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Very fast transient overvoltages (VFTOs) and the associated very fast transient currents (VFTCs) 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 gasto-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. Index Terms-Electromagnetic interference (EMI), finitedifference time-domain (FDTD), gas-insulated substation (GIS), switching, very fast transient currents (VFTCs), very fast transient overvoltages (VFTOs). I. INTRODUCTION I N A GAS-INSULATED substation (GIS), very fast transient overvoltages (VFTOs) are generated due to switching operations. These transient overvoltages and the associated very fast transient currents (VFTCs) could have a rise time of the order of 4 ns and above [1]. The peak magnitude of the transient current may be about a few kiloamperes depending on the location of the switch operated, the substation layout, and the observation point. These transient voltages/currents radiate electromagnetic (EM) fields during their propagation along the coaxial highvoltage (HV) bus of the GIS, as the associated frequencies are in the range of a few megahertz to about a few hundreds of megahertz. The transient EM fields, in turn, leak into the external environment through apertures like SF 6 gas-to-air bushing, gas-to-cable termination, nonmetallic viewing ports, insulated flanges, etc., and couple to the control equipment/data cables present within the GIS. This coupling produces transient current/voltage on the shield of the control cable. Depending on the

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