Modeling of electrical transformers and seismic performance evaluation of high voltage transformer bushings
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Electrical transformers are complex structures, crucial for the operation of a power system, an important lifeline for community resilience. Damage sustained in past strong earthquakes rendered many of them unusable, leading to severe economic losses. Of particular interest for the overall performance is the dynamic response of high voltage bushings, the failure of which is closely associated with the loss of functionality of the transformer. This study deals with the modeling variations and structural modifications of commercial finite element models of an existing transformer in view of identifying its dynamic characteristics and the significant interactions between various components and the high voltage bushings. Detailed sensitivity analyses are performed on a finite element model which comprises three identical 196 kV bushings mounted on top of a 230 kV transformer. Bushings are modeled using properties provided by manufacturers while the cover plate is modeled considering only the important structural features. The base of the structure is excited with 3-D wide band random motion (white noise) and the IEEE-693 spectrum compatible ground motion. Transfer functions are calculated to determine the relevant properties, even when the structure has nonlinear elements. The interaction between the bushings and several main parts of the transformer is assessed through cross-spectra and coherences. Sensitivity of interaction is determined by modifying properties of interacting critical parts on the transformer, especially the cover, radiators, surge arresters and the core assembly. Moreover, the response quantities of interest (accelerations and displacements) are measured at specific locations on the bushings and the spectra at the corners of the cover and at the base of the turrets are obtained, to which, the response of the bushings is associated.