Electrical power has long been a concern for the city of San Francisco, with its relatively old infrastructure (by California standards, anyway) and its high-tech, tourist-driven economy.

For decades, high-voltage surface cable lines connected to California's power grid have taken up the slack for whatever outside generation this West Coast metropolis required. The problem was, these lines all come from the same direction: the San Mateo Peninsula, to San Francisco's south. This leaves the city susceptible to any disruption in that area, such as an earthquake, which could knock out all power lines at once. For example, substation malfunctions occurred there in 1998, causing a widespread failure that knocked out San Francisco's power for hours. That chaotic scene was memorable, as the city boasting the largest fleet of electric trolleys in the nation was gridlocked by immobilized transit vehicles.

Now, however, a new frontier to the east has been opened. The city is able to import energy from across the San Francisco Bay, thanks to an innovative new system that came on-line just before the end of last year. Approved in 2005 and completed in November 2010, the new Trans Bay Cable (TBC) has connected the city of San Francisco to the power grid on the eastern side of the Bay and delivers up to 400 MW, or 40% of power requirements, for San Francisco, helping to ensure the reliability of the Bay Area's power system.

Technology to the Rescue

TBC is an energy transmission infrastructure project chosen by the California Independent System Operator (CAISO) to provide reliable energy to the city of San Francisco. It consists of an approximately 53-mile (85-km)-long ±200-kV high-voltage direct-current (HVDC) transmission cable that runs under the San Francisco Bay to transfer energy from an electric transmission system substation in Pittsburg, California, to one in San Francisco. TBC does not involve any new generation, but rather accesses an array of potential sources available to the grid from the eastern side of the Bay.

San Francisco is on the tip of the peninsula, so, up until now, it had to create its own power or get it from the south. TBC has opened up a new power avenue and a new source of security. What is more, the undersea cable should help lower energy costs, relieve congestion and improve reliability throughout the Bay Area. TBC is owned by SteelRiver Infrastructure Partners.

Reinforcing Reliability

In the wake of rolling blackouts in the early 2000s and the 1998 power outages, CAISO deemed that the Bay Area's northern peninsula needed additional transmission sources to ensure energy reliability for this critically important region of California. In September 2005, after a lengthy stakeholder process, the CAISO selected TBC over other alternatives as the best transmission solution for the northern peninsula.

Construction of the cable was completed in December 2009. The system underwent testing throughout 2010, and working with the CAISO, SteelRiver was able to put the cable into service by the end of 2010 — before CAISO's expected shutdown of one of San Francisco's oldest power plants in 2011.

SteelRiver made history with the TBC project by successfully developing the country's first purely privately proposed and financed solution for meeting the reliability needs of a regional utility grid. The firm's highly experienced transmission team, along with its partners at Prysmian Cables & Systems, Siemens and Pattern Energy, developed a world-class transmission asset that will help ensure the reliability of the San Francisco Bay Area's current and future power needs.

Project Partners

TBC is a point-to-point, controllable land- and submarine-based transmission cable system consisting of two high-voltage transmission cables and a fiber-optic communication cable bundled together. The entire cable bundle is about 10 inches (254 mm) in diameter and was installed primarily using an underwater hydro plow operated from the world's largest cable-laying ship, the Giulio Verne. A smaller barge was used in selected areas where the minimum water depth for a ship is not available. The cable-laying activity had less impact on the Bay than a winter storm or typical tidal action.

TBC was designed and constructed by Siemens Energy and Prysmian Construction Services Inc. The two converter stations, one in Pittsburg and the other adjacent to PG&E's Potrero Station in San Francisco, were built by Siemens and use voltage source converter (VSC) technology, trade named HVDC Plus. The project design specifications enable each converter station to produce at least ±140 MVA of reactive power when active power transmission is at 400 MW.

Technology in Use

Following many years of successful and broad application in traction and medium-voltage drives systems, the VSC uses modern insulated-gate bipolar transistors, which have become important in HVDC transmission and flexible AC transmission system applications. In comparison with classic thyristor-based line commutated converters (LCC), the VSC self-commutated converters provide additional and advanced features that are advantageous in meeting the power transmission challenges of today's systems:

• Both real power flow and reactive power exchange can be controlled independently.

• Dynamic response can be achieved that surpasses the grid operation requirements.

• Reliable operation is exhibited whether the receiving system is strong, weak or even passive.

• Station layout is compact and flexible.

A new modular multilevel converter topology has been introduced by Siemens into the Trans Bay HVDC application. Based on the concept of this topology, the converter arms act as a controllable voltage source with a high number of possible discrete voltage steps, which allow forming an approximate sine wave in terms of adjustable magnitude of the voltage to the AC terminal.

By serially connecting many modules, an elegant multilevel topology can be constructed. It is possible to individually and selectively control each of the individual sub-modules in a converter arm. The total voltage of the two converter arms in one phase unit equals the DC voltage, and by adjusting the ratio of the converter arm voltages in one phase module, the desired sinusoidal voltage at the AC terminal can be achieved.

In addition, the TBC converter system has a simplified design and is comprised of standard and, essentially, commercially available components.