The new Transformer Bank 73 at the City Light Shoreline Substation may not look very different to you than any other heavy piece of equipment in our system – until you look under it.
It sits on a set of rollers designed to allow the heavy machine to slide back and forth during an earthquake, and to return it to its original position when the shaking stops.
The Shoreline transformer is only the second transformer in the United States to use this base isolation technology. The first was installed in Vancouver, Wash., by the Bonneville Power Administration. Leon Kempner, principal structural engineer and seismic program manager at BPA, helped to introduce the technology to City Light.
The specific device used in Shoreline is called a triple pendulum bearing – a series of concave plates and a center post that are free to slide during strong ground motion. These bearings act as a type of shock absorber, allowing the transformer to glide back and forth in a pendulum motion, trading movement for force. This reduces the stress on the transformer and its connections during an earthquake, and helps to keep it balanced.
Past earthquakes in the region have toppled transformers, damaging radiators, conservators, gaskets and cracking porcelain parts. Each transformer costs more than a million dollars and takes about a year to replace, so losing multiple transformers in an earthquake could be devastating for the Seattle area.
This upgrade was developed and designed by Robert Cochran, a senior civil engineer at City Light. “Incorporating base isolation with high voltage transformers is something I have been working on for years and finally came together this year,” he said.
Robert will present a paper on this topic at the 2015 ASCE’s Electrical Transmission and Substation Structures Conference in Branson, Mo.