Corona stabilization and breakdown in SF6 and SF6/N2 mixtures in concentric cylindrical gaps
Corona and breakdown characteristics of compressed SF6 and SF6/N2 mixtures are studied experimentally and theoretically in the pressure range 0-8 bar for a wide range of gap nonuniformities under static and impulse voltages. The dielectric strength of SF6 is compared with that of SF6/N2 mixtures for the factors important in the design of CGI systems. A comprehensive literature survey is given in the first chapter. Basic gas discharge mechanisms and associated processes are reviewed. Factors degrading the dielectric strength of the gases, especially those of SF6, are delineated and proposed models are discussed. Finally, a systematic review is made of recent findings on corona stabilization and corona discharge phenomena. The results of series conditioning tests performed in SF6 and SF/N2 mixtures for various gas and gap combinations are demonstrated in the third chapter. The conditioning is found to be prolonged both by increasing the electrode surface roughness and N2 concentration. The mixtures are more polarity dependent with rough electrodes. The statistical assessment of conditioning results for area effect by applying the Weibull type extreme value distribution is made in the fourth chapter. The Maximum Likelihood Method is adapted for the estimation of the distribution parameters, and a method is described to determine the tolerance bounds on each parameter for a given quantile. The area effect is shown to be more pronounced with increasing N2 concentration. The slopes of distributions for rough electrodes are found to lie in the range 6-20. The conditioned breakdown levels of successive breakdown tests is used for the evaluation of the breakdown performance of SF6 and SF6/N2 mixtures in the fifth chapter. The limiting field strength at which the deviation from the similarity starts is ~ 180 kV/cm for polished electrodes and ~ 150 kV/cm for rough electrodes. Breakdown calculations made with an isolated triangular model of 2μm height, show good agreement with experimental results obtained for polished electrodes. The compressibility factor calculated for mixtures is used in breakdown estimations. The corona stabilized breakdown characteristics of SF6 and SF6/N2 mixtures are studied in the sixth chapter. The voltage pressure characteristics exhibit the same anomaly as is observed in point-plane gaps. The rising part of the curve is dependent upon the radius of the outer electrode and the mixture ratio, and at breakdown the space charge field strength achieves a value in the range 35 - 45 kV/cm. The downcurving part of the curve is rather reproducible. The critical pressure is independent of the mixture ratio. The seventh chapter deals with corona discharge phenomena SF6 and SF 6/N2 mixtures. The corona is identified as a streamer corona, although in mixtures over the downcurving part of the characteristic, stepwise streamer/leader discharge activity is observed. The ion mobilities is found to exhibit pressure and field dependence at pressures less than 1 bar.