Upgrading its Miguel Substation, San Diego Gas & Electric (SDG&E; San Diego, California, U.S.) is using gas-insulated switchgear (GIS) technology for the first time—installing a 500-kV GIS and step-down transformer in San Diego County. After market deregulation, more than 8000 MW of new generation was added in Arizona, creating more than 8000 hours per year of transmission congestion at SDG&E’s Miguel Substation in the southeast region of San Diego County in Bonita, California. State and federal regulators identified Miguel as one of the most congested transmission hubs on the state’s power grid. SDG&E responded by initiating a fast-track upgrade, marking the first time it had used GIS technology to expand a substation.

In May 2003, SDG&E proposed four projects to ease the congestion at Miguel Substation:
� Add a new GIS with a 500/230-kV 1120 MVA transformer
� Upgrade the Miguel-Imperial Valley series capacitors from 1400 A to 2600 A
� Re-conductor 4.2 km (2.6 miles) of 138-kV transmission line
� Install a new 230-kV line from Miguel to SDG&E’s Mission Substation in central San Diego.

The first three projects, which were recently completed, have increased the import capability of Miguel from about 1100 MW to 1500 MW. When the 230-kV transmission line is in place, which could be as early as September 2005, the congestion at Miguel should drop to about 1000 hours per year, increasing SDG&E’s import capability to 2000 MW along the path.

Innovative Approach

SDG&E solved the congestion problem by installing GIS and step-down transformers within boundaries of the existing transformer yard. A new 6-m by 12-m (20-ft by 40-ft), 500-kV control house was built between the current line and a future tie line A-frame in the existing yard. The control house contains new relays for the two transformers, tie line and reactor-protective relays, and two dc batteries. The 500-kV equipment is now on its own dc system.

An emergency bypass was added to allow the two transformer banks to be energized in case of a failure in the GIS tie line. This allows restoration in less than one day. Two existing 230-kV bays were also expanded and two breakers were replaced.

Construction Team

The engineer procure construct (EPC) contract for the GIS portion of the substation upgrade was awarded to Mitsubishi Electric Power Products Inc. (Warrendale, Pennsylvania, U.S.) in October 2003. Other alternatives, including an air-insulated design, were considered. However, given the size restraints and room needed for future expansion, the best fix was the GIS and step-down transformer supplied by Mitsubishi Electric.

In preparation for the substation upgrade, SDG&E electricians were trained at Mitsubishi’s factory and assembled the GIS onsite under Mitsubishi Electric’s direction. Siemens USA (New York, New York, U.S.) was contracted to set up the shunt reactors. Civil and grade work was done by ROEL Construction Co. Inc. (San Diego).

The total cost for the substation upgrade, construction of additional buildings and ancillary equipment refurbishing was approximately US$30 million. Compared to similar projects that have taken three years to complete, the GIS substation upgrade was put into service in less than 12 months—an unprecedented time frame.

The California Public Utilities Commission approved SDG&E’s proposal to accelerate construction of the new 230-kV line from Miguel Substation to its Mission Substation in Mission Valley. The original in-service date of June 2006 is being moved up to September 2005. l