Seismic Evaluation and Retrofit of 230-kV Porcelain Transformer Bushings, PEER Report 1999-14


Two identical 230-kV porcelain transformer bushings and two retrofit details were selected for testing and evaluation. The class of the bushing tested was built in the mid-1980s and was known to be vulnerable to earthquake shaking. Resonance-search tests were conducted to determine the dynamic properties of the bushings. Static testing was undertaken to determine the limit states of the bushing and to assess the effectiveness of a retrofit detail. For static testing, the bushing was vertically mounted on a stiff frame and four target displacement orbits were used: unidirectional, unidirectional with offset, bidirectional, and monotonic. During the static tests, oil leaked from the gasket connection, and the porcelain unit immediately above the fl ange slipped at a displacement of 10 mm. Reduction of the bushi ng post-tensioning force reduced the uplift displacement at which the bushing leaked oil. The addition of the first retrofit detail (Ring-1) did not prevent the oil leakage or the slip of the porcelain unit. Triaxial earthquake-simulator testing was conducted to evaluate the seismic performance of the bushings mounted on rigid and flexible frames and to evaluate the efficacy of a second re trofit detail (Ring-2). Two sets of spectrum- compatible ground motion records, derived from motions recorded during the 1978 Tabas earthquake in Iran, were used for testing. The fundamental frequency and damping ratio of the bushing were 18 to 20 Hz, and 2 to 3 percent of critical, respectively, when mounted on a rigid frame. When subjected to motions compatible with the IEEE 693-1997 spectrum for High Level qualification, there was no structural damage and no oil leakage. The fundamental frequency of the bushing-support system was reduced to approximately 7 Hz when the bushing was installed on a flexible plate. This reduction in frequency substantially increased the response of the bushing. The bushing leaked oil, the porcelain unit immediately above the flange slipped, and the gasket extruded from the porcelain to flange interface when the bushing was subjected to severe shaking. The addition of Ring-2 reduced the slip of the porcelain unit, prevented the extrusion of the gasket, and delayed oil leakage.

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Amir S. Gilani
Andrew S. Whittaker
Gregory L. Fenves
Eric Fujisaki
Publication date: 
December 3, 1999
Publication type: 
Technical Report
Gilani, A. S., Whittaker, A. S., Fenves, G. L., & Fujisaki, E. (1999). Seismic Evaluation and Retrofit of 230-kV Porcelain Transformer Bushings, PEER Report 1999-14. Pacific Earthquake Engineering Research Center, University of California, Berkeley, CA.