Comparison of Mitigation Techniques for Very Fast Transients in a 1000kV GIS

Gas Insulated Substations (GIS) are gaining popularity over Air Insulated Substations (AIS) due to less space requirements and increased reliability. However very fast transient overvoltage (VFTO) occurring in gas insulated substations is one of the major concerns for its insulation design. In Gas Insulated Substations SF6 Gas insulation is subjected to electrical stress produced by high frequency switching transients called VFTOs (Very Fast Transient Over Voltages). VFTOs arise within a GIS any time, when an instantaneous change in voltage occurs in the system. Mostly these changes occur with the result of closing or opening operations of disconnecting switches in the system or Line to ground fault. The slow operating speed of disconnector moving contacts will induce the great number of pre-strikes and re-strikes, is the main cause of VFTOs in GIS. The Very Fast Transient currents associated with VFTOs will produce Electromagnetic fields causes electromagnetic interference in secondary control circuits, this will lead to maloperation of the secondary equipment such relays, Data acquisition systems equipment. In this paper VFTOs are analysed in a 1000KV UHV system, the disconnector switching operations are performed in EMTP Software and computed the peak values of VFTOs at different locations. Various mitigating methods are incorporated to reduce the peak value & rise time of VFTOs.