In-Situ Measurements of SF₆ Leak Rates in Indoor Gas-Insulated Switchgears (GIS)

Abstract

All SF₆-insulated equipment leaks to some degree. These losses of SF₆ gas are usually low and usually less than 2% of the whole gas content of typical GIS. They occur slowly and almost undetected for many years, until a low gas pressure alarm sounds. Over the last fifteen years, manufacturers have continuously improve the gas tightness of their metal-clad equipment which now exhibit leak rates roughly four time lower. Nevertheless, environmental concerns have been expressed in the last five years, peaking at the Kyoto meeting, about the cumulative effects of SF₆ in the atmosphere. Remembering the PCB problems, electric utilities start evaluating their SF₆ losses, since 80% of the SF₆ gas lost into the air annually come from electrical equipment. At Hydro-Quebec, we realized an evaluation program aimed at the direct measurement of the SF₆ leak rate of indoor GIS. Two substations were fitted with a sensitive SF₆ trace analyzer and monitored during 9 and 13 months respectively, in two consecutive periods.SF₆ gas was always present in the GIS surrounding air, at concentrations varying typically between 20 and ≈100 ppbv, but sometimes higher. These fluctuations were not due to the GIS leak rate changes but resulted mostly from the variations of the air exchange rate of the building due to external (climatic) and internal (heating, ventilation, human activities) factors. Interestingly, these measurements have shown that the two 20-years-old GIS studied exhibits average SF₆ lost rates of only 0.55% or less.