Siemens Power Transmission and Distribution (Erlangan, Germany) has secured an order to construct an undersea high-voltage direct current (HVDC) transmission link between San Francisco's City Center electrical power grid and a substation near Pittsburg, California, U.S. The order was placed by Trans Bay Cable, LLC (San Francisco), a wholly owned subsidiary of the project developer Babcock & Brown. Siemens' share of the total order is more than US$150 million.

From March 2010, the HVDC Plus system will transmit up to 400 MW at a dc voltage of ±200 kV. The main advantages of the new HVDC Plus link are the increased network security and reliability due to network upgrade and reduced system losses.

Today, the major electrical supply for the city of San Francisco is coming from the south side of the San Francisco peninsula. The city relies mainly on ac grids that run along the lower part of the bay, with the new HVDC Plus interconnection link power flows directly in the center of San Francisco. The Trans Bay project will decrease the overall transmission grid congestion and increase overall security and reliability of the electrical system. Furthermore, it will reduce the need to build additional power plants in the city of San Francisco.

The project will be implemented by a consortium of Siemens and Milan-based Prysmian Energy Cables and Systems. As consortium leader, Siemens was awarded a turnkey contract that comprises the converter stations for the HVDC Plus system, including engineering, design, manufacturing, installation and commissioning of the transmission system. The company will deliver all high-voltage components including transformers, converter submodules, converter reactors and breakers, and is responsible for the control and protection, civil works and building systems. Furthermore, Siemens will fulfill all major requirements, which have to be considered for the electrical components as well as for all buildings for a highly seismic zone like San Francisco. The HVDC Plus solution uses a minimum amount of space — crucial for converter sites in urban areas — and minimizes environmental impact. The consortium partner Prysmian will provide the submarine power cables that will be installed in the San Francisco Bay.

The Trans Bay Cable Project will be the premier installation of Siemens' new HVDC technology, HVDC Plus (Power Link Universal System), an advance in HVDC transmission systems that opens new fields in proven HVDC technology. And with the implementation of voltage-sourced converters, HVDC Plus is the preferred solution in space-constrained installations.