Statistical assessment of very fast transient overvoltages in gas insulated substations

Summary form only given. VFT in GIS can be divided into internal and external. Internal transients can produce overvoltages between the inner conductor and the enclosure, external transients can cause stress on secondary and adjacent equipment. The magnitude of internal transients is in the range of 1.5 to 2 per unit of the line-to-neutral voltage crest, but they can also reach values as high as 2.5 per unit. External VFT can be dangerous for secondary and adjacent equipment. These external transients include transient voltages between the enclosure and ground at GIS-air interfaces, voltages across insulating spacers in the vicinity of GIS current transformers, when they do not have a metallic screen on the outside surface, voltages on the secondary terminals of GIS instrument transformers, and radiated electromagnetic fields (EMF). Disconnector operations in GIS can cause a wide range of problems inside and outside the enclosure. Each disconnector operation usually generates a large number of ignitions between contacts. The travelling wave which is originated by an ignition propagates in the GIS and can result in overvoltages whose level and waveshape depend on the GIS configuration, the measuring point and the disconnector characteristics. This paper presents an introduction to the origin of VFT in GIS. These transients and their magnitude are random by nature; a significant effort has been performed to determine the statistical distribution of VFT overvoltages in a GIS. The document includes a summary of procedures for statistical calculation of VFT overvoltages originated by disconnector operations in GIS