Calculation of temperature dependent critical field strength of mixtures containing C5-FK, CO2 and O2

Many of today's novel SF6-free gas mixtures for high-and medium-voltage insulation use low-percentage admixtures of perfluoroketone (C5-FK) or perfluoronitrile (C4-FN) in mixtures of CO2, O2, and/or N2.Mixture compositions vary widely based on target operating conditions, with several different mixtures being reported in literature.Testing each mixture composition in real equipment is time consuming and financially too expensive.While carrying out classical breakdown tests is simple, it requires numerous tests to cover all relevant technical geometries, including those with defects and consequently high field non-uniformities.Adding to the complexity, the breakdown mechanism in non-uniform fields depends on the ambient pressure, field utilization factor and gas properties, among other factors.Models for characterizing the insulation properties of pure gases under arbitrary electrode configurations and arbitrary voltage waveshapes exist [1 -4].Important input for these models is the temperature dependent critical field strength and thermodynamic properties of the gas.A first step towards using these models to investigate the insulation properties of gas mixtures is an accurate chemical kinetic description of the gas mixtures.Recently, chemical decomposition and recombination pathways of pure C5-FK and C4-FN have received much attention [5].In the present contribution we study kinetic models for mixtures containing C5-FK, CO2 and O2 based on ab-initio calculations.We include reactions of the carrier gas molecules, CO2 and O2 and present a method for calculating the temperature dependent critical field strength of mixtures containing C5-FK, CO2 and O2.Results are given for a selected mixture over a wide pressure and temperature range, and we motivate the need for more thorough kinetic modelling of the C5-FK decomposition process.