Study of the withstand voltage of sulfurhexafluoride under sharp oscillating waves

Gas insulated switchgears are now widely used in high voltage power network. The success of this technology is mainly due to a high level of performances. So some unexpected failures in dielectric withstand which appeared during operation of disconnectors have brought suspicion about the viability of this kind of H.V. apparatus. It is now well known that a lot of fast transient voltages with very high front steepnesses and significant peak values may arise inside GIS with disconnector operation. Indeed during this operation a lot of sparks are taking place between the contacts. Futher-more the structure of GIS, that is generally a coaxial one, facilitate propagation and reflections of transients with a quite low attenuation. In this way steepened over-voltages with rise time typically less than 100 nanoseconds are generated. So these surges could be reasonably supposed to be responsible for the breakdowns. Hence manufacturers and electrical boards have shown the greatest interest for studies which could attempt a better knowledge of the behaviour of GIS under this kind of stress. Obviously this behaviour is principaly dependent on dielectric properties of sulfurhexafluoride in the short time region (time to breakdown less than 1 μs). This work deals with this point.