Seismic evaluation and retrofit of 230-kV porcelain transformer bushings

Static and earthquake-simulator testing of two 230-kV, 3000A, Type-U transformer bushings manufactured in the mid 1980s by the General Electric (GE) Company was undertaken to evaluate both the seismic performance of bushings known to be vulnerable to damage from moderate earthquake shaking, and the efficacy of simple retrofit details. Both bushings survived earthquake shaking compatible with the IEEE spectrum for High Level qualification when mounted on a rigid frame. When mounted on a flexible frame, one 230-kV bushing was unable to sustain the High Level qualification shaking without oil leakage and slip of the porcelain units. Two retrofit details utilizing two semicircular ring plates and epoxy filler were prepared by utility representatives with the objective of limiting slip of the UPPER-1 porcelain unit over the gasket connection. The second detail, Ring-2, permitted the 230-kV bushing to sustain substantially larger accelerations and displacements than those of the unretrofitted bushing and eliminated the permanent slip of the UPPER-1 porcelain unit over the gasket connection for extreme earthquake shaking.