Thermal Decomposition Characteristics Of SF₆under Overheating Fault In SF₆Insulated Equipment

Insulation gas SF6has stable chemical property and strong electronegativity, withexcellent insulation and arc-suppressing performance. However, exploratoryexperiments find that SF6will decompose into various components when temperature ofSF6insulated equipment reaches to some degree, such as SO2, SOF2, SO2F2, and HF. Onone hand, these characteristic decomposed components is highly corrosive. They canaccelerate aging speed and cause reduction of insulation strength, which will furtherjeopardize the safety of the SF6insulated equipment. On the other hand, varianceregularity of SF6thermal decomposed component is close related with thermal faulttype and status of SF6insulated equipment. Hence, by detecting decomposedcomponents and clarifying their variance regularity under local thermal fault, it can notonly ameliorate the deficiency of SF6decomposition theoretical system, but alsoimprove the method of using decomposed component to monitor and diagnose localthermal fault in SF6insulated equipment. As a result, it can lay solid foundation torealize early warning of thermal fault in SF6insulated equipment.This thesis conducted serials of experiments on the designed SF6thermaldecomposition stimulation testing system, Gas Chromatograph, Chromatography-massSpectrometry, and Fourier Infrared Spectroscopy were used to quantitatively measuredecomposed components. Based on this, decomposition beginning temperature of SF6under thermal fault was investigated, concentration variance regularity of eachcomponent with different temperature was analyzed, the most significant characteristiccomponent under thermal fault was identified, and the formation mechanism ofdecomposed component was explained. Besides, this thesis established three types offeature parameters, namely, total decomposition amount, effective formation rate, andcharacteristic component ratio, to reflect thermal fault property and severity of SF6insulated equipment, the representation ability of each feature parameter was alsodiscussed.Result shows that: SF6begins to decompose at300℃and contains CO2, SO2F2,SOF4, SOF2, SO2, H2S and HF; the improvement of fault temperature will promote theformation of decomposed components, but the influence regularity of each componentis different; SOF2and SO2are the most stable characteristic components and H2S is thespecial component which occurs only when the temperature is above340℃；total decomposition amount TDCcan reasonably represent the overall degradation status ofinsulation medium SF6; component’s formation rate can effectively reflect the severityand development tendency of thermal fault of SF6insulated equipment; Combining IECstandard, the obtained thermal fault maximum safe time TMScan accurately assess thelargest duration time when equipment occurs thermal fault; component characteristicratio C(SO2)/C(SO2F2) and C(SO2F2+SOF4)/C(SOF2) can qualitatively judge the faultproperty whether it belongs to thermal fault; the defined thermal correlation ratio TRcan availably to differentiate the temperature point of thermal fault among300~400℃.