Analysis of decomposition products of sulfur hexafluoride in negative dc corona with special emphasis on content of H2O and O2

We are able to analyse the SF₆ decomposition directly during a discharge by means of a FT-IR (Fourier transform infrared) spectrometer and a 2.51 stainless steel discharge cell with infrared windows and a path length of 0.175 m. With this cell we investigated the influence of water vapour and oxygen on the production and persistence of the most important by-products from negative DC corona in mixtures based on “pure” SF₆ (99.995%) at a total pressure of 100 kPa. SF₆ mixtures without oxygen addition ([O₂ ]<10 ppm_v), with 1% (volume) oxygen and with different moisture contents (280 ppm_v<[H₂O]<4350 ppm_v) were analysed. 12 hours after preparation of the mixtures, when a certain equilibrium between residual water vapour and the walls had been reached, the moisture was measured and then the negative corona discharge was started in a point to plane gap. With a mean current of I=50 μA the discharge was maintained for a duration in the range from 29 h to 55 h. The moisture and the decomposition products (S₂F₁₀, SF₄, SOF₂, SOF₄ , SO₂F₂, SiF₄, COF₂, CF₄, CO and HF) were monitored by IR-absorption during and after the discharge. Quantitative analysis was made, by simple spectra subtraction, for the concentrations of H₂O and of the decomposition products S₂F₁₀, SF₄, SOF ₄, SO₂F₂, and SOF₂. The results indicate an important effect of water and oxygen on the production of SF₆ decomposition products. In general, the S ₂F₁₀ yield was reduced with increasing the initial moisture content. In the presence of 1% oxygen the production rate of S ₂F₁₀ under “very dry” conditions (initial [H₂O]<280 ppm_v) is reduced by a factor of 10 compared to mixtures with no oxygen added, [O₂]<10 ppm_v. The experiments show a good stability of S₂F ₁₀ at least for the first 30 hours after the discharge for all mixtures investigated