Drift-diffusion modelling of breakdown of SF6 mixtures

Summary

In recent years, SF6 mixtures have been considered as a replacement for pure SF6 due to economic and environmental issues. In particular, SF6-N2/CO2 mixtures are considered as suitable replacements as they demonstrate strong synergistic effects. The synergistic effects of SF6 gas mixtures are determined by the ability of the additive gas to slow down the free electrons sufficiently to enable their removal by the SF6 molecules. Thus, the synergistic effect is time dependent and affected by the dynamic behaviour of the gas discharge. A drift-diffusion model is used to explore the dynamic behaviour of gas discharge in a mixture, where a Boltzmann equation solver, the electron energy distribution function, the Townsend coefficients and the transport coefficients are used to provide input to the gas discharge model. The model clearly indicates the synergy effect in the mixtures and the impact of the additive gas on the dynamic behaviour of the gas discharge. The SF6-N2 mixtures show increasing breakdown voltage with increasing pressure and the greatest increase in breakdown strength occurring for lower SF6 content (0% - 30%) while SF6-CO2 mixtures, show the greatest increase in breakdown strength at higher SF6 content (>50%). Experimental testing of the synergy effects of the SF6-N2/CO2 mixtures under positive and negative lightning impulse for uniform electric field conditions with pressures from up to 300 kPa and SF6 content from 0% to 100% validated the model. The results were used to determine the influential factors and increase the accuracy of the model for future studies of alternate gas mixtures.

Additional informations

Authors

ZOMERS, DE GRAAFF

Keywords

SF6-N2, SF6-CO2, modelling, drift-diffusion