Research On Microscopic Parameters And Electrical Breakdown Characteristics Of SF₆ Alternative Dielectric Arc

From the perspective of environmental protection,the higher greenhouse effect of SF₆ gas has not met the long-term development benefits,the search for SF₆ replacement gas with excellent performance and environmental protection has become a hot spot at the moment.In recent years,C₄F₇N-N₂ has been widely concerned by domestic and foreign researchers due to its excellent insulation performance and environmental protection characteristics.There is relatively little research on its arc extinguishing characteristics,especially when the switch breaks a small current,the airflow field simulation under no-load requires the thermodynamic parameters of the medium under normal conditions.In addition,the study of the airflow field distribution when the arc occurs during the switching process also requires the thermodynamic parameters and the transport coefficient of the medium as the support.In order to further study the insulation characteristics of thermal C₄F₇N-N₂ during the recovery process of the back-arc medium,SF₆-N₂ is used as the comparative research object to calculate its electrical breakdown characteristics.The main research work of this paper is as follows:（1）The density,constant pressure specific heat capacity,viscosity and thermal conductivity of C₄F₇N and its mixed gas are calculated based on the actual gas state equation,and the saturated vapor pressure and global warming potential of C₄F₇N and its mixed gas are obtained.A physical property parameter database of C₄F₇N-N₂ and C₄F₇N-CO₂ with different mixing ratios in the range of pressure 0.1 MPa-1.1 MPa and temperature-20℃-630℃is established.（2）The density functional theory B3LYP method and 6-31G basis set are used to optimize the structures of the reactants and products in the C₄F₇N decomposition path.The main reaction path is obtained under the minimum energy requirement of the reaction path.The vibration frequency and the rotation constant of the reactants and products are also solved.The results solve the problem of missing gas partition function parameters and provides basic conditions for the calculation of arc plasma micro parameters.（3）Using the law of mass action to calculate the equilibrium components and contents of C₄F₇N-N₂ and SF₆-N₂ plasmas.Through standard statistical thermodynamic relationships,the plasma mass density,molar mass,specific enthalpy constant pressure specific heat capacity and other thermodynamic parameters are obtained,and the influence of pressure on the thermodynamic parameters of plasma are analyzed.The transport coefficients such as plasma viscosity coefficient,thermal conductivity,and electrical conductivity are calculated using Chapman-Enskog method.A database of thermodynamic parameters and transport coefficients of equilibrium C₄F₇N-N₂ plasma in the pressure range of 1 atm-15 atm and temperature of 300 K-30000 K is established.（4）Two approximate methods are used to solve the Boltzmann equation.Through this method,the electron energy distribution function of the thermal SF₆-N₂ is calculated.The total reduced ionization coefficient and the total reduced adsorption coefficient of the thermal SF₆-N₂ under different reduced electric fields are obtained.Then the critical breakdown field strength of thermal SF₆-N₂ is obtained.A critical breakdown field strength database of thermal SF₆-N₂ at the pressure of 1 atm to 10 atm and the temperature of 300 K-3500 K is established.