23 rd International Conference on Electricity Distribution Lyon, 15-18 June 2015 Paper 0926 CIRED 2015 1/5 DIELECTRIC PROPERTIES OF GASES SUITABLE FOR SECONDARY MEDIUM VOLTAGE SWITCHGEAR Magne SAXEGAARD Martin KRISTOFFERSEN Patrick STOLLER ABB – Norway ABB – Norway ABB - Switzerland magne.saxegaard@no.abb.com martin.kristoffersen@no.abb.com patrick.stoller@ch.abb.com Martin SEEGER Maik HYRENBACH Henrik LANDSVERK ABB – Switzerland ABB – Germany ABB - Norway martin.seeger@ch.abb.com maik.hyrenbach@de.abb.com henrik.landsverk@no.abb.com ABSTRACT 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. INTRODUCTION Medium voltage (MV, 1- 72 kV) gas insulated switchgear and ring main units (RMU) are key components in the primary and secondary distribution network. In RMUs, a collection of switches is typically placed in a single, gas- filled compartment. The gas serves as a medium for electrical insulation, cooling and current interruption of load currents. SF6 provides all these features in compact equipment and has dominated the power distribution industry for several decades. Unfortunately, SF6 has a very high GWP of ~23,000 CO2 equivalents. MV switchgear is sealed for life with less than 0.1% emissions per year, but unwanted emissions can occur due to improper handling. The industry is therefore eager to find a suitable alternative. The EU has further targeted secondary MV switchgear specifically and aims to assess the situation by July 2020 [1]. If it is found that a viable alternative exist, a proposal will be made to restrict the use of SF6 in these products. The dielectric strength of alternative gases is of particular interest because it determines the minimum electrode separation and the external dimensions of the RMU. Backwards compatibility with existing installations is a strict requirement in RMU design and increasing the outer dimensions is therefore not an option. Dry air offers the lowest possible environmental impact at a very low cost, but has a limited dielectric strength. As recently demonstrated by Bjørtuft et.al [2,3], 12 kV ratings can be achieved with dry air alone, but higher ratings will require larger outer dimensions. Several SF6 alternatives with reduced GWP have been investigated over the years [4,5], including SF6/N2 mixtures [5], perfluorcarbons (PFC) [6], fluoroxiranes (FO) [7] and fluoronitriles [8]. Excluding SF6/N2 mixtures, the common drawback for these synthetic fluids is their high boiling point. In order to utilize them in switchgear with operating temperatures as low as -25°C, they would typically have to be mixed with a background gas, such as air or CO2. Another promising alternative that offers high dielectric strength and a very low GWP of 1 was recently presented by Mantilla et.al. [9]. The gas is a mixture of perfluorinated ketones (PFK) and technical air or CO2. In this paper, we compare key properties of the above-mentioned SF6 alternatives and present studies of the −25°C PFK/air mixture at 1.3 bar, suitable for secondary MV switchgear.