Prebreakdown studies in binary mixtures of electronegative and buffer gases

Abstract

Electronegative gases of high dielectric strength such as SF?, CCl?F?, etc., are extensively used as a gaseous insulating medium in high voltage transmission systems and power apparatus. Such gases are expensive, and hence research in recent times has been directed towards evolving suitable gas mixtures by admixing electronegative gases with low-cost buffer gases like N?, Air, CO?, etc. Although a fair amount of data is available on the breakdown strength of various gas mixtures, very little information is available on the fundamental physical processes that occur prior to breakdown. The work reported in the thesis is an attempt to determine pre-breakdown swarm coefficients ?, ?, ?, electron D/? in SF?, CCl?F?, CO?, mixtures of N?-O?, N?-O?-CO?, CCl?F?-N?. A simple technique has been proposed for the first time to evaluate attachment (?) in SF? and CCl?F? at low E/N where ionisation is negligible. Data obtained in all the gas mixtures have been used to propose an empirical relationship using which breakdown voltages could be evaluated for new gas mixtures with varying concentrations of the constituents. It has been shown that the constant which occurs in various proposed empirical relationships depends solely on the nature of the electronegative gas and has been termed as the Synergism Factor. It represents the change of physical processes occurring in electrically stressed binary mixtures under uniform electric fields. This study clearly establishes the significance of the pre-breakdown processes in the determination of the characteristics of gaseous mixtures as high voltage insulants. This would be very valuable information for design engineers using gaseous mixtures as insulants.