Construction of Reax FF Force Field for SF6 Gas Insulation Medium under Over-thermal Fault

The SF6 gas insulation medium will decompose under the partial over-thermal fault (POF) inside the GIS equipment, which declines the insulation performance, thereby endangering the safety of the equipment. Although the decomposition characteristics under several degrees of POF are obtained, the problem of describing the continuous process of SF6 decomposition under POF has not been solved. The difficulty lies in the lack of a force field that can effectively describe the decomposition process of SF6. To this end, this paper first calculates the main molecular systems of fluoride-sulfides based on density functional theory, and obtains a training set for establishing a force field file. Based on the training set, the Reax FF force field is optimized using a fitting algorithm, and the correctness of the force field is verified. Then, the molecular dynamic simulations of SF6 under the over-thermal state is studied. The breaking and bonding process of SF6, SF5, SF4, SF3, and SF2 is clarified and compared with the macro experimental data of SF6 decomposition under partial over-thermal fault. The results show that the established force field can accurately describe the breaking and bonding process of SF6, SF5, SF4, SF3 and SF2 molecules. The simulation data is in good agreement with the actual experiments, and the research work in this paper lays a foundation for the systematic study of over-thermal decomposition of SF6.