Electromagnetic Field Emission From Gas-to-Air Bushing in a GIS During Switching Operations

Very fast transient overvoltages (VFTO) and the associated very fast transient currents (VFTC) generated during switching operations in a gas-insulated substation (GIS) radiate electromagnetic (EM) fields, which in turn can leak into the external environment through apertures like SF 6 gas-to-air bushing. Therefore, it becomes necessary to characterize and quantify the EM fields when sensitive control devices are used for the operation of such substations. Keeping this in view, a numerical model using a finite-difference time-domain (FDTD) technique was developed to compute the transient field emission from a 245-kV-rated gas- to-air bushing. Shielding effectiveness (SE) of the bushing for these transient fields was evaluated using a derivative Gaussian current excitation of the high-tension (HT) conductor. The variation in EM field levels along the axis of bushing at various radial distances for different frequencies of the transient current were analyzed. The enhancement of the field levels due to the presence of a metallic structure on the ground plane, which simulates the control cubicle or the grounded enclosures of the GIS, were studied. Finally, the emission levels from different bushing models were calculated for the VFTC generated during a switching event, and the dominant frequencies were identified.