Study of switching behaviour of g³ based gas mixtures

Abstract Ten years after its invention, g 3 — the mixture of C 4 F 7 N, CO 2 and O 2 — remains the most promising candidate for the substitution of SF 6 as an insulating and switching gas. The extension and optimization of g 3 applications at a high-voltage level, especially in circuit breakers, requires knowledge of the properties and dynamics of arc discharges in this gas. The features of the arc plasma can be obtained from appropriate magneto-hydrodynamic simulations. To predict the arc behaviour, a consistent set of thermo-physical and radiation properties is necessary. The corresponding database is calculated for the case of local thermodynamic equilibrium and covers the temperature range 250–40 000 K, the pressure range 0.01–150 bar and various contents of C 4 F 7 N, O 2 and Teflon (nozzle material). This contribution presents examples of plasma composition, thermo-physical and radiation properties for g 3 mixtures. The feasibility of the new database is demonstrated on an example of simulation results for a g 3 -filled high-voltage circuit breaker. Predicted arc voltage and pressure build-up in selected volumes are compared with available experimental data. Furthermore, to explore the differences in switching behaviour between g 3 and SF 6 , simulations for these two gases at the same operation conditions were performed. The space- and time-dependent temperature and pressure profiles are presented and discussed.