The influence of small amounts of N2 in SF6 on prebreakdown discharges

Pre-breakdown discharge activity in SF₆ and SF₆ +2%N₂ at 1 bar absolute has been investigated to assess the feasibility of adding small amounts of N₂ to SF₆ to improve the luminosity of the pre-breakdown discharges. It has been shown that small amounts of N₂ does not significantly alter the breakdown strength of the mixture but will have a profound influence on the pre-breakdown discharge activity under 50 Hz voltages and lightning impulse voltages. This influence is attributed to the following three mechanisms: (1) the increased availability of free electrons due to the lower ionisation potential of the N₂ compared to SF₆; (2) the removal of high energy electrons from the discharge by electron scattering due to negative ion resonance in N₂, thus effectively increasing the attachment of SF₆ ; (3) the creation of longer lived SF₆^- ions by the attachment of the scattered low energy electrons from N₂ , thus effectively enhancing the negative ion space charge