Decomposition products from negative and 50 Hz ac corona discharges in compressed SF6 and SF6/N2 (10:90) mixtures. Effect of water vapour added to the gas

Abstract

SF6/N2 mixtures with a majority of nitrogen are currently highly recommended, at the international level, in gas insulated transmission lines as an alternative to pure SF6; indeed, these mixtures are much more friendly to the atmosphere and particularly cheap. Among the areas of investigation of such gas mixtures, their electrical decomposition as a function of impurity content and type of discharge must be studied. The present study concerns the decomposition rate of SF6 and SF6/N2 (10:90) mixtures at 400 kPa under negative and 50 Hz alternating current corona discharges carried out without and in the presence of 0.3% H2O added, up to 10 C of transported charge. The corona discharges were generated at 23 °C in a 340 cm3 experimental cell between a stainless steel point (radius of curvature 10 µm) connected to high voltage and a plane layer of aluminium (gap space 2.3 mm for pure SF6 and 3.4 mm for SF6/N2). The gaseous decomposition products SOF4, SO2F2, SF4+SOF2, SO2, S2OF10, S2F10, S2O2F10, S2O3F6, SF5NF2, NF3 and (SF5)2NF were assayed by gas chromatography at the end of each run. The comparison of the formation rates of the byproducts detected in SF6 and SF6/N2 mixtures led to the following conclusions. (i) The use of wet SF6/N2 presents an additional advantage with respect to pure wet SF6 besides those mentioned above; the production rates of all the usual SF6 decomposition products are much lower with the wet SF6/N2 (10:90) mixture than with wet SF6. (ii) More exactly the addition of 0.3% H2O to SF6/N2 greatly affects the production rates of all the compounds whose formation needs SF5 radicals as SOF4; so the lower quantity of products formed in SF6/N2 is in fact more due to the particularly inhibiting effect of water than to the low percentage of SF6 in the mixture. (iii) The production of NF3, SF5NF2 and (SF5)2NF remains very low compared to the other gaseous byproducts.