After five years of construction—which is considered fast for this type of project—the Trans Bay Cable (TBC) under the San Francisco Bay went into commercial service in late November. The 53-mile underwater cable runs from the city of Pittsburg in Contra Costa County at the outer portion of the East Bay to the city of San Francisco. The 400-megawatt (MW), high-voltage direct current (HVDC) submarine line can transmit up to 40 percent of peak-power needs for the entire city of San Francisco.

About 10 inches in diameter, the DC copper-core transmission cable contains cables for fiber optic communications and metallic return cable.

The cable installation was done with underwater hydro-plow operated from the world’s largest cable ship, the Giulio Verne. For protection, the cable was buried between 4 to 6 feet beneath the bottom of the bay. In some areas, where plow depth was not possible, such as where crossing over other utilities or where soil was too hard, the cable lays on the bay floor and is covered with concrete mattresses.

“The Trans Bay Cable provides a new source of power for the city of San Francisco, helping to lower long-term energy costs, relieve congestion and provide transmission reliability for the entire Bay Area, while shutting down the last remaining old power plant in San Francisco,” said Mike Garland, CEO of the Pattern Energy Group, which developed the project and managed construction.

The area’s geography made other transmission options implausible.

“The city of San Francisco is pretty isolated at the end of a long peninsula and has not been able to get power from north of the city,” said Matt Dallas, spokesperson for Pattern. “This cable really opens up a new avenue to bring power into San Francisco, including clean power from the many renewable-energy sources north of the city.”

Since alternating current (AC) power does not travel efficiently underground and underwater over long distances, an AC-to-DC converter station was built in Pittsburg, and a DC-to-AC converter station was built in San Francisco.

“Pattern created history with the Trans Bay Cable by successfully developing the country’s first purely privately proposed and financed solution for meeting the reliability needs of a regional utility grid,” said Michael Cyrus, COO of SteelRiver Infrastructure Partners. “Pattern’s highly-experienced transmission team, along with SteelRiver, Prysmian [cable manufacturer], Siemens and the Trans Bay Cable team, has developed a world-class transmission asset that will help ensure the reliability of the San Francisco Bay Area’s current and future power needs.”

TBC is owned and operated by SteelRiver Transmission Co., an affiliate of SteelRiver Infrastructure Partners. For more information, visit www.transbaycable.com.