Simulation Of The Substitutes For SF₆ And Breakdown Characteristics Of Typical Gas

With the excellent dielectric and arc extinguishing properties, SF₆ has been widely used in high voltage apparatus. While it is a consensus that the usage of SF₆ should be reduce due to its greenhouse effect. Therefore,looking for a new environment-friendly dielectric gas instead of SF₆, has become the core research topic in the field of high voltage insulation.The ionization energy, electronic affinity and energy gap of gas molecules are significant parameters affecting the gas breakdown strength.Hence a theoretical investigation on the ionization parameters for SF₆ and its potential alternative gases was explored by using density functional theory（DFT） at the atomic and molecular levels has been done in this study. In order to investigate the influence factors of molecular structure, element composition, valence bond and F atomic number on gas ionization parameters.The results shown that, the insulation performance could be influenced greatly by configuration and elements of the gas molecule. In the same cycle, with the increase of the atomic number, the electric strength of the gases increases. The electric strengths of the different fluoride gases formed by the same atomic increase with atomic valence increasing. The electric strength of the gases in the same molecular configuration increases with the number of F atom.The breakdown electric field of the gas is related to ionization coefficients, electron attachment coefficient, gas density and collision cross section. The critical reduced electric field strength of several typical gases and SF₆ was calculated by solving two-term Boltzmann equation to find breakdown strength of SF₆ and its potential alternative gases.The results shown that,the critical reduced electric field strength of c-C₄F₈ and CF₃ I was significantly higher than that of other gases, which indicated a good insulating ability of these two gases. Moreover, we mixed these two gaseswith N₂ or CO₂ respectively, then obtained correlation of gas breakdown strength changes and gas content.Finally, the c-C₄F₈ mixed gas has been tested under DC and AC voltage to verify its insulation capability in non-uniform electric field. The results revealed that, two elements mixed gas of c-C₄F₈ gas and CO₂ or N₂ in nonuniform electric field still exhibits a good insulating ability. But when the distance become shorter, the breakdown voltage appears hump and saturation in long gap.