Effect of Various $\mathrm{C}_{4} \mathrm{F}_{7} \mathrm{N} / \text{CO}_{2}$ Mixtures on Surface Charge Accumulation on Basin Insulator with Metal Particle Under AC Voltage

C<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf>F<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</inf>N/CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> mixture is believed to be a promising and environmentally sustainable substitute to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{SF}_{6}$</tex> in gasinsulated switchgear (GIS). Surface charge accumulation, induced by metal particles in GIS, is a crucial trigger for unexpected flashover failures. In <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{C}_{4} \mathrm{F}_{7} \mathrm{N} / \text{CO}_{2}$</tex> mixtures under AC voltage, charge behavior is intricate and lacks investigation. This study investigated the behavior of surface charges on a down-scaled basin-shape insulator equipped with a metal particle, under different pressures and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{C}_{4} \mathrm{F}_{7} \mathrm{N} / \text{CO}_{2}$</tex> gas blends. Following the application of a 40 kV AC voltage for a duration of 5 minutes, measurements of surface potential were conducted, and the distribution of charges was obtained. The findings showed that without the presence of metal particles, few charges were accumulated. When a metal particle attached to the central conductor on the insulator's surface, negative charge speckles were observed on matching areas of both surfaces near the particle. At 0.1 MPa, the charge accumulation reduced as the ratio of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{C}_{4} \mathrm{F}_{7} \mathrm{N}$</tex> increased. Under the same ratio, the charge accumulation increased with the increase in gas pressure. This study offers valuable information for selecting gases in environmentally friendly GIS.