Because of its excellent dielectric properties, sulfur hexafluoride (SF6) is currently the most used gas in gas insulated substations (GIS). However, its greenhouse capacities are well known, with a global warming potential of 23900 times that of CO2 over a 100 years period, and an approximately 3200 years lifetime in the atmosphere. Thus, since the Kyoto summit (COP 3), international authorities tend to reduce or ban its use. For the past several years, manufacturers investigated more eco-friendly alternative solutions, showing equivalent or superior physical properties in comparison with SF6. One of the suggested solutions is the fluoronitrile (C4F7N), a gas having alone a dielectric withstand 2.7 times higher than SF6. However, its high liquefaction temperature requires to dilute it for use, mixed with CO2. O2 can also be added to address issues regarding the gas mixture decomposition during arc interruption. The resulting gas mixture can be used at relatively low temperature, with dielectric properties close to that of SF6. To correctly use this new mixture in GIS, its characterization must be completed by the study of its dielectric behavior at the interface with solid insulators used in the stations. Based upon previous studies found in literature, a first set of experiments was conducted under radial geometry in order to measure the discharges extensions without flashover, to be compared to those obtained in SF6. A second comparative study was then conducted in tangential electric field geometry, allowing to measure streamers propagation velocities during flashover. O2 influence on the gas mixture dielectric strength was also examined. Finally, in order to give sizing criteria for GIS using fluoronitrile/CO2 mixtures as insulating gas, flashover voltages measurements were made under various experimental conditions, including particle pollution on surface, under lightning impulse and alternative voltage.
Pr Abderrahmane Beroual
In collaboration with Laboration Ampère, Ecole Centrale de Lyon