Long-term stability of synthetic air in gas-insulated systems and the impact of gas quality on the electric and ageing performance

SF6. Synthetic air (clean air) is now applied as a fluorine-free insulating gas in GIS. It complies with European and other F-gas regulations, is chemically stable, non-toxic, and allows easy gas handling. For GIS with long service life (40-50 years), aspects like required gas quality and long-term stability must be known and respective investigations were conducted. The gas composition is of influence on the electric performance. This was investigated in detail. It was found that clean air is insensitive to O2 ratio tolerances in a wide range, and traces of other gases had minor impact on the electric performance. Tests with filtered ambient air instead of synthetic air showed promising results in terms of the electric performance. Over time, gas permeation through sealings may alter gas pressure and the mixing ratio. Permeation tests and calculations were conducted. It was found that N2 and O2 permeate similarly, considering the relation between permeation coefficients and partial pressures, so there are no significant changes in composition or electrical properties, even after decades. During operation, small amounts of nitrogen oxides can form due to electrical arcs or partial discharges, but analyses and testing showed that these byproducts do not significantly affect materials, electric function, or health. Special handling measures are only needed for rare events like internal arc.