Research On SF₆ Gas Decomposition Characteristics And Characteristic Components Under Partial Discharge Of Different Insulation Defects

For the excellent insulating and arc-quenching performance, SF₆ can significantly reduce the equipment size and improve the dielectric strength. It has been widely used in gas-insulated switchgear (hereinafter referred to as GIS) as insulating medium. Pure SF₆ is non-toxic and non-flammable inert gas. It has good stability and is difficult to decompose. However, partial discharge, spark discharge, electric arc and local overheating can cause SF₆ decomposition. At present, it has become a hot area of research that by way of analyzing the decomposition components to diagnose the insulation condition of SF₆ gas-insulated electrical equipment.PD (Partial Discharge, referred to as PD) is the main indicator of insulation performance of equipment. However, the traditional measurement method is difficult to get rid of the electromagnetic disturbance in field. There are limitations. Researches show that PD can course SF₆ decomposition. And the decomposition characteristics has differences under various PD. Intensive study on the formation, mechanism and the relative content of SF₆ decomposition products in order to propose the judgments theory and method of insulation condition based on quantitative detection of decomposition components is significant to the realization of early diagnosis and prediction of GIS catastrophic failure caused by insulation defects. This paper mainly research on SF₆ decomposition characteristics under various PD of different insulation defects in GIS.The typical insulation defects in GIS include high voltage metallic outshoot, free conductive particles, metallic contamination on insulator, insulator external gas-gap, insulator internal bubble defects. Based on the SF₆ discharge decomposition test equipment and combined with electrical measurement techniques and gas chromate- graphic analysis, SF₆ decomposition compositions under different PD were tracked and measured in this paper. The SF₆ decomposition characteristics, the development of insulation defects and influencing factors under different PD were studied in order to find effective characteristic parameters of fault types. Therefore, five kinds of physical model of typical insulation defects were constructed and the distribution of electric field of these models was analyzed by Finite Element Simulation. Simulation results showed that the defect models can raise PD in scheduled location and acceptable threshold voltage.96 hours PD test was conducted for different insulation defects to cause SF₆ decomposition. The results showed that: The characteristic decomposition components of SF₆ under PD were CF₄, CO₂, SOF₂, and SO₂F₂, and due to different discharge energy, intensity, location, etc. in various PD the decomposition quantity and rate had significant differences. High voltage metallic outshoot PD had stable frequency and large energy. The decomposition rate of SF₆ was rapid and the gas production rate was high. Under high PD intensity of free conductive particles defect, SOF₂/SO₂F₂ was higher; Owing to involving solid insulating material in metallic contamination on insulator PD, CF₄ content increased gradually; In insulator external gas-gap defect PD intensity was high, but the total production was low. SOF₂ content is lower than SO₂F₂. Insulation defects can be preliminarily identified by analyzing SOF₂/SO₂F₂, CO₂/CF₄ and (SOF₂+SO₂F₂)/(CO₂+CF₄). Further, use SOF₂/SO₂F₂ to assess intensity and development trend of PD. Changes of CF₄ content can reflect degradation of the insulating material in GIS to some extent.