Calculation of electrical insulation of C ₄ F ₇ N/CO ₂ mixed gas by avalanche characteristics of pure gas

Abstract

Abstract C 4 F 7 N/CO 2 mixed gas is one of promising alternative gases for SF 6 in high-voltage equipment. The electrical insulation of C 4 F 7 N/CO 2 mixed gas is critical for design of the insulation system. The contradiction of liquefaction temperature and insulation strength requires a balanced mixing ratio and pressure. The synergistic effect is the key technique to optimize the mixing ratio of C 4 F 7 N/CO 2 from electrical insulation aspect. In this paper, the synergistic effect between C 4 F 7 N and CO 2 is studied and applied to calculate the electrical insulation and optimize the content of C 4 F 7 N in the mixture. It is found that the synergistic coefficient is related to avalanche parameters of pure gas. A graphic method based on Wieland approximation is proposed to calculate the critical reduced electric field of mixed gas. Based on the derived formula for calculation of the critical reduced electrical field of binary mixed gas, the theoretical optimal mixing ratio is given out.