By By Steven M. Brown, editor in chief
Since the first commercial implementation of ABB’s high-voltage direct current (HVDC) technology in Sweden in 1954, the technology’s benefits have been sufficiently demonstrated and are well-known in the power industry. However, HVDC’s high cost, relative to typical AC transmission, has been a deterrent to widespread adoption, relegating HVDC’s use to very specific applications where AC lines won’t suffice, such as long submarine installations, long-distance bulk power delivery and interconnections between asynchronous grids.
Some power industry participants and HVDC experts believe we’ll see a rise in domestic and international implementations of the technology in the future as the needs increase, the power electronics at the core of HVDC technology evolve and the cost of HVDC falls. In the meantime, three current projects ongoing in the United States highlight some of HVDC’s applications and benefits.
Sharyland’s Cross-border Tie
Sharyland Utilities in south Texas is unique in several ways. When it was authorized by the Texas Public Utility Commission in 1999, it became the first greenfield electric utility in the United States since the Reedy Creek Improvement District was created to serve Disney World in the late 1960s. Its south Texas service territory consists of Sharyland Plantation, a 6,000-acre industrial park and real estate development, and all of its customers have automated metering at their premises, measuring usage at 15-minute intervals.
Sharyland is also unique in that it is about to begin construction on an interconnection between the Texas and Mexico power grids that the company says will be the first of its kind to support both emergency power and commercial business activity in Texas and Mexico.
Demonstrating HVDC’s ability to connect asynchronous grids, this 150-MW tie will serve to match the electric grids between Texas and Mexico, allowing power to flow in the direction and at the magnitude dictated by the contracting utility. This is one of HVDC’s benefits: Power in a DC system is controllable, whereas in an AC system, the laws of physics dictate direction and magnitude of power flow. In this application, the ability to control power flow with HVDC was necessary because of the commercial transactions the tie is expected to support.
The HVDC tie equipment for the project will be owned and operated by Sharyland Utilities. This particular implementation is what’s known as a “back-to-back” HVDC tie, meaning that there is one converter station connecting the two grids. In other HVDC installations, including the two submarine installations discussed later in this article, two converter stations are employed: one to convert AC power to DC and the other, at the other end of the HVDC cable, to convert DC back to AC for delivery to customers. The back-to-back converter station, which serves to match the asynchronous Mexican and U.S. power systems, will be located in Mission, Texas, with transmission lines connecting facilities in Reynosa, Tamaulipas, Mexico.
Construction on the Sharyland tie is expected to begin by mid-2006 with commercial operations supported by about August 2007. Sharyland Utilities has contracted with American Electric Power’s T&D Services department to provide project management and transmission system design. AEP T&D Services will oversee the design and operational requirements of the project and coordinate construction of the site with the HVDC vendor, ABB.
As a non-merchant facility, the line will be available to all Texas interconnected energy providers as an “open access tie.” Mark Caskey, Sharyland’s general manager, explained that the Electric Reliability Council of Texas (ERCOT) will coordinate open-access operations of the tie when it is completed, and the Public Utilities Commission of Texas (PUCT) will regulate electrical transmission rates. Caskey noted that the DC Tie will be open to all generators and retail electric providers in ERCOT and Mexico, and they will negotiate actual energy prices at open-market rates in accordance with the open-access operational standards developed by ERCOT. The PUCT will regulate pricing for transmission of that power.
The HVDC tie will also provide black start capability, an important reliability-enhancing feature in which normal operations can be suspended and a safe flow of power provided to help restore affected areas.
Caskey said the HVDC tie with Mexico is an investment meant to support not only power reliability but also future economic growth in the area Sharyland Utilities serves. “Many Fortune 500 executives are surprised by the financial impact a power outage has on their businesses,” he said. “With this HVDC tie, we are ensuring that state-of-the-art electrical infrastructure will be available for the critical operations at those businesses that choose to relocate to Texas.”
Neptune’s New Jersey to Long Island Link
Siemens Power Transmission & Distribution is involved in two projects on opposite sides of the country which both demonstrate HVDC’s submarine application. One project, which is currently under way, is being developed by Neptune Regional Transmission System (RTS) to connect Long Island to the PJM grid.
Neptune RTS awarded Siemens Power T&D a contract to construct an undersea HVDC transmission link between Sayreville, N.J., and North Hempstead in Long Island, N.Y. The project will be implemented by Siemens, which will engineer and supply the HVDC system, and Prysmian Cables & Systems (formerly Pirelli Cables & Systems), which will provide the 65-mile-long submarine power cable. The total contract value is in excess of $400 million.
Lindsay Martin, business development manager for Siemens high-voltage systems division, said that while demand is increasing on Long Island, there is little opportunity to build new generation there to meet increased demand. If new generation were to be built on Long Island, it likely would be natural gas-fired-not an attractive option given the current high cost of natural gas. The Neptune HVDC link will give the Long Island Power Authority (LIPA) access to the PJM grid, which has a more diverse, and therefore more affordable, fuel base.
Besides access to more affordable generation, the HVDC link will also provide reliable, consistent and predictable transmission of power to Long Island. The controllable nature of the HVDC system is a key benefit here, just as it is in the Sharyland installation.
“If LIPA wants to dial in 500 MW starting at 8 a.m., 550 MW starting at 9 a.m. and 600 MW starting at 10 a.m., they can issue those orders to the operator of Neptune, and at each hour, on the hour, we can ramp up the system and deliver the exact amount of power on demand,” Martin said. “It’s a very controllable power system.”
The Neptune HVDC system, once completed, will consist of a converter station in Sayreville, N.J., where AC power from the PJM grid is converted to DC power, which then will be sent through the 65-mile-long underwater HVDC cable to a mirror image converter station on Duffy Ave. in Long Island. There, the power will be converted back to AC for delivery to LIPA customers.
Construction on the Neptune project began in August 2005, and the scheduled commercial operation date is July 1, 2007. Both converter sites were being prepared at the time of this writing, and some cable routing on Long Island had already begun.
Trans Bay Project
Siemens and Prysmian are also involved in another submarine HVDC project on the opposite side of the country. That West Coast project is scheduled to start later this year and is being built to deliver power to the city center of San Francisco, which, while on a peninsula geographically speaking, is on an island in terms of electric power.
Trans Bay Cable, a subsidiary of the project’s developer Babcock & Brown, has received approval from the California ISO for a proposed submarine HVDC transmission line to connect San Francisco businesses and residents with power generated across the bay. As in the Neptune project, Siemens Power T&D will provide converter station technology, engineering and procurement, and construction management, and Prysmian will supply the HVDC cable.
Martin explained why the Trans Bay HVDC project is necessary. San Francisco does not generate enough power for its businesses and residents, but building a large generating station in the city is not an option. Further complicating the problem, building an overhead line in northern California is almost impossible from a permitting standpoint, Martin said. Most of the power in the San Francisco bay area is generated on the Oakland side of the bay. The existing AC grid runs along the lower part of the bay and comes up the peninsula to the city’s center. The submarine HVDC project Siemens is building will bypass that circuitous route and deliver power under the bay, straight to the center of San Francisco.
The Trans Bay project will consist of installation of 59 miles of submarine HVDC cable transmitting up to 400 MW of power from Pittsburg, which has excess power capacity but a congested transmission grid, to San Francisco. A new 7.5-acre converter station in Pittsburg will convert AC power from the grid to DC power. The power will run through the submarine cable to the other side of the bay where another 6.1-acre converter station on the San Francisco side will convert DC back to AC for distribution to city residents.
Martin said that in both the Neptune and Trans Bay projects, submarine HVDC cable provides the best, and perhaps the only, vehicle for delivery of much needed energy. “The loads in both places are growing and building local generation isn’t going to happen,” he said. “They both need energy and capacity. The submarine cable provides a right of way that’s not visible and that doesn’t cross over people’s property or existing infrastructure.”
Construction on the Trans Bay project is scheduled to start in 2006 and is targeted to come on-line in 2009. The project’s total price tag is in the neighborhood of $300 million.
While these three projects don’t quite represent a boom in HVDC deployment in the United States, both Siemens’ Martin and Sharyland’s Caskey believe the technology will see greater adoption in years to come.
Martin noted that China, in particular, is planning a number of HVDC projects to support its rapidly growing economy. With much of China’s economic development occurring on the country’s east coast, Martin noted that China has been building HVDC links to deliver bulk power over long distances from coal and hydro generating sources in Western China to the burgeoning coastal areas. Martin estimated that in the last half-decade or so, China has built six to eight 2,000- to 3,000-MW HVDC systems to move power from west to east and that the country has plans for 15 more.
He believes we’ll see more HVDC systems being built in the United States, too, particularly as California seeks to import large amounts of clean coal and renewable power from the east.
“The U.S. grid is a tremendous machine that has evolved over time,” Siemens’ Martin said. “HVDC is newer, and it continues to evolve even more rapidly as power electronics evolve. We’re going to see more and more HVDC systems. They are more expensive than a typical AC system, but they have applications where they are the only vehicle that can be used or there’s a great distance involved.”
Sharyland’s Caskey agrees that HVDC will be adopted on a broader scale in the future, and credits reliability as a driver of that growth. He noted that one of the important benefits of HVDC in the Texas-Mexico tie is the ability of the technology to buffer each system from faults or disturbances on the interconnected grids.
“It (HVDC technology) has significant benefits that cannot be ignored,” Caskey said. “For instance, this type of technology may have helped mitigate the cascading blackouts in the Northeastern U.S. and Canada on Aug. 14, 2003. HVDC technology acts as an isolating firewall against transmission line disturbances. Energy companies should evaluate strategic locations where HVDC technology may be appropriate while balancing cost recovery options.”<<