Monitoring of SF6 gas admixtures in gas-insulated electrical equipment

Mixtures of SF
₆
and N
₂
are used in gas-insulated lines to reduce the amount of SF
₆
in use and thus minimize environmental impact. Outdoor-installed electrical equipment operating in low-temperature regions currently also entails the use of mixtures of SF
₆
gas with N
₂
or CF
₄
. The dielectric strength of SF
₆
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.