Shielding Effectiveness of the Gas-Insulated Bus Duct for Transient EM Fields Generated in a GIS During Switching Operations

Characterization and quantification of the transient electromagnetic (EM) fields generated in a gas-insulated substation (GIS) during switching operations has become important in recent years. This is due to increased use of sensitive electronic devices in the control circuitry of such substations. In this paper, a numerical model based on the finite-difference time-domain technique has been developed to compute the EM field emission from the gas-insulated bus duct through nonmetallic flange of the support insulator during switching operations. The shielding effectiveness (SE) of the bus duct/section for the EM fields has been evaluated using a derivative Gaussian current source and a short time sinusoidal current source. The effect of variations in dimensions of the slot formed by the nonmetallic flange as well as electromagnetic properties of the insulator/bus duct enclosure on SE has been studied. The variation in field pattern along the axis of bus duct at various distances due to the transient currents of different frequencies has been analyzed. The enhancement factor for the EM field levels due to the presence of a metallic structure has been reported. Finally, emission levels are calculated for the very fast transient currents generated in a 245 kV GIS and their dominant frequencies have been identified.