Procedures and Results of Assessment of Seismic Performance of Seismically Isolated Electrical Transformers with Due Consideration for Vertical Isolation and Vertical Ground Motion Effects

Keywords: Earthquake engineering; electrical transformers; seismic isolation; three-dimensional seismic isolation; risk assessment; probabilistic analysis; vertical ground motion

Abstract: This report presents an analytical study of the response of seismically isolated electrical  transformers with particular emphasis on comparing the performance of equipment that are non-isolated to equipment that are isolated only in the horizontal direction or are isolated by a three-dimensional isolation system. The failure characteristics of transformers were determined by calibrating a failure model using fragility data of non-isolated transformers that were based on field data and are used in the seismic performance of electrical equipment by utilities in the Western U.S.

The performance was assessed by calculating the probability of failure as a function of the seismic intensity with due consideration of (a) horizontal and vertical ground seismic motion effects, (b) displacement capacity of the seismic isolation system, including uplift capacity, (c) acceleration limits for failure of electrical bushings, (d) details of construction of the isolation system that allow or restrain rocking of the isolated structure, (e) weight of the isolated transformer in the range of 320 to 520 kip, (f) bushings with as-installed frequencies of 2.6 to 11.3 Hz, (g) inclined and vertical placed bushings and (h) various details of the isolators, including upper and lower bound properties and details of construction of the isolators.