Decomposition of high-pressure (400 kPa) SF6–CO2, SF6–CO, SF6–N2–CO2and SF6–N2–CO mixtures under negative dc coronas

Sulfur hexafluoride, SF 6 , is successfully employed by the electric power industry for gas insulated equipment. However, it is a potent greenhouse gas and mixtures of SF 6 with more friendly environmental gases have recently been proposed. It was demonstrated that SF 6 mixed with N 2 and/or CO 2 could be a good substitute for some insulation applications such as gas-insulated transmission lines (GILs). Considering the possible occurrence of corona discharges in GILs, the decomposition of these gas mixtures under such stress must be studied. In this paper, the chemical stability under negative corona of SF 6 and SF 6 –N 2 (10 : 90) or (5 : 95) mixtures was investigated at 400 kPa on adding percentages of CO 2 up to 80%. The corona discharges were generated at 23°C in a 340 cm 3 experimental cell between a stainless steel point connected to a high voltage source and a plane of aluminium. The gaseous decomposition products were assayed by gas chromatography at the end of each run carried out over a range of transported charges covering 8–17 C. The presence of CO 2 in SF 6 and SF 6 –N 2 mixtures submitted to negative coronas leads to: a considerable increase in the formation of the major compound group (SOF 4 +SO 2 F 2 ), formation of similar large quantities of COF 2 , abundant production of CO with diluted SF 6 in nitrogen, decreased formation of S 2 F 10 .