Electrical Insulation Characteristics of LN2/CF4 Mixture at Cryogenic Temperatures

High-temperature superconducting (HTS) power equipment and tests at present require operating temperatures higher than the liquid nitrogen (LN
₂
). LN
₂
is an important cryogenic refrigerant and liquid insulation material, which is widely used in HTS equipment. Although pressurized LN
₂
can achieve temperatures higher than 77 K, it requires the use of sealed pressure vessels. Thus, a liquid cryogen that can offer temperatures higher than LN
₂
at atmospheric pressure, excellent electrical insulation performance, and high thermal conductivity has to be explored. Tetrafluoromethane (CF
₄
) is another cryogenic refrigerant and electrical insulation material. The mixture of LN
₂
and liquid CF
₄
has excellent thermal properties. When mixed in appropriate proportions, it has a melting point of 50 K and a boiling point of 145 K. The LN
₂
/CF
₄
mixture 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 LN
₂
/CF
₄
mixture 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 LN
₂
/CF
₄
liquid 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 LN
₂
/CF
₄
mixture is lower than that of pure LN
₂
. As the proportion of CF
₄
increases, the breakdown field strength of liquid mixture first decreases and then increases. When the mole fraction of CF
₄
is 60%, the average breakdown strength decreases to the lowest value (14.07 kV/mm), about half of that of pure LN
₂
(28.26 kV/mm). The results of this article can provide a reference for the applications of LN
₂
/CF
₄
liquid mixture in HTS power equipment and a scheme to expand the working temperature range of superconducting applications.