Siemens has won an order to supply its most advanced high voltage direct current (HVDC) technology to a project that will link the city of San Francisco with a substation near Pittsburgh, California. The 88 km subsea transmission link will increase the reliability and security of electricity supplies to San Francisco.

Siemens will supply its HVDC Plus technology for the 400 MW link, which is being developed by Trans Bay Cable LLC, a subsidiary of Babcock & Brown. The project will mark the first installation of this technology and will be commissioned in 2010.

The project will be implemented by a consortium of Siemens and Milan-based Prysmian Energy Cables and Systems. It will allow power to flow directly into the centre of San Francisco from the East Bay, rather than through a network of AC grids and will help the city meet future electrical power needs.

Under the $150 million contract, Siemens will supply converter stations and 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. The consortium partner Prysmian will provide the submarine power cables that will be installed in the San Francisco Bay. The system will reduce the need for new power plants in San Francisco city and reduce transmission grid congestion in the East Bay area.

In contrast to line-commutated converter technology, HVDC Plus operates with power semiconductors which have turn-on and turn-off capability. It also makes use of the advantages of multilevel voltage-sourced converter technology, which allows active and reactive power to be set independently. The capability of very rapid intervention in the power converter for control and protection purposes makes the system highly dynamic, significant for protection against system faults and malfunctions in transmission systems.