Computation of EMI fields in a high voltage gas insulated substation during switching operations

Numerical characterization of the transient electromagnetic field generated in a gas insulated substation during the operation of a disconnector switch or a circuit breaker is essential when sensitive control devices using VLSI circuits are being used in modern gas insulated substations. The quantification of emission levels from the gas insulated equipment during switching events is found to be important for EMC design of control devices operating in such harsh EM environment and hence to ensure reliable operation of the systems. In the present study, transient electromagnetic field emission from gas to air bushing through the silicon rubber insulated housing has been quantified for Very Fast Transient Currents (VFTC) which is generated during switching events in a 245 kV GIS. For this purpose, the modified version of finite difference time domain (FDTD) technique has been adopted. For the estimation of transient field emission levels, a computational model has been developed for the 245 kV gas to air bushing. The variation of EM field pattern along the axis of the bushing at various radial distances is plotted and the effect of parameters like frequency of transient current, physical positioning of the bushing above the ground plane etc., on emission levels is analyzed as part of the present study. The range of enhancement factor of the transient field levels due to the presence of a metallic structure on the ground plane, which simulates the control cubicle in a GIS is established. Finally, the transient EM field emission levels from the gas to air bushing for the VFTC waveform calculated at the entrance of the bushing are evaluated.