Monitoring of SF6 gas admixtures in gas-insulated electrical equipment

Mixtures of SF₆ and N₂ are used in gas-insulated lines to reduce the amount of SF₆ in use and thus minimize environmental impact. Outdoor-installed electrical equipment operating in low-temperature regions currently also entails the use of mixtures of SF₆ gas with N₂ or CF₄. The dielectric strength of SF₆ mixtures depends on the density and the mixing ratio which can change as a result of leakage events, condensation, adsorption or decomposition. A general leak can be detected when the total density drops, but no information on the mixing ratio can be obtained. We have designed and experimentally verified a straightforward and easily implementable solution to this problem. Measurement of pressure, temperature and density allows the calculation of the partial pressures and hence of the mixing ratio of a two-component gas. The determined ratio can be processed analogously to today´s density tracking, enabling early detection of faults in which one gas is lost preferentially. We report on the performance of such a sensor system in the laboratory.