In the late 1800s, Thomas Edison and George Westinghouse faced off in what has been called the “War of the Currents,” with Edison trumpeting direct current (DC) for its efficiency and safety and Westinghouse backing Nikola Tesla’s alternating current (AC) for its usefulness in large distribution systems. Westinghouse won that war with AC’s adoption by developers of Niagara Falls’ first generating station. However, a range of new demands is forcing planners to take a new look at DC’s possible advantages.
In fact, DC power is becoming a player again, in settings ranging from high-voltage transmission down to commercial-building and data-center distribution systems (see sidebar, page 50). In many cases, its revival also has brought back the Edison versus Westinghouse either/or debate, as though DC’s newly reappreciated qualities will somehow push AC aside. But what we likely will see moving forward will be both/and, rather than either/or, as electrical designers explore the benefits both technologies have to offer.
Take long-distance, high-voltage power transmission, for example. AC has been the predominant technology in these applications for decades because it can be transformed to lower voltages relatively easily. However, several factors now are raising the profile of high-voltage DC (HVDC) transmission technology across the globe, as well as here in the United States, because DC lines can be more efficient in carrying especially large volumes of electricity across particularly long hauls. In these applications, AC wires face challenges because of the need to introduce reactive power into the line to enable the transmission of “real” power.
“With very long transmission lines, you need more reactive power, and that leaves less room for real power,” said Wayne Galli, transmission and technical services vice president for Houston-based Clean Line Energy Partners, adding that this means you need multiple conductors to carry the same amount of electricity that could be transported in a single DC line. “With DC lines, there’s no reactive power component.”
Additionally, AC lines suffer from heavy line losses in undersea and underground installations. Europe now depends on undersea and underground DC cables to connect offshore wind farms to onshore demand centers. European planners envision the possibility of a continent-wide grid fed, perhaps, by North African wind farms, while Chinese energy planners face the need to transport electricity generated from coal and wind resources in its midsection to high population zones along its coast.
“China has been building HVDC like gangbusters the last few years,” Galli said. “They’ve got a lot of resources inland, but all their population centers are on the coast. They were the first country to go to plus-or-minus 800 kilovolts [kV], and they’re now pushing vendors to go up to 1,100 kV, at up to 10,000 megawatts [MW], for up to 800 to 900 miles.”
Blowin’ in the wind
This same desire to move electricity from the nation’s windy midsection to the East and West is driving the ambitious plans that Clean Line Energy is developing for four separate HVDC transmission lines. The projects would originate in Iowa, Kansas, Oklahoma and New Mexico and serve wind farms yet to be built.
These wouldn’t be the first U.S. HVDC transmission lines, Galli said. Several projects were constructed in the late 1970s and early 1980s, tying Canadian hydropower resources to New England and connecting Utah and the Pacific Northwest to California.
DC cable also has been used in several underground or undersea transmission projects. The 25-mile Cross Sound cable was installed in 2003 to connect Long Island, N.Y., to New Haven, Conn., and the 65-mile Neptune Project, completed in 2007, links Long Island to Sayreville, N.J.
Currently under development is the Champlain-Hudson Power Express, a 333-mile line intended to carry 1,000 MW of Canadian hydropower-generated electricity down to New York City. As proposed, the line would run under Lake Champlain and the Hudson River, with a land portion buried under highway or railroad rights-of-way. In this way, developers may be able to avoid many of the not-in-my-backyard-style protests than can bring overhead AC-line projects to a standstill. (As an additional benefit, DC lines generate static electric and magnetic fields similar to the levels produced by the Earth itself.)
The new overhead lines Galli’s firm plans to develop would be significant additions to this country’s DC transmission lineup, running between 500 and 900 miles each and capable of handling up to 3,500 MW of connected load. Just as the early railroads provided a way to market for farmers interested in settling what is now the nation’s breadbasket, these new transmission lines will provide valuable access to distant utilities for wind farm developers needing faraway power purchase agreements to make their investments worthwhile.
“It is kind of a ‘build it and they will come’ type of scenario,” Galli said. “There’s a really good resource in the middle of the continent, and the demand is on the right and left coast.”
However, Clean Line Energy’s bet seems reasonable renewable-energy demand is growing, even as natural gas continues its downward price trend because numerous states require electric utilities to include green-energy sources in their portfolios. But developing resources adequate to this demand can be almost impossible in densely populated service territories, so utilities often have limited nearby options.
“Our utility system tends to be very Balkanized, so utilities don’t look beyond their service territory for resources,” Galli said, adding that public utilities commissions also limit options for electric companies interested in developing generation resources outside their territorial boundaries. “If they try to build outside their service territory, it’s hard to get those projects paid for.”
The lines Galli’s company is planning are strictly point-to-point. Tying HVDC lines together to create interconnected transmission systems poses challenges, not least of which is the ability to safely and quickly disconnect and reconnect individual line segments that might be carrying the equivalent of a nuclear generating station’s output without damaging equipment or injuring workers. Swiss-based manufacturer and engineering firm ABB announced recently that it had developed a circuit breaker that could accomplish this feat within 5 milliseconds.
“As you start doing more and more point-to-point, you start asking, ‘Is there a better way?’” said Aftab Kahn, ABB’s senior vice president and general manager for North American grid systems. “Or, would it be smarter to move to a DC grid?”
The company is a longtime leader in DC transmission systems. It recently announced landing a $130 million HVDC project connecting the Finnish mainland to the country’s Åland archipelago. According to press releases describing this new breaker, the company views the device as a “historical breakthrough” in the possible adoption of DC transmission grids. The unit combines two electronic switches on either side of a mechanical switch, and its development was, for years, a flagship effort in a larger research and development program on which the company spends more than $1 billion annually.
“The reaction has been extremely positive,” Kahn said, commenting on the international attention the new device is receiving. “I think [many people] see this as a game changer.”
While success was achieved in the lab, the company’s next step is to test the new circuit breaker in a utility pilot installation. This could take a while, cautioned Anders Sjoelin, president of the ABB’s North American power systems division, because new transmission projects take years to plan and the right application might still be early in the approval process. But he’s confident that such an opportunity will arise in the not-too-distant future, given the winning hand this former loser in the war of the currents now is playing.
“If you look at what’s been proposed in the last five to six years, you can see that DC has gotten a bigger part of the planning,” Sjoelin said. “DC is slowly getting a bigger share of the transmission build.”