Three 550 kV porcelain transformer bushings were evaluated for their response to severe earthquake shaking. The first bushing was similar to bushings currently in service in the United States; the other two bushings were modified versions of the first bushing. The modifications to the second and third bushings were intended to enhance seismic performance and included added tiers of springs, increased preload, and stiffer gaskets. The dynamic properties, vibration frequencies, and damping ratios of the bushings were evaluated from the experimental data. Tri-directional earthquake simulator testing was undertaken to investigate the dynamic response of the bushings, to qualify one of the modified bushings for moderate earthquake shaking (per IEEE 693-1997), and to evaluate the response of the other two bushings to extreme shaking effects. For earthquake testing, the bushings were mounted at 20° to the vertical in a stiff support frame. Two sets of spectrum-compatible ground motion records, derived from motions recorded during the 1978 Tabas earthquake in Iran, were used for testing. None of the bushings met the IEEE criteria for Moderate Level qualification. However, the response of the modified bushings was superior to the response of the unmodified bushing.
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