Influence of Three Buffer Gases on Dielectric Strength of C4F7N Mixtures

In order to study the potential of C4F7N (perfluoroisobutyronitrile) mixed with three buffer gases CO2, N2 and air respectively as insulation mediums to substitute SF6, the power frequency insulation performances of C4F7N/CO2, C4F7N/N2 and C4F7N/air mixtures were tested under uniform electric field. The gas pressures of mixtures range from 0.1 to 0.7 MPa, and the mixing percentages of C4F7N in mixtures are 5% to 20%. The insulation properties of C4F7N mixtures containing different buffer gases were compared, and the effects of gas pressure and mixing ratio on the breakdown voltage of different C4F7N mixtures were analyzed. The experimental results demonstrate that the breakdown voltage of C4F7N/CO2 and C4F7N/air mixtures increases linearly with gas pressure, whereas the breakdown voltage of C4F7N/N2 mixtures show weak saturation trends at higher gas pressures. The breakdown voltages of three kinds of C4F7N mixtures increase linearly with mixing ratios of C4F7N. The dielectric strength relative to SF6 of C4F7N/CO2, C4F7N/N2 and C4F7N/air mixtures varying with pressure, is not constant, and reaches the minimum value near 0.4 MPa. The carbon precipitation of C4F7N/N2 mixture under discharge is more obvious. When carbon heavily deposits on the electrode surface, the breakdown voltage of C4F7N/N2 mixture will decrease significantly. When the insulation performance, liquefaction temperature and carbon precipitation under discharge conditions are taken into consideration, CO2 and air are suitable buffer gases of C4F7N. © 2020, High Voltage Engineering Editorial Department of CEPRI. All right reserved.