On April 4, 2010, an earthquake with a magnitude of 7.2 occurred in northern Mexico and was felt as far away as Los Angeles, California, U.S. The earthquake caused significant damage to the 500-kV yard of the San Diego Gas & Electric Co. (SDG&E) Imperial Valley substation.
Damage to substations can have significant economic impacts on a utility and its customers. Recognizing these impacts, SDG&E developed a post-earthquake mitigation plan involving a detailed study of the flexible bus conductor in its Imperial Valley substation’s 500-kV yard to determine the possible causes of damage, identify weaknesses and study seismic-strengthening methods.
SDG&E has several 500-kV and 230-kV substations. The 500-kV Imperial Valley substation is located 10 miles (16 km) southwest of El Centro, California, within the Imperial Valley. Jointly owned by SDG&E and the Imperial Irrigation District, the Imperial Valley substation is a key facility along SDG&E’s 500-kV Southwest Powerlink, through which bulk power deliveries flow from both the desert southwest and the Imperial Valley into the SDG&E system.
Southwest Powerlink and the Imperial Valley substation are the primary paths for delivery of Imperial Irrigation District’s power purchases from Arizona. In addition, the Imperial Valley substation serves as a key point of interconnection between SDG&E and the Comisión Federal de Electricidad of Mexico, and it is a point of interconnection for several large independent power producers of both conventional and renewable generation.
The Damage Done
In 2010, the 7.2-magnitude El Mayor-Cucapah earthquake struck Southern California. The event lasted roughly 90 seconds, with about 40 seconds of strong shaking felt as far away as Los Angeles. Because of its importance to the area and significant knowledge gained through research performed by the Pacific Earthquake Engineering Research Center, the Imperial Valley substation had been seismically hardened before the 2010 event.
Although the Imperial Valley substation was forced out of service by the earthquake, the extent of physical damage to the substation was significantly reduced as a result of the seismic hardening SDG&E had completed at the facility. There was no interruption in service to SDG&E customers and minimal disruption of off-system interchanges between SDG&E and Imperial Irrigation District, Comisión Federal de Electricidad of Mexico and Arizona utilities.
The Imperial Valley substation contains both 500-kV and 230-kV switchyards with a transformer bank between the two switchyards. The damage in the 500-kV switchyard indicated in the post-earthquake survey report included the 500-kV and 230-kV transformer porcelain bushings, porcelain post insulators, porcelain surge arresters, bus support connectors, A-frame tie-down springs, freestanding surge arresters and extended corona rings on disconnect switches.