The Analysis Of Discharge Process In SF₆, N₂and Mixture Gases In Short Gap

Gas insulation dielectrics are widely used in the power system, and SF₆iswidely used in gas-insulated equipment, because of its high dielectric strength,excellent arc extinguishing and well cooling. But the pollution of SF₆aboutenvironment can not be ignored, so the urgent problem is to find other gases todisplace it, SF₆/N₂mixture has been widely applied and concerned.In this dissertation we use two-dimension fluid model to finish the simulationof SF₆, N₂and SF₆/N₂mixture gases discharge processes. N₂-fluid model is non-negative electric fluid model, it includes the electron and positive ion continuityequation and the coupled Poisson equation, SF₆and SF₆/N₂fluid model are theelectric negativity fluid model and are composed of electron, positive ion andnegative ion continuity equation coupled with the Poisson equation. The modelsinclude the impact ionization, diffusion, adsorption and photoionizationprocesses, and three-dimension photoionization model is simplified for reducingcomputation. FCT and overrelaxation iterative method are used to solve theparticle continuity equation and Poisson equation, respectively. We completenumerical simulations of the SF₆, N₂and50%-50%SF₆/N₂mixture gasdischarge process under the same pressure and humidity conditions. The resultsshow that the narrow gap gas discharge can be divided into three stages: theavalanche stage, the avalanche transferred to streamer phase and the streamerdischarge stage; the role of photoionization only can be observed in the streamerstage, a large number of secondary electron accelerates the process of streamerdischarge; compared with SF₆gas, the50%-50%SF₆/N₂is easier to bebreakdown under the same conditions, but its breakdown voltage only decreasedabout10%, and much higher than the pure N₂gas’, finally this defect can becompensated by increasing the pressure.The simulation results can present every time and space distributions ofcharged particles and electric field, understand deeply the different roles of the various physical processes in discharge stages, observe the dynamics behavior ofdischarged partials and plasma channel parameters; provide the theory basis forfurther studying discharge mechanism, can also optimize the gas dielectricapplying in electrical equipment.