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.

Station Design

The two stations are identical in design and ratings, except that at the converter station in Pittsburg, the AC system voltage is 230 kV, and at the converter station in San Francisco, the AC system voltage is 115 kV. The single-line diagram is simplified in that it shows the major subsystems and does not include equipment such as the measuring devices, grounding switches, disconnects and surge arresters.

Each converter station consists of the following equipment:

  • A circuit breaker that connects the converter to the AC network bus

  • A main power transformers with three single-phase, three-winding units

  • A pre-insertion resistor for initial energization

  • Six converter arms, three for 200 kV, the other three for -200 kV

  • Grading resistors to establish the DC-side ground reference, as the DC side is essentially ungrounded

  • Converter reactors to limit the rate of rise of current and limit interphase circulating currents.

The Cables

The two converter stations are connected by DC cables, and the TBC system uses AC cables to connect to the network. Operating as one complete system, the San Francisco converter station (the inverter) takes DC from the cable system and converts it to AC for use in the network, and the Pittsburg converter station (the rectifier) takes AC power from the grid and converts it to DC for transmission to San Francisco.

The Pittsburg 230-kV AC cable system, the Potrero 115-kV AC cable system and the DC cable system were all built by Prysmian. The Pittsburg 230-kV AC cable system, which connects the Pittsburg converter station with the grid's East Bay, and the Potrero 115-kV AC cable system, which connects the Potrero converter station with the substation on the San Francisco side, are both underground and use cross-linked polyethylene (XLPE) insulation technology. The DC cable system, which connects the two converter stations, is rated 200 kV and uses DC XLPE insulation technology.

The advantages of using XPLE cables are the lower cost, ease of handling and ease of repair.

TBC was developed by Pattern Energy's transmission experts from the napkin-sketch stage, so it takes on special meaning to see the project completed just five years after the mandate was received in 2005.


The hope and expectations are that by providing a new source of power for the City of San Francisco, TBC will ultimately help to lower long-term energy costs, relieve congestion and provide transmission reliability for the entire Bay Area.

Now that TBC, whose US$529 million price tag is considerably smaller than many of California's other transmission projects currently in the works, has formally joined the electrical grid, CAISO has indicated it will move forward with steps to close the Potrero generating plant, which sits near the converter station in San Francisco.

According to a New York Times article published in spring 2010, underwater cables are becoming an increasingly popular alternative to land-based overhead transmission lines using steel towers. The article said that utilities “are finding a remarkably simple answer” to the political problems the towers often create. “They are putting power lines under water, in a string of projects that has so far provoked only token opposition from environmentalists and virtually no reaction from the larger public,” stated the New York Times article. Still, the Trans Bay Cable marks California's first successful foray into this new solution. The TBC project will be a significant asset to San Francisco and the entire Bay Area for many years to come.

James Alligan (james.alligan@transbaycable.com) oversees the asset management program for Trans Bay Cable LLC. He has more than 31 years of power industry engineering, operations, maintenance, and research and development experience across generation, transmission and distribution. Alligan is a graduate of the UK Central Electricity Generating Board power engineering program.

Companies mentioned:

California ISO www.caiso.com

Pacific Gas and Electric Co. www.pge.com

Pattern Energy www.patternenergy.com

Prysmian www.prysmian.com

Siemens www.siemens.com

SteelRiver Infrastructure Partners www.steelriverpartners.com

Trans Bay Cable www.transbaycable.com