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

Steep-wavefront impulse overvoltages may be generated, including fast-front overvoltage (FFO) with wavefront time of 0.1~20 μs, during the operations of disconnector switch in gas insulated equipment (GIE), threatening its insulation. To study the breakdown characteristics of SF
₆
alternative C
₄
F
₇
N/CO
₂
mixtures under FFO, the influence of FFO wavefront time, pressures and polarities on the breakdown characteristics of C
₄
F
₇
N/CO
₂
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 C
₄
F
₇
N/CO
₂
and SF
₆
are similar. In the slightly non-uniform electric field, the insulation performance of 9%C
₄
F
₇
N/91% CO
₂
reaches 78% SF
₆
under the same conditions. The 50% breakdown voltage of gas under impulse voltage (U
_50%
) of the C
₄
F
₇
N/CO
₂
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%
-Tf) 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 C
₄
F
₇
N/CO
₂
tends to decrease first and then stabilize with the increase of wavefront time. With the increase of pressure, the U
_50%
of C
₄
F
₇
N/CO
₂
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 C
₄
F
₇
N/CO
₂
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 C
₄
F
₇
N/CO
₂
rises with the increase of pressure and tends to be saturated.