Breakdown behavior of insulating gases for HVDC applications

Since the early 1960ies, Sulfur Hexafluoride (SF₆) has been used as insulating and arc-quenching gas in gas insulated high voltage equipment. SF₆ has nearly ideal dielectric, physical and chemical insulating properties. It has, however extremely high global warming potential as well as large atmospheric life time. Due to those facts the need of new, environmentally compatible gaseous dielectrics as an alternative to SF₆ is given. Particularly since the publication of the Kyoto Protocol in 1997, the research on SF₆ substitutes has been intensified. Various gas mixtures, above all those- with major amount of Nitrogen gas (N₂), has been tested. N₂ is abundant, cheap, inert, non-toxic, non-flammable and unquestionably environmentally acceptable. This paper contains the discussion and the comparison of results of experimental investigations on breakdown behavior of SF₆, N₂/SF₆ (80/20) gas mixture and pure N₂ in homogenous and inhomogeneous DC fields. The experiments were carried out at the Institute of High Voltage Engineering at the Graz University of Technology. Both, the insulation qualities and the switching behavior of ecologically friendly gases are important. So far, no acceptable substitute has been found. Therefore extensive research and experiments under a great variety of different surrounding conditions are necessary to find a new insulation gas mixture with comparable breakdown and withstand voltages than SF₆ not only for DC, but also for AC voltages.