Fluoronitriles/ CO2 gas mixture as promising substitute to SF6 for insulation in high voltage applications

This paper is aimed at the breakdown characteristics of Fluoronitriles – CO2 gas mixtures in different experimental conditions; these mixtures constitute promising substitutes to SF6 gas in high voltage applications especially gas insulating switchgear (GIS). Fluoronitriles chemical gas compound based on 3MTM NOVEC 4710TM have a high dielectric strength, more than 2 times that of SF6 and a low Global Warming Potential (GWP). Mixed with CO2 as gas carrier, the obtained mixtures offer interesting dielectric properties and the possibility to be used for low temperature applications. The experiments are conducted with different electrodes geometries namely plane-to-plane, sphere-to-sphere, sphere-to-plane and rod-to-plane (i.e. in homogeneous, quasi-homogeneous and inhomogeneous electric field distribution) and different field utilization factors, under AC and lightning impulse voltages. The same experiments are reproduced for pure SF6 for the comparison. The comparison of breakdown voltages results of mixtures with different concentrations of Fluoronitriles in CO2 in sphere-to-sphere electrodes arrangement shows that the 3.7 % Fluoronitriles / 96.3% CO2 mixture constitutes a good compromise and an appropriate gas mixture for high voltage apparatus insulation in point of view of pressure and  low ambient temperature application (-30°C).

These geometry differences result in changes in dielectric spectra that are analysed. The impact of the series resistance at high frequency due to the different contact resistance resulting from the different counter-electrodes is in particular highlighted.