Numerical analysis of very fast transient overvoltage in GIS

Very fast transient over voltage (VFTO) is a kind of phenomenon which commonly occurs in gas insulated switchgear(GIS), and is also receiving more and more attention since it may endanger GIS internal insulation and secondary equipment. For this reason, combining with the VFTO phenomenon in GIS, this paper completed the numerical simulation calculation of VFTO transient process gone through by gas gap in GIS, established an one-dimensional self-consistent hydrodynamic model of gas discharging, in which the electron and ion continuity equations and the Poisson equation of coupled electric field are contained, furthermore, utilized the improved SG algorithm and finite difference method to get numerical solutions of the above equations. This not only provided a new calculation method for VFTO simulation study, but also the breakdown process of GIS gas gap caused by VFTO can be clearly described by simulation results. At the same time, this paper as well showed the numerical simulation results of breakdown characteristics under different atmosphere pressure and different SF₆/N₂ partial pressure ratio, and obtained VFTO time-frequency relations image and three dimensional graphic of gas-gap electric field distribution which is evolved over time. The results show that the frequency bands that VFTO appears mostly are those between 50MHz to 270MHz; when pressure is increased to 0.4~0.6Mpa, the recovery strength of GIS insulation is greatly improved; without increasing the pressure, most of the insulation strength of SF₆ gas mixture are less than that of pure SF₆.