Study on Power Frequency Breakdown Characteristics of Low-Content C₄F₇N Gas Mixtures Under Different Electric Field Uniformities

Sulfur hexafluoride (SF₆), widely used in high-voltage switchgear, possesses a strong greenhouse effect, prompting an urgent need for environmentally friendly alternatives. Among them, C₄F₇N/CO₂ gas mixtures are considered highly promising. As environmentally friendly power equipment is developed for higher voltage levels, the insulation performance of gas mixtures with low C₄F₇N content requires further investigation. This study explores the power frequency breakdown characteristics of low-content C₄F₇N gas mixtures under different electric field uniformities. The results show that the power frequency breakdown voltage exhibits a strong dependence on the electric field distribution. Under highly inhomogeneous fields, the gas mixtures display a nonlinear saturation effect in breakdown voltage, with the pressure-dependent increase reduced by 30%–50% compared to that under slightly inhomogeneous conditions. In slightly inhomogeneous fields, adding 4%–8% O₂ improves insulation performance by up to 9.35%. However, under highly inhomogeneous conditions, increasing the O₂ concentration to 12% is required to enhance the dielectric strength effectively.