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Steep-wavefront impulse overvoltages may be generated, including fast-front overvoltage (FFO) with wavefront time of 0.1~20 &#x03BC;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 (U50%) 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 (U50%-Tf) and the 50% breakdown voltage curve with gas pressure (U50%-P) in the slightly non-uniform electric field. In the extremely non-uniform electric field, the U50% of the C4F7N/CO2 tends to decrease first and then stabilize with the increase of wavefront time. With the increase of pressure, the U50% of C4F7N/CO2 increases linearly in the slightly non-uniform electric field. Under the positive FFO, in the extremely non-uniform electric field, the U50% 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 U50% of C4F7N/CO2 rises with the increase of pressure and tends to be saturated.

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

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