Chemical Decomposition of High Pressure SF₆/N₂ (5:95) Mixtures under Negative DC Corona Discharges

Abstract

Sulfur hexafluoride (SF₆) is the insulator gas used in most of equipments handling high and very high voltages.Owing to its high global warming potential, SF₆ has been classified during the Kyoto conference on climate change among the greenhouse gases. Its emission in the atmosphere must therefore be reduced1. In order to comply with this aim, one of the solutions adopted by the manufacturers and the users of some of these equipments, consists in reducing the quantity of SF₆ used by mixing it with zero (or very much lower) risk gases.For instance, as SF₆/N₂ mixtures with SF₆ concentrations lying between 5 and 15% realise a satisfactory compromise between dielectric performances and cost, Electricité de France plans to use these mixtures to fill its future gas-insulated lines2. Under the effect of the corona discharges which can occur in these equipments, part of the SF₆ and of N₂ will dissociate and lead, in the presence of even low concentrations of H₂0 and/or 0₂, to the formation of a lot of other gaseous compounds: (SF₄+SOF₂), SOF₄, S0₂F₂, S₂F₁₀, NF₃....Previous studies on the decomposition of SF₆/N₂ (10:90) mixtures under coronas3 showed that, with no impurity added, the total quantity of by-products formed is not very different from that measured in pure SF₆ for the same charge transported value. This indicates that the percentage of SF₆ consumed increases as its concentration in the gas phase decreases 3,4.This result led us to extend our experiments to SF₆/N₂ mixtures only containing 5% of SF₆.