Dynamic Breakdown Characteristics of GIS Disconnecting Switch Operation and Its Calculation of Operating Overvoltage in SF₆/N₂ Medium

From the perspective of engineering application, the SF₆/N₂ mixture gas is still the most effective alternative to the pure SF₆ gas. To provide a reference for the insulation coordination of the electrical equipment with the SF₆/N₂ gas mixture as a medium, the overvoltage of the disconnecting switch under different mixing ratios and pressure conditions of the SF₆/N₂ gas mixture is studied in this paper. The breakdown characteristic of the SF₆/N₂ mixture is the basis of the overvoltage calculation. The results show that the critical breakdown voltage of the SF₆/N₂ mixture has a linear relationship with its gas density distribution. In this paper, a multi-physical field coupling model is established to analyze the dynamic breakdown characteristics of the SF₆/N₂ gas mixture during the operation of the disconnecting switch. In order to simulate the dynamic operation process of the disconnector swichgear, the dynamic grid partitioning technology is used. Subsequently, the breakdown voltages of gas mixtures under different conditions are fitted, and the goodness of fit and significance test are used to evaluate the fitting function, and finally the quadratic polynomial function is used to fit. Based on the above research, a SF₆/N₂ gas discharge model is established by using the dynamic breakdown characteristics and the calculation of the rise time constant in the single discharge model of the gas mixture is solved according to the dynamic breakdown characteristics of the gas mixture. Finally, taking a typical 550kV GIS spacer circuit as an example, the operating overvoltage of the SF₆/N₂ mixed gas medium GIS disconnecting switch is calculated. Calculation results show that the VFTO_SF6/N₂ waveforms and amplitudes generated by the repeated breakdowns of the SF₆/N₂ gas mixture with the same insulation strength are similar to those of the pure SF₆ during the operation of isolation switch, and they have the same breakdown times. When the SF₆ volume proportion in the SF₆/N₂ gas mixture increases from 20% to 40% under the 0.6MPa chamber pressure, the breakdown times of the disconnecting switch and the duration of the VFTO_SF6/N₂ process reduce by about 50%. Under the condition of 30% SF₆ volume ratio, when the pressure of the gas chamber increases from 0.6MPa to 0.8MPa, the breakdown times of the disconnecting switch are reduced by half, and the duration of the whole process of the VFTO_SF6/N₂ during switching is shortened by about 30%.