Seismic Response Amplification Mechanisms and Base-Isolation Retrofit Evaluation of a 500 kV Three-Phase Transformer with Steel Supports

The effects of steel supports on the dynamic characteristics and seismic demands of large three-phase power transformers remain insufficiently quantified. A three-dimensional finite-element model of a 500 kV three-phase transformer was developed to compare configurations without steel support (NS) and with steel support (WS). Modal characteristics and seismic bushing responses were evaluated under seven three-component ground motions. Frequency-response characteristics and global tank rotations were then analyzed, and a double friction pendulum (DFP) base-isolation retrofit was assessed. The lowest natural frequency of the WS model was 1.47 Hz, lower than the 1.84 Hz obtained for the NS model. For the representative X-direction response of high-voltage bushing A, the dominant low-frequency peak of the frequency-response function shifted from approximately 1.89 to 1.64 Hz, while its magnitude increased from approximately 8 to 19. The mean peak maximum principal tensile stress at the root of bushing A increased by 83.1%. The WS model also exhibited greater tank rocking and torsional responses, whose peaks were positively associated with the peak root stresses of bushings A and B. For representative high-voltage bushing B, the DFP retrofit achieved a mean isolation efficiency of approximately 60%. These results elucidate the mechanisms by which steel supports amplify the seismic response of the large 500 kV three-phase power transformer and provide a numerical evaluation of the effectiveness of a DFP base-isolation retrofit for this transformer.