Comparison of the Breakdown Strength of N2, CO2 and SF6 using the Extended Up-and-Down Method

Nowadays in gas insulated systems (GIS), SF₆ is often used as insulating medium. Due to the fact that SF₆ is a greenhouse gas with a very high global warming potential (GWP), alternatives have been searched for over the last years. In particular, SF₆/N₂ gas mixtures have been extensively investigated for the use in gas insulated lines (GIL) and is being used in several applications. However, in these cases SF₆ is still part of the insulating gas and the search for alternatives continues. In this respect, nitrogen (N₂) and carbon dioxide (CO₂) are interesting alternatives. In this paper, investigations into the breakdown strength of these gases will be described. Both gases were investigated in a plane-plane configuration, with a distance of 10 mm between the planes. The gas pressure was varied in steps of 2 bar, from 7 to 11 bar. To get to the 5% breakdown value under positive and negative lightning as well as switching impulses, the extended up-and-down method was applied and will be discussed in the paper. The results are compared to 3 and 4 bar SF₆ to get the required data that can be used for design purposes