Comparison of gases for electrical insulation: Fundamental concepts

Several fluorinated compounds have been introduced by chemical and equipment manufacturers to replace the widely used sulfur hexafluoride (SF6) as electrical insulation gas. Unlike SF6, these compounds are admixed to the atmospheric gases nitrogen, oxygen or carbon dioxide to avoid liquefaction at typical operating temperatures. The aim of this paper is to give a direct and clear comparison of the two key parameters, namely the breakdown voltage and the minimum operating temperature, of recently proposed insulation gases as well as to support the selection process of novel compounds. This article covers the basic physics and essential models necessary to characterize the electric strength and the liquefaction temperature of gas mixtures. This includes the description of electron avalanches, negative ion formation by electron attachment and the phenomenon of three-body attachment.