SF6 gas decomposition analysis under point-to-plane 50 Hz AC corona discharge

As one of the most commonly used insulating mediums, in electrical discharge, sulfur hexafluoride (SF₆) will decompose in the presence of water vapor and oxygen, producing various gaseous by-products. It is shown that the concentrations and production rates of SF₆ gaseous decomposition by-products heavily depend on electrical field to gas density ratio (E/N) under corona discharge. Regarding the difficulty in measuring E and N, applied voltage and the SF₆ pressure, are proposed in this paper to describe E and N, respectively. Several experiments of 50-Hz AC discharges were carried out under different applied voltages and different SF₆ pressures using point-to-plane electrodes. The concentrations of SF₆ gaseous by-products were measured by a gas chromatograph-mass spectrometer system (GS/MS). Experimental data show that the concentrations of SF₆ gaseous by-products increase while the ratio (SOF₂+SO₂)/SO₂F₂ decreases, with increasing applied voltage. The concentrations decrease with increasing pressure of SF₆. The formation mechanisms of SF₆ gaseous by-products with the variations of applied voltage and the SF₆ pressure are discussed, such as SO₂F₂, SOF₂+SO₂, CF₄, and CO₂ and so on. An experimental expression is derived which connects the SF₆ pressure with the concentration of the decomposition by-products.