Synergistic effect of c-C<inf>4</inf>F<inf>8</inf>/N<inf>2</inf> gas mixtures in slightly non-uniform electric field under lightning impulse

Abstract

Perfluorocyclobutance (c-C 4 F 8 ) is a potential alternative insulating gas to SF 6 . It is nontoxic and has lower global warming potential (GWP) compared with SF 6 . Because of the high boiling point, the c-C 4 F 8 and N 2 mixture gas has been suggested as a replacement to SF 6 in gas insulated switchgear (GIS). However, the synergistic effect of c-C4F8/N2 under lightning impulse is seldom researched. This paper studied the breakdown characteristics of the binary mixture gas with low c-C4F8 volume fraction in slightly non-uniform electric field under both positive and negative lightning impulse and analyzed how the synergistic effect of c-C4F8/N2 mixtures affected by the impulse polarity and the pressure. In order to determine the synergistic effect we proposed a coefficient and found the pressure has little influence on the coefficient. The experimental results also show that the discharge characteristics of c-C4F8/N2 gaps present an obvious polarity effect in slightly non-uniform field. The breakdown voltage under negative lighting impulse is lower than that under positive lightning impulse. It is due to the polarity of lightning impulse influence the velocity and kinetic energy of free electrons. Free electrons under negative lightning impulse have higher speed and make c-C 4 F 8 molecules absorb only little free electrons, so the breakdown voltage under negative lightning impulse is lower than that under positive lightning impulse.