Charge Accumulation on Epoxy Insulator Surface in C4F7N/CO2 Mixtures under AC Voltage

C
₄
F
₇
N is a potential alternative gas of SF
₆
due to its higher dielectric strength and lower Global Warming Potential (GWP). In this paper, the charge accumulation phenomenon on insulator surface in C
₄
F
₇
N/CO
₂
mixtures is investigated under needle-plate electrode. The characteristics of charge accumulation are compared with those in air. The influence of AC voltage frequency, gas pressure, and the ratio of C
₄
F
₇
N in the mixed gas on charge accumulation are analyzed. The results show that the charge distribution on the insulator surface presents a three-tier concentric circle structure. The distribution range in C
₄
F
₇
N/CO
₂
mixture is smaller than that in air, and it is mainly composed of negative charges. The charge density and distribution range decrease with the growing of the gas pressure or the ratio of C
₄
F
₇
N in the mixed gas, while the positive charges on the insulator surface increases with the growing of voltage amplitude or the decrease of AC frequency. It is suggested that the characteristics of charge distribution in the C
₄
F
₇
N/CO
₂
mixtures are due to the large diameter and strong electronegativity of C
₄
F
₇
N gas molecules, which are easy to attach free electrons.