Dielectric Properties of Gases Suitable for Secondary Medium Voltage Switchgear

SF6 is the dominating technology for gas insulated switchgear due to its excellent dielectric, thermal and arc quenching properties. Unfortunately, SF6 suffers from a very high global warming potential (GWP). Even though medium voltage switchgear is sealed for life with less than 0.1% emissions per year, the EU has specifically targeted SF6 for replacement in medium voltage switchgear for secondary distribution. The dielectric strength of alternative gases is of particular interest due to its impact on minimum electrode separation and external dimensions of the switchgear. A promising alternative with very low GWP and high dielectric strength is given by a mixture of perfluorinated ketones (PFK) and air or CO2. In this paper, we present studies of the dielectric properties of a PFK/air mixture with a dew point below -25°C, making it suitable for secondary medium voltage switchgear. The withstand level of an optimized design is predicted for air and PFK/air using the streamer theory and compared with the results of full-scale tests of lightning impulse and power frequency withstand levels. The measured lightning impulse withstand level is found to be ~60% higher for the PFK/air mixture than for pure air. Finally, we show that switchgear designed for 12 kV rated voltage insulated with dry air can be scaled up to 24 kV rated voltage by replacing the dry air with the -25°C PFK/air mixture.