Study On Corona Discharge Decompositioncharacteristics Of SF₆and The Related Influencing Factors Under The Needle-plate Electrode

Gas insulated switchgear is widely used. It has many advantages such as smallfloor space, high reliability, low maintenance and friendly electromagnetic environment.Under normal circumstances, SF6gas inside the chamber of the gas insulated switchgearhas stable chemicalproperty, and does not easily decompose. However, when the gaschamber exists arc discharge, spark discharge or corona discharge, etc., SF6gas willdecompose because of the heavy current magnetic effect. It will generate low sulfurfluorides such as SF5, SF4, SF3, SF2and SF1. Some low sulfur fluorides will react withoxygen, water and other impurities inside the gas chamber. The reactions will producesome stable compounds such as CO2, SO2F2, SOF2, SO2and so on. The products notonly reduce the dielectric insulation properties, but also corrode insulators and otherinsulating equipment. It seriously endangers the safe operation of equipment for a longtime.Gas decomposition characteristics of SF6are closely related with insulation defects’type and severity inside the gas chamber. The analysis about the decompositions ofSF6is used to diagnose internal insulation fault of equipment. However, gas compositionanalysis based decomposition of SF6is still in the preliminary study stage. Many issuesare not fully resolved. Defect type, discharge energy, electrode materials, gas pressureand other influence factors will affect decomposition characteristics of SF6gas. Theeffect is unclear. Therefore, it is necessary to understand the impact of various factorson the decomposition characteristics of SF6. We should correct these influence on thecharacteristics and the ratio of the components. It is order to accurately use thecomponent analysis of SF6gas to on-line monitor and diagnose the insulated equipment.Combined existing research achievements this paper carries out somedecomposition experiments of SF6under corona discharge. These experiments arecarried out on the existing test platform. These experiments’ conditions include differentneedle-plate electrode spacings, different electrode materials and different gas pressures.Weanalysed decomposition components of SF6gas under the different discharge time bygas chromatography and gas chromatograph-mass spectrometer. Through comparativelyanalysing the test datas we get some conclusions. Needle-plate electrode spacing doesnot affect the generation of CO2, SOF2and SO2. With the increase of the needle-plateelectrode spacing, SO2F2level declines. The two kinds of characteristic ratios,includingφ(SO2F2+SOF2+SO2)/φ(CO2) and φ(SOF2+SO2)/φ(SO2F2), were all associated with needle-plate electrode spacing. Under the needle-plate defect model,plate electrode material does not affect the generation of CO2. The active chemicalactivity of plate electrode material will promote the generations of SOF2and SO2, andrestrain the generation of SO2F2. The chemical activity of Al is higher than the chemicalactivities of Cu and stainless steel. The chemical activity of stainless steel is least. Thepoint electrode material affects the generations of CO2，SO2F2，SOF2and SO2.φ(SOF2+SO2)/φ(SO2F2) is obviously associated with the plate electrode. It isn’tassociated with the point electrode. φ(SO2F2+SOF2+SO2)/φ(CO2) is obviouslyassociated with the plate electrode and the piont electrode. With the increase of gaspressure the concentrations of CO2, SO2F2, SOF2, SO2and φ(SOF2+SO2)/φ(SO2F2) willall decrease inordinately. Gas pressure inconspicuously affects the ratio ofφ(SO2F2+SOF2+SO2)toφ(CO2). When considering the effect of gas pressure,φ(SO2F2+SOF2+SO2)/φ(CO2) is more suitable as a troubleshooting feature amount.