Effect of the percentage of SF6 (100%-10%-5%) on the decomposition of SF6-N2 mixtures under negative dc coronas in the presence of water vapour or oxygen

Low SF6 content SF6-N-2 mixtures have recently been proposed as a replacement for pure SF6 in the insulation of gas insulated lines (GIL). Among the areas of investigation of such gas mixtures, their electrical decomposition under corona discharges must be studied considering the possible occurrence of such stress in GIL. This paper presents data concerning the decomposition of high-pressure SF6-N-2 (5 : 95) mixtures (400 kPa) submitted to negative dc coronas in the absence or presence of 0.3% H2O or 0.3% O-2. The chemical stability of these mixtures is compared with that of SF6-N-2 (10: 90) mixtures or undiluted SF6 investigated in the same conditions in a previous paper. The corona discharges were generated with a point-to-plane set-up and the gaseous by-products were assayed by gas chromatography at the end of each run carried out over a range of transported charge covering 0-13 C. The following by-products were detected and assayed: SOF4, SO2F2, (SF4 + SOF2), SO2, S2F10, S2O2F10, S2O3F6, (SF5)(2)NF, NF3 and N2O. Whatever the type or the concentration of impurity added to the SF6-N-2 mixtures the major compound groups, (SOF4 + SO2F2) and (SF4 + SOF2 + SO2) were formed with 5% SF6 in quantities very close to those observed with 10% SF6. However, the lesser production rates of S2F10, (SF5)(2)NF and NF3 measured with the most dilute SF6-N-2 mixtures makes the use of SF6-N-2 (5: 95) more advantageous than that of SF6-N-2 (10: 90) in all cases. Considering the overall quantity of by-products formed in the presence of water or without any impurity added, it appears that SF6-N-2 mixtures are, from this point of view, preferable to pure SF6.