Decomposition of high-pressure (400 kPa) SF6 and SF6/N2(10:90) mixtures submitted to negative or 50 Hz ac corona discharges in thepresence of water vapour and/or oxygen

Abstract

From the point of view of environmental protection, as SF6 has astrong greenhouse effect, high-pressure SF6/N2 (10:90) mixtures could bea good alternative to pure SF6 for use in gas-insulated lines (GIL); thus,the decomposition of these gases should be compared following stress such ascorona discharges, susceptible to occur in such electrical equipment.So, the present study concerns the decomposition of sulphur hexafluoride(SF6) and the mixture SF6/N2 (10:90) at high pressure (400 kPa) underthe effect of negative dc or 50 Hz ac corona discharges in the presence orabsence of added impurities (H2O and/or O2). The chemical stability ofSF6/N2 was compared to that of pure SF6 over a range of transportedcharge covering 0-12 C in the presence and absence of various proportionsof impurities added to the gas.The discharges, generated with a point-to-plane set-up, led to the formationof the following compounds: SOF4, SO2F2, (SF4 + SOF2), SO2, S2F10, S2OF10,S2O2F10, S2O3F6, NF3, SF5NF2, (SF5)2NF, N2O and CO2, which were assayed by gaschromatography at the end of each run.In the absence of impurities added to the gas phase, the overall quantities ofby-products formed in SF6 and SF6/N2 (10:90) were very close. In thepresence of 0.3% added oxygen, greater amounts of degradation product werefound in SF6/N2 (10:90) than in pure SF6. On addition of largequantities of water (⩾0.3%) the opposite was observed, even when 0.3% oxygenwas added at the same time as the water. So, the use of SF6/N2 (10:90)instead of pure SF6 was beneficial in the dampest conditions studied. Thiscan be accounted for by the inhibitory action of water, especially in diluteSF6, on the formation of by-products that require the SF5 radical such asSOF4 and S2F10, to mention only the most important. It follows that in theseconditions (damp, dilute SF6) the pair of degradation products (SF4 + SOF2)becomes preponderant over (SOF4 + SO2F2).However, the quantity of gaseous by-products formed in SF6/N2 (10:90)was much greater than suggested at first sight by the proportion of SF6 inthe mixture. This is mainly due to the fact that the dilution of the SF6fragments in the N2 at the same time inhibits the SF6 regenerationreactions and enhances the hydrolysis/oxidation of the SFx fragments whichlead to the formation of sulphur oxyfluorides. This dilution effect is alsoresponsible for the very low levels of S2F10 formed in the mixture.