Ternary gas mixtures for high-voltage industrial insulation

New gas dielectrics for insulating power apparatus (e.g., gas insulated transmission lines (GITL) are evaluated. The comprehensive Oak Ridge National Laboratory gas dielectrics program has found that a mixture of gases, chosen via the fundamental physics of electron density and energy control and other chemical and physical properties, is best for insulation in a given application. Particular attention was given to mixtures using large proportions of an electron-moderating gas (viz., N/sub 2/, CHF/sub 3/, or 1,1,1-C/sub 2/H/sub 3/F/sub 3/) and smaller quantities of two electron attaching gases: SF/sub 6/ and one fluorocarbon (e.g., c-C/sub 4/F/sub 8/). The proportions were sought at which the three gases function best as a team, moderating free electrons from higher to lower energies and there attaching them. Small amounts of the electron attaching additives can drastically raise the dielectric strength of the moderator. Certain combinations of SF/sub 6/ and fluorocarbons do not produce the undesirable spark by-products associated with either SF/sub 6/ or fluorocarbons alone and also show improved negative polarity impulse breakdown characteristics over pure SF/sub 6/. Small scale breakdown measurements were made of various ternary mixtures in the GITL geometry: concentric cylinders, with intentionally roughened surfaces. It was found that nitrogen moderates electrons more » to extremely low energies, where SF/sub 6/ attaches; the dipolar gases moderate to somewhat higher energies, where some fluorocarbons attach best. Optimum ternary mixtures are identified (e.g., 20% SF/sub 6/ + 10% c-C/sub 4/F/sub 8/ + 70% N/sub 2/; 20% SF/sub 6/ + 15% c-C/sub 4/F/sub 8/ + 65% CHF/sub 3/). These will be discussed for possible use in power apparatus. « less