Experimental study for SF6 decomposition byproducts formation mechanisms in 50Hz AC corona discharge and flashover

As an excellent insulating medium, Sulfur hexafluoride (SF₆) has been widely used in gas-insulated equipment. With the increase of SF₆-insulated equipment applying in ultra-high voltage or extra-high voltage power grids, the tools that can efficiently diagnosis electrical insulation faults are explored. Previous researches and actual operation experiences have shown that SF₆ gaseous decomposition byproducts are directly related to the insulation faults. Therefore, nowadays the analysis on gaseous decomposition by-products has been regarded as a powerful tool in diagnosing the internal condition of SF₆-insulated equipment. In this paper, the experiments regarding to two different types of insulation faults are carried out, including corona discharge and flashover. In the experiments, the electrode was set to point-plane electrode, and the total gas pressure was set to 200kPa. In addition, eight gaseous decomposition by-products are taken into account, that is, SOF₂, SO₂F₂, SO₂, H₂S, COS, CS₂, CF₄ and CO₂. As a source of information on identification of discharges, the contents of the gaseous decomposition by-products were measured by gas chromatograph-mass spectrometer system (GCMS). The results of the experiments show that the contents are apparently different, though the gaseous decomposition by-products under the two discharge types are similar. Based on the results of phenomenological observations, the decomposition mechanisms leading to the formation of decomposition by-products in various modes of electrical discharges are analyzed. In particular, applied voltage and corona inception voltage are proposed to assess electrical field intensity in the glow region of corona discharge. Finally, the identification criteria under two discharge types are introduced, which can effectively distinguish the two discharge types.