Simulation Study On Partial Discharge Characteristics Of SF₆/N₂ Gas Mixture

With the continuous improvement of power system voltage level,GIS equipment is more and more used in power grid with high voltage level because of its many advantages.Because of the good insulation performance and high chemical stability,SF₆ gas is widely used as insulating gas in GIS equipment.However,SF₆ gas also has many disadvantages,such as high price,high greenhouse effect and easy liquefaction in low temperature environment.Thus,replacing pure SF₆gas with SF₆/N₂ mixture can not only improve the liquefaction temperature of pure SF6 gas,but also improve its discharge characteristics on the premise of ensuring that the insulation level of the gas does not decrease too much.It is widely used SF₆/N₂ mixture instead of pure SF6 gas in gas insulated switchgear.As the gas in which the charges of positive and negative particles cancel each other in space after partial or complete ionization,plasma appears to be electrically neutral macroscopically.The technology of plasma is widely used in various fields of industry.So far,most of the discharge phenomena of SF₆/N₂ mixture are from the perspective of macro experiments,and there are few studies on the discharge mechanism of SF₆/N₂ mixture through micro theories such as plasma.In order to further explore the relevant characteristics of partial discharge in SF₆/N₂ mixture and make some guidance for better use of this gas mixture,the plasma model of finite element simulation software COMSOL is used to complete the following two parts in this paper:In the first part,the negative streamer discharge of SF₆/N₂ gas mixture in uniform electric field is simulated.The spatial distribution of particle number density and spatial electric field distribution in the discharge process are compared and analyzed from the microscopic point of view,and the breakdown field strength in the corresponding case is obtained.The research shows that the change of SF₆ gas mole fraction will have an impact on the number density distribution of various particles in space,especially on electrons and SF₆^-.At the same time,the difference of SF₆gas mole fraction will also have an impact on the development speed of streamer,the distribution of spatial electric field and the energy of average electrons.In addition,through the comparative analysis of the breakdown field strength of the mixed gas,it is found that the breakdown field strength will increase with the increase of the mole fraction of SF₆ gas,and it is attainable to increase the mole fraction of SF₆ gas if you want to improve the insulation performance of SF₆/N₂mixture.The second part is to simulate the corona discharge of SF₆/N₂ gas mixture in uneven electric field.The research shows that in the process of corona discharge,due to the strong electric field around the needle electrode,a large number of electrons generated by electron avalanche are gathered at the head of the needle tip.These electrons will distort the electric field at the head in the process of discharge development,while the electric field near the plate electrode is more uniform.The greater the number density of SF₆^-ions closer the needle electrode,and the smaller the number density of SF₆^-ions closer to the plate electrode.In addition,the magnitude of the applied voltage will also affect the discharge characteristics of the gas mixture.With the increase of the applied voltage,the more intense the corona discharge in the space and the greater the distance of corona discharge development;Meanwhile,the higher the position of the sudden increase of electron number density,the greater the increase of electron number density,and the faster the development of corona discharge.