Experimental studies of breakdown characteristics of SF|6/CF|4 mixtures in highly non-uniform fields for high-voltage applications

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Toufani, Kamran
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Pure SF$\sb6$ liquefies at approximately $-$33$\sp\circ$C at increased pressure range of the practical high-voltage gas-insulated switchgears (3.0-5.0 bar). The use of SF$\sb6$/CF$\sb4$ mixtures with lower condensation temperature and comparable dielectric properties, is proposed to overcome the SF$\sb6$ limitations for operation in extreme cold climates (i.e. Canada). Recent studies have shown that the SF$\sb6$/CF$\sb4$ mixture when mixed in certain proportions exhibits strong positive synergism for application of ac and dc voltages in highly divergent fields. The present thesis aimed to investigate thoroughly the dielectric strength of SF$\sb6$/CF$\sb4$ mixtures over the range of pressure used in practical high-voltage apparatus using highly divergent fields and a wide range of mixture compositions. The SF$\sb6$/CF$\sb4$ mixtures were studied under ac, dc and standard lightning impulse voltages (positive and negative polarities). The time lags for lightning impulse breakdown voltages and corona inception for ac and dc voltages were recorded. The occurrence of strong positive synergism was validated for dc and ac voltage applications in non-uniform fields.