Seismic Protection of Electrical Transformer Bushing Systems by Stiffening Techniques

Keywords: High voltage bushing structures.  Seismic performance.  Stiffening techniques.  Vulnerability.  Damage mitigation.  Numerical simulations.  Shaking table tests.  Flexural stiffeners.  Linear dynamic time-history analyses.  Transformers.

Abstract: This report investigates the seismic response of high voltage bushing structures both "as installed" on a flexible base and on a rigid base as well as attempts to identify feasible approaches of stiffening the base of the transformer bushings as a measure to mitigate their vulnerability under strong seismic excitation.  Finite element models of our 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 a rigid base as well as investigate the efficiency of several stiffening techniques to ensure the integrity of the bushings during strong earthquakes.  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 rigid 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, incorporating flexural stiffeners on the cover plate of the transformer tank appears to be the most efficient approach.