Influence of electric field non-uniformity on breakdown characteristics in SF6/N2 gas mixtures under lightning impulse

In recent years, SF
₆
/N
₂
was getting used as insulation medium in gas-insulated equipment for replacing pure SF
₆
which causes greenhouse effect. For the application of SF
₆
/N
₂
in power equipment such as GIS and GIL, the breakdown characteristics of SF
₆
/N
₂
in varying electric field non-uniformity under standard lightning impulse were studied in this paper based on an impulse experimental platform. The experimental results demonstrates that the 50% breakdown voltage of SF
₆
/N
₂
rises linearly with rise of gas pressure when the electric field is slightly non-uniform, and the saturated trend appears when the electric field turns to be more non-uniform. The reversal phenomenon of polarity effect was found in SF
₆
/N
₂
with different mixing ratio. That is, when the pressure is relatively low, the 50% breakdown voltage under positive polarity is lower than which under negative polarity. When gas pressure reaches the critical pressure, the situation would be opposite. The augment of SF6 mixing ratio or decrease of electric field non-uniformity would both result in the decline of the critical pressure. Meanwhile the change of polarity effect with SF
₆
/N
₂
is later than that with pure single gas when slightly non-uniform electric field turns to be strongly non-uniform. In addition, the synergistic effect of SF
₆
/N
₂
weakens obviously with strengthening of electric field non-uniformity, and the negative synergistic effect even appears with strongly non-uniform electric field.