The very fast transient over voltages (VFTOs) generated due to switching operations of gas insulated substations (GIS) are dangerous to the equipment’s connected to the substations. In special cases these transients may cross the Basic Insulation Level (BIL) of the GIS. In this paper, these transient voltages are investigated at the different sensitive locations of a 400 KV GIS. Different techniques for mitigation and analysis of very fast transient over voltages (VFTOs) in the 400KV GIS are applied and the results are analyzed in these paper. Hence, the suppression of the VFTOs is also an important field of interest. The Different suppression techniques to reduce the VFTOs are also discussed.

Keywords : Very Fast Transient Over Voltages (VFTOs),Basic insulation level (BIL), Gas insulated substation (GIS),Air insulated substations (AIS).