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

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, Electricite 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.