Influence regularity of O2 on dielectric and decomposition properties of C4F7N–CO2–O2 gas mixture for medium-voltage equipment

Abstract

Fluorinated nitrile (C4F7N) gas mixture has been introduced as the most promising candidate to replace sulfur hexafluoride using in gas-insulated equipment. In this study, the authors explored the influence of oxygen on the dielectric and decomposition properties of C4F7N–CO2–O2 gas mixture. The authors found that the dielectric strength of the C4F7N–CO2–O2 gas mixture with 2, 4, 6, 8 and 10% O2 was increased by 4.85%, 6.49%, 7.70%, 3.21% and 2.74% compared with C4F7N–CO2. The addition of 2–6% O2 to the C4F7N–CO2 gas mixture could effectively reduce the content of most of the decomposition byproducts such as CF4, CO, C2F6, C3F6, C3F8, CF3CN, C2F5CN, (CN)2. While high content of oxygen (>6%) results in higher decomposition of C4F7N, which has a negative effect on the stability of C4F7N gas mixture. Generally, it is recommended to add 2–6% O2 in the C4F7N–CO2 gas mixture to improve its insulation properties as well as inhibit the decomposition of C4F7N in the discharge for medium-voltage engineering application.