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The Grid

High Voltage: Watch Out AC / DC Is Getting Its Groove Back

Dorothy Pomerantz
November 02, 2016
America’s largest machine — the power grid — has been pumping lifeblood electricity from power plants to our homes and businesses for more than a century. The vast network of wires, switches, transformers and other technology has gone through periodic upgrades, but the infrastructure is aging and increasingly prone to blackouts. Unfortunately, the stress on the network is starting to show at exactly the time when we need it to shoulder and move thousands of megawatts from new wind farms and solar installations popping up all over the country. You could say, in fact, that the grid may be the weakest link in the nation’s drive to produce a quarter of its energy from renewable sources by 2025.
But one company in Texas believes it has found a remedy. It is developing new transmission lines around the country to channel electricity generated by renewable energy sources over hundreds of miles with improved efficiency in comparison with typical long-distance power transmission. The company, Clean Line Energy Partners, is doing it with technology that has been gathering momentum in Europe and is now ready to make a powerful comeback in the U.S. — direct current (DC).

Ever since Westinghouse and Tesla beat Edison in the “current wars,” alternating current (AC) has been the dominant method of shipping power over long distances. But Clean Line is now working with GE — the company Edison co-founded — to change that.

Take Oklahoma, where wind farms generate more than 23 percent of the electricity produced in the state, enough to power 1.3 million homes. Clean Line is developing one of the country’s longest highest-voltage direct-current (HVDC) transmission lines — known as the Plains and Eastern Clean Line — to bring 4,000 megawatts over 720 miles from the windswept Panhandle region to the southeast. GE Energy Connections will provide the HVDC technology crucial to making the plan work.

Panoramic view of a back to back HVDC Converter station, Al Fadhili_copyright GE Top image: A bank of AC/DC GE converters in Rio Madeira, Brazil. Above: A panoramic view of a back to back HVDC Converter station in Al Fadhili, Saudi Arabia. Images credit: GE Energy Connections

HVDC is a much more efficient way to move energy over long distances than alternating current. “If you used AC, you would need more wires in the air to get the same amount of power the same distance,” says Neil Kirby, HVDC business development manager at Grid Solutions from GE Energy Connections.

Wind turbines will send electricity to GE-built stations, where it will be converted from AC to DC. The electricity will flow across HVDC lines and will be converted back into AC before it goes into homes and factories. This method conserves more energy and is more economical and environmentally friendly than transporting the electricity as AC the entire way. The use of HVDC also allows the power flow to be closely controlled and coordinated with the varying wind power being generated, which eases the burden on the AC networks at each end of the converter terminals. “This line will enable what will effectively be the second-largest power project in America after the Grand Coulee Dam built in the 1930s,” says Michael Skelly, president and founder of Clean Line Energy Partners. (When Grand Coulee opened on the Columbia River in Washington State, it was the largest hydropower plant in the U.S. It still has more than 6,000 megawatts of capacity.)

Clean Line will transport electricity from Oklahoma to a station in Pope County, Arkansas, where GE technology will convert the electricity to AC. This connection will make 500 megawatts of capacity available to the Midwest Independent System Operator. A further connection to a station in Shelby County, Tennessee, will provide 3500 megawatts of capacity to the Tennessee Valley Authority’s grid.

Bina Substation, India_copyright GE An HVDC substation is Bina, India. Image credit: GE Energy Connections

Clean Line’s Skelly says the new transmission line should allow for the construction of more than 4,500 megawatts of new wind projects that would not otherwise be built.

The Plains and Eastern line is more complex than a typical HVDC installation because it includes more than one point where the power leaves the transmission line. It will use special software to monitor and control the power flow, making sure the converters are sending energy in the right direction. The converters can be controlled on-site or remotely, from a common control room. “There’s a lot of complex timing and coordination,” says Andrew Goodman, general manager of sales and commercial operations for GE Energy Connections’ Grid Solutions North America. “As the world’s premier digital industrial company, we are using our software capabilities to ensure our customers have the ability to efficiently control and monitor how the energy is flowing in their network.”

Clean Line has already spent $100 million on the project and now has more than 50 people in the field, working to secure rights-of-way for the new line. In total, the project — including the renewable energy that will connect to it — will cost about $10 billion.

A few decades ago, GE was among the first companies to use HVDC technology based  on the use of thyristors (called Line Commutated Converters or LCC). The equipment is still operating at locations across North America. GE later sold the technology to Alstom, only to have HVDC return to the fold after GE bought Alstom last year. The individual heritages of GE and Alstom can both be traced back to the late 19th century, and their recent combination has created a true powerhouse in the world of HVDC, with the strength to address this exciting and growing market.

Clean line says the new HVDC link should start ferrying power in 2020.