Influence of Trace H2O and O2 on SF6 Decomposition under Corona Discharge and Spark Discharge based on Oxygen Isotope Tracer

Lower fluorides of sulfur are primary byproducts of SF6 when insulation failures occur, which decompose into final long-lived by-products in the presence of oxygen or moisture, such as SO2F2, SO2, SOF2 and SOF4. The decomposition mechanism of SF6 is complicated, and the decomposition products are different under different insulation failures. Therefore, summarizing a variation of SF6 decomposition products under different faults and different discharge conditions can provide a reference for fault diagnosis of SF6 gas-insulated electrical equipment. Based on oxygen isotope tracer technique, different contents of H2¹⁸O and ¹⁸O2 were injected into the text vessel to simulate an environment containing trace moisture or oxygen. Then, a series of corona discharge and spark discharge experiments were carried out. The decomposition gases were detected by gas chromatography-mass spectrometry (GC-MS). The decomposition gases S¹⁶O2F2, S¹⁶O¹⁸OF2, S¹⁸O2F2, S¹⁶O2 and S¹⁶O¹⁸O indicate that moisture and oxygen participate in the decomposition reaction of SF6. Furthermore, the influence of trace moisture and oxygen on the contents of three isotopic compounds of SO2F2 were analyzed, and the ratios of SO2F2 to (SO2+SOF2) were investigated in the trace moisture environment. The results show that both H2O and O2 involve into the formation of SO2F2. In the trace moisture environment, the peak area ratio of SO2F2/ (SOF2+SO2) gradually decreases with the increase of water injection. The change of the ratio of SO2F2 to (SOF2+SO2) may provide an index for detecting the moisture content of SF6 gas-insulated electrical equipment.