SYSTEMS TASK FORCE D1.03.10 Members: W. Boeck (Convener), T.R. Blackburn, A.H. Cookson, A. Diessner, F. Dorier, F. Endo, M. Ermel, K. Feser, A. Giboulet, A. Girodet, S. Halliday, B.F. Hampton, W. Koltunowicz*, H.-G. Kranz, J. Lopez-Roldan, L. Lundgaard, Li Ming, S. Meijer, C. Neumann, R. Pietsch, U. Riechert, W. Rutgers, U. Schichler, H. Slowikowska *CESI Via Rubattino, 54 20134 Milano SUMMARY N2/SF6 gas mixtures possess good insulation properties, even at low SF6 contents. Adequate dielectric properties can be achieved with SF6 contents of 10 to 20%, which are regarded as reasonable for GIL applications when technical, economic and environmental aspects are considered. Only a modest pressure increase of about 45 to 70% is necessary to recover the dielectric strength of pure SF6, and the amount of SF6 required, and the SF6 leakage rate, will be considerably reduced by approximately 70 to 85%. The influence of field enhancements caused by electrode curvatures and surface roughness can easily be taken into account for the design of the equipment. The breakdown voltage in the presence of defects in such mixtures is slightly lower than in pure SF6 of equal dielectric strength. However, all existing diagnostic systems can be applied in these mixtures, with an equivalent or higher detection sensitivity than in pure SF6. The discharge current and signal emission of fixed defects at live parts are similar to those in pure SF6. The signal emission from mobile particles is independent of the gas type and mixture ratio. All methods for defect diagnosis can be applied in the same way as for SF6 insulation in GIS. The heat transfer capability and cooling properties are similar for SF6 and N2/SF6 mixtures of equal dielectric strength. Such mixtures therefore permit the same power transfer capability for a GIL as pure SF6. The pressure increase by an internal arc is higher than in pure SF6, but in GIL there are usually large gas volumes which restrict the overpressure. The resulting roots of internal arcs are much smoother than in pure SF6. The arc quenching ability at current zero, and the resulting current interruption performance in such mixtures, is poor. Even the switching capability of disconnectors for small bus charging currents is considerably degraded. The leader channel changes its direction more often, and there is a much higher risk for leader branching and flashover to ground during arcing between contacts in such mixtures than in SF6. Therefore these mixtures are not suitable for any switching duties. The total reduction in SF6 used in a partially N2/SF6 insulated GIS where all compartments with switching devices are filled with pure SF6 would be minimal. In GIS, therefore, the substitution of SF6 by N2/SF6 mixtures would lead to uneconomical technical solutions and would have no ecological advantage.