Insulation characteristics of fluoronitriles / CO2 gas mixture under DC electric field

Environmentally friendly DC gas insulated transmission line (GIL) is currently one of the hot topics in advanced electric power equipment. Recently, fluoronitriles have been widely studied and considered to replace SF6 in GILs. In this paper, the breakdown voltage, insulator flashover voltage, and the partial discharge inception voltage of fluoronitriles/CO ₂ gas mixtures are reported in a DC electric field. The mole ratio of the mixed gases are 4 : 96 % and 8 : 92 %, and the gas pressure is in the range 304.7 to 717.83 kPa. Based on the results, the breakdown field strength of fluoronitriles/CO ₂ gas mixture shows a growth tendency with increasing gas pressure in a uniform electric field. At 717.83 kPa, the positive breakdown field strength is higher than the negative and reached a maximum. In a non-uniform electric field, the negative breakdown voltage of fluoronitriles/CO ₂ gas mixture increases with increasing gas pressure. In the contrast, the positive breakdown voltage decreases with the rise of gas pressure, which exhibits a significant polarity effect. The flashover voltage of fluoronitriles/CO ₂ gas mixture increases with increasing gas pressure. At the same gas pressure, the negative flashover voltage is slightly higher than the positive. The negative flashover voltage of fluoronitriles/CO ₂ gas mixture at 717.83 kPa is equal to 96.06 % of SF6 gas at 0.5 MPa abs. The negative partial discharge inception voltage (PDIV) of the fluoronitriles/CO ₂ gas mixture is greater than the positive, and both 4 : 96 % and 8 : 92 % gas mixtures showed higher PDIV than the SF6. Considering the insulation characteristics, the fluoronitriles/CO ₂ mixture has the potential to replace SF6 in DC gas insulated transmission lines.