Dynamic current-zero behaviour of wall-stabilised arc in different arc-quenching media

SF₆ gas is the medium currently used to quench the arc in gas circuit breakers (GCBs) and gas insulated switchgear (GIS). It has become increasingly necessary to look at alternatives to SF₆ for the following reasons: (i) high cost; (ii) sensitivity to particle contamination; and (iii) concerns regarding global warming and hence the possibility of imposed rules for decreased SF₆ production. From a practical standpoint, the complete elimination of SF₆ as an arc-extinguishing medium is improbable. On the other hand, a reduction in the amount of SF₆ and the inclusion of a buffer gas is worthy of consideration. In order to determine the suitability of a buffer gas, it is necessary to understand the buffer gases´ fundamental plasma properties. A comparative prediction of the thermal breakdown strengths of the following gases: SF₆, N₂, O₂, air, CO₂, H₂, He and N₂-PTFE after current-zero is presented. The thermophysical properties of a N₂-PTFE mixture are also calculated. A one-dimensional wall-stabilised arc model is solved numerically to determine the thermal breakdown strength of the aforementioned buffer gases in the presence of a uniform electric field intensity having a certain rate of rise. Calculations are done for two typical arc radii: 2.5 and 1.5 mm. Predictions confirm the suitability of He, CO₂, N₂ and air as a buffer gas to SF₆ and helium is found to be the best buffer choice.