200 kV DC extruded cables crossing the San Francisco Bay

摘要

In the Bay of San Francisco, in California, there is the first case in the world of commercial transmission of electrical energy in direct current at 200 kV with Multilevel VSC technology and cables with extruded insulation: it is the "Trans Bay Cable" (TBC) project. The innovative submarine HVDC link consists of two cables in bipolar configuration and has the capability to transmit 400 MW over a length of 55 miles i.e. 88 km. The connection between converter station in Potrero - in San Francisco - and Pittsburg - in the East Bay - allows the closure of the loop of the already existing "Greater Bay Area" transmission system, thus reinforcing the power flow directed to the center of San Francisco. This will decrease transmission grid congestion in the East Bay and will also avoid the need of building additional power plants in the City of San Francisco. Cables are manufactured by Prysmian in a factory close to Naples, in Italy, which is fully dedicated to submarine cables. Design for this specific project includes a copper conductor with a cross section of 1100 mm~2, extruded insulation, lead sheath and armour of steel wires. The two HVDC cables (+/- 200 kV) are laid in bundle and buried at a depth of 6 ft (1.8 m), in a safe corridor separate from any existing AC cables. Laying operations are carried out by two different vessels: the "Giulio Verne" (one of the largest cableships in the world) for most of the route and a suitably equipped barge for the very shallow waters in the East Bay. The use of extruded insulation cables in the HVDC links showed a large increase during last years, due to some advantages that polymeric insulation can offer in comparison to traditional laminated mass impregnated cables and since the large technological progress happened, which involved materials, manufacturing process, system integration.