Breakdown Characteristics of C4F7N/CO2 Mixtures under Steep-Wavefront Impulse Voltages

Abstract

Steep-wavefront impulse overvoltages may be generated, including fast-front overvoltage (FFO) with wavefront time of .1\sim 20~\mu \text{s}$ , during the operations of disconnector switch in gas insulated equipment (GIE), threatening its insulation. To study the breakdown characteristics of SF6 alternative C4F7N/CO2 mixtures under FFO, the influence of FFO wavefront time, pressures and polarities on the breakdown characteristics of C4F7N/CO2 in slightly non-uniform and extremely non-uniform electric field were investigated. The results show that the effects of FFO wavefront time, polarities and pressures on the insulation properties of C4F7N/CO2 and SF6 are similar. In the slightly non-uniform electric field, the insulation performance of 9%C4F7N/91% CO2 reaches 78% SF6 under the same conditions. The 50% breakdown voltage of gas under impulse voltage ( $U_{50\%}$ ) of the C4F7N/CO2 decreases first then rise with the FFO wavefront time showing a U-curve. It is also found that the voltage polarity has little effect on the trend of the 50% breakdown voltage curve with wavefront time of impulse voltage ( $U_{50\%}$ - $T_{\mathrm {f}}$ ) and the 50% breakdown voltage curve with gas pressure ( $U_{50\%}$ - $P$ ) in the slightly non-uniform electric field. In the extremely non-uniform electric field, the $U_{50\%}$ of the C4F7N/CO2 tends to decrease first and then stabilize with the increase of wavefront time. With the increase of pressure, the $U_{50\%}$ of C4F7N/CO2 increases linearly in the slightly non-uniform electric field. Under the positive FFO, in the extremely non-uniform electric field, the $U_{50\%}$ of the C4F7N/CO2 changes with the rise in pressure showed a tendency to increases first, then decreases and rises again, resembling the $N$ letter. Under the negative FFO, the $U_{50\%}$ of C4F7N/CO2 rises with the increase of pressure and tends to be saturated.