Dynamic behavior and structural vulnerability analysis of molded transformers in power plants during earthquakes

This study assesses the structural performance and seismic fragility of molded transformers in power plants using triaxial shaking table tests together with three-dimensional finite element analyses. Tests conducted in accordance with ICC-ES AC 156 showed up to an 8.33% reduction in natural frequency during the resonance search, indicating stiffness degradation. At higher excitation levels, bolt detachment occurred at 100% PGA and component separation at 150% PGA. The numerical model, validated against measured natural frequencies and response forces, demonstrated high reliability and was used to construct period-sensitive fragility curves. To evaluate the potential progression of damage in the transformer body, fragility of the overturning-type displacement at the top (TD) and the effective stress at the bottom steel base channel (SB) was examined over a wider PGA range, representing conditions where the foundation is strengthened or boundary conditions differ from the as-tested configuration. The results show that relative displacement is more vulnerable to long-period motions, whereas structural stress is more sensitive to short-period motions. These findings provide quantitative indicators of seismic performance and a practical technical basis for improving seismic design standards and enhancing the safety of power-plant equipment.