Influence Mechanisms Of Trace H₂O,O₂ And Pressure On SF₆ Spark Discharge Decomposition Characteristics

SF6-insulated electrical equipment(such as GIS,GIL,etc.)gradually receives worldwide attention in modern power and UHV system for less land occupation,long maintenance cycle and small electromagnetic radiation,and plays an important role in building up modern city power supply system.However,there are some inevitable defects in GIS due to the inappropriate production process,installation process,operation and maintenance of the equipment.The defect may induce spark discharge in the later stage.Additionally,the combination effect of strong electromagnetic field and thermal field lead to the serious metal ablation,insulating solid material vaporization and gas decomposition.Thus,spark discharge fault is a dangerous “malignant tumor” in the equipment,which would bring great loss to the power system if it is not recognized and eliminated in time.At present,there is no effective monitoring method for spark discharge fault in SF6-insulation equipment.Researches show that SF6 spark discharge decomposition characteristics can intuitively reflect the development of spark discharge severity.However,trace content of H2 O,O2 inside equipment and the working pressure are both the key factors remarkablly influencing SF6 decomposition.The recognition rate of the monitoring and fault diagnosis system based on gas composition method would decline significantly,or even fail due to the changing of the factors.Therefore,the paper will focus on the influence and related mechanisms of different trace H2 O,O2 contents and pressures on SF6 decomposition characteristics of spark discharge to perfect the expert decision-making system for SF6 spark discharge fault,and the work and research results carried out by this paper are as follows:(1)A series of SF6 spark discharge experiments were conducted under SF6 spark discharge fault simulation experiment platform,obtained spark discharge decomposition characteristics under SF6/H2 O and SF6/O2 mixed gases with different concentrations were,and the influence mechanisms of pressure on SF6 spark discharge decomposition was studied.Based on the mechanism of decomposition,determine the main characteristics quantity representing spark discharge fault properties and states without influence by the environmental factors of the gas chamber,and the formation path of each characteristic decomposition component is revealed.The content of SOF2,SO2F2,SO2,SOF4 and CO2 show varying degrees of growth trend with H2 O content increasing.The content of SO2F2,SOF4 and CO2 increases,and that of SOF2 remains stable,SO2 drop slightly with O2 increasing.The decomposition productions gradually decline with pressure rising due to the reduction of mean free path.The content of CF4 decreased with the rising trace H2 O,O2 contents and pressures,but the production were really low.(2).On the basis of the SF6 decomposition characteristics,analyze the species,the relative content and valence state of the elements on the surface layer of metal electrode materials after SF6 spark discharge by XPS(X-ray photoelectron spectroscopy)technique,and obtain the formation process of decomposition products of SF6 spark discharge.Research shows that: Al2(SO4)3,Al2(SO3)3 and AlF3 are detected in the the surface material on condition that trace H2 O content is relatively abundant.The main forms of Al in the surface material are Al2O3 and AlF3,S mainly takes the form of metal sulfides when there exists much O2,which indicates free states S* or S exists under spark discharge.The relative content of carbon in the solid product decreases sharply,indicating the carbon elements in CO2 and CF4 are mainly derived from metal materials.According to the decomposition characteristics,CO2 and SOF4 originate from oxygen impurities in the chamber,and SOF2 and SO2F2 mainly originate from moisture.(3)The characteristics parameters of the severity of PD were improved for spark discharge,and gain the characteristics parameters–SF6 total decomposition,effective gas rate and ratio,and illustrates their physical significance respectively.Furthermore,the effects of different trace H2 O,O2 contents and pressures were studied on the key characteristic parameters which represent the spark discharge energy(severity of fault)on the key characteristic parameters which represent the spark discharge energy(severity of fault).Results show that the increase of trace H2 O and O2 content significantly promote SF6 total decomposition and the effective gas rates of the components(except CF4),however pressure rising has the inhibition effect on SF6 total decomposition and effective gas rates.On the contrary,the characteristic ratios gradually decrease with trace H2 O and O2 content,but the increase of pressure they showed the trend of “rising-saturation”.(4)The chemical kinetics theory was adopted to explore the SF6 spark discharge decomposition process under different trace H2 O and O2 contents,and the mathematical relation between the characteristic ratios ?(SOF2+SO2)/?(SO2F2+SOF4),?(CF4+CO2)/?(SOF4+SO2F2+SOF2+SO2)and the initial H2 O,O2 content were derived,which can be unified in the form of power function.What's more,the mathematical relation between the pressure and the characteristic ratios can be expressed as the form of exponential function based on the impact ionization theory.On the basis of characteristic component experimental data,least squares fitting were used to get the mathematical model to correct the influence of H2 O,O2 content and pressure on characteristic ratios.Additionally,the accuracy of the calibration models are verified by the calculation of partial correlation coefficients and repeated experiments,indicating the feasibility of the proposed calibration methods of trace H2 O,O2 and pressure.