superconducting (HTS) power equipment and tests at present require operating temperatures higher than the liquid nitrogen (LN2). LN2is an important cryogenic refrigerant and liquid insulation material, which is widely used in HTS equipment. Although pressurized LN2can achieve temperatures higher than 77 K, it requires the use of sealed pressure vessels. Thus, a liquid cryogen that can offer temperatures higher than LN2at atmospheric pressure, excellent electrical insulation performance, and high thermal conductivity has to be explored. Tetrafluoromethane (CF4) is another cryogenic refrigerant and electrical insulation material. The mixture of LN2and liquid CF4has excellent thermal properties. When mixed in appropriate proportions, it has a melting point of 50 K and a boiling point of 145 K. The LN2/CF4mixture is used in the insulating layer of the superconducting energy pipeline to provide a low-temperature environment (85-100 K) for the HTS dc cable. However, the insulation characteristics of the LN2/CF4mixture at cryogenic temperatures have not been studied, quantitatively. In this article, a test platform is developed to measure the breakdown voltage of the liquid mixture. The dc breakdown voltages of LN2/CF4liquid mixture under varying mixing proportions are obtained at cryogenic temperature. The insulation failure probability is estimated using a two-parameter Weibull statistical method. The results reveal that the breakdown strength of the LN2/CF4mixture is lower than that of pure LN2. As the proportion of CF4increases, the breakdown field strength of liquid mixture first decreases and then increases. When the mole fraction of CF4is 60%, the average breakdown strength decreases to the lowest value (14.07 kV/mm), about half of that of pure LN2(28.26 kV/mm). The results of this article can provide a reference for the applications of LN2/CF4liquid mixture in HTS power equipment and a scheme to expand the working temperature range of superconducting applications.