Seismic protection of electrical transformer bushing systems by stiffening techniques
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The past few decades, the electrical substation equipment has shown vulnerable behavior under strong earthquakes resulting in severe damages of the electric power networks. High voltage bushings, which are insulated conductors providing electrical connections between high voltage lines and oil-filled transformers, are the most critical as well as the most vulnerable components of the electrical substations. Rehabilitation of existing bushing structures and proper design of new ones could considerably reduce potential damages on them. Several experimental and numerical studies conducted to investigate the seismic performance of transformer bushing structures have shown that improved seismic performance may be achieved for bushings mounted on rigid base compared to those mounted on actual transformer tanks (bushings as installed), which appear to be very flexible. This thesis investigates the seismic response of bushing structures both as installed and on fixed base as well as attempts to identify feasible approaches of stiffening the base of the transformer bushings as installed as a measure to mitigate their vulnerability under strong seismic excitation. Finite element models of four different high voltage transformers were used for performing modal and linear dynamic time history analyses in order to compare the response of the bushing structures as installed and on fixed base as well as investigate the efficiency of several retrofit techniques to ensure the bushings integrity during strong earthquake. In addition, a two stage experimental investigation, consisting of system identification testing and shake table testing, was conducted to verify the response trends identified by the numerical studies. Both numerical and experimental studies clearly showed that the bushing structures as installed are very vulnerable to seismic excitation as well as very flexible compared to the ones mounted on a fixed base. Moreover, these studies showed that stiffening the base of the bushings can be beneficial for their response against ground shaking. From the stiffening techniques investigated, installation of flexural stiffeners on the cover plate of the transformer tank appears to be the most efficient approach.