In the early days of electrical generation and transmission, much hullabaloo was made over the respective value of alternating current (AC) versus direct current (DC) technology, and the reigning geniuses of the 1880s, Nikola Tesla and Thomas Edison, waged a metaphorical war over the issue. AC has predominated ever since, but a shifting electric-generation landscape offers new opportunities for high-voltage DC-based (HVDC) transmission systems, and the industry is drawing new battle lines.
HVDC transmission systems aren’t new. In fact, such designs have been in service since the 1950s in Europe and since the early 1970s in the United States. The first U.S. project, the Pacific DC Intertie, was developed for much the same reason today’s developers are turning to the technology—an interest in transmitting large amounts of remotely generated electricity over distance to a population center. In this case, the Bonneville Power Administration (BPA) wanted to reach the largest possible market, Los Angeles, for its hydrogenerated electricity.
The initial design, which began operations in May 1970, featured a capacity of 1,440 megawatts (MW) and stretched more than 800 miles to supply the Los Angeles Department of Water and Power. That still-impressive capacity has been upgraded several times as the Portland, Ore.-based BPA has sought to meet the needs of the ever-growing L.A. region with carbon-free hydropower. The most recent effort, a total reconstruction of the Oregon converter station that ties the line into the BPA’s grid, was completed this year. The line now features a capacity of 3,220 MW, and the new converter station was designed to enable system expansion up to 3,800 MW.
Including their ability to connect to otherwise incompatible grids, HVDC systems offer a number of advantages over AC transmission at such scale and distance. AC transmission lines can only connect to grids with electrical phases that are in sync with each other. HVDC systems, on the other hand, use converter stations at either end of their runs that sync operations with the respective connected grids. This means they can transfer power between asynchronous grids, such as those of the United States and Canada, which is an important bonus in several projects now on the drawing board.
HVDC is much more efficient than AC transmission in long-distance applications, which is even more important for projects now in development. HVDC systems feature lower line resistance, which leads to significantly reduced line losses in long-distance applications. This also means HVDC designs can make full use of their conductors’ carrying capacity, so they can move more power with similarly sized conductors. While AC transmission requires three conductors, HVDC plans only need two, which means narrower right-of-way requirements for transmission towers and other equipment.
“Generally, a DC line of the same transmission capacity is cheaper to build,” said Roger Rosenqvist, business development vice president in ABB’s HVDC and HV cable division, based in Cary, N.C. “And, the value you put on the energy that’s basically burned off to the environment also plays into the picture.”
This added efficiency, along with reduced right-of-way needs, is driving a surge in HVDC development in China, Europe and the United States. China, especially, is turning to the technology to carry enormous amounts of electricity from resource-rich rural areas to its dense population centers. The country has recently completed a 1,600-mile, ultra-high- voltage DC line with a capacity of 1,100 MW at 1,100 kilovolts (kV)—current U.S. AC transmission systems top out at 765 kV. A large network of similar 800–1,100-kV lines is in the planning and construction stages. In the wake of Japan’s Fukushima nuclear disaster, Germany also is developing an HVDC network to use wind-power sources to replace the nuclear generation it shut down.
Largely due to the fact that interstate transmission lines require approval from each state they cross, the United States has been somewhat slower in such efforts. Planning and permitting long-distance transmission across multiple states is difficult and expensive. However, several U.S. projects that promise to enable bulk transmission of wind-generated electricity from the West and Midwest to the East, and hydropower from Canada to the Northeast, are slowly moving forward.
The key technology for HVDC systems, and the point where the greatest advances have come, is at the converter station. It is at this facility that AC power from the supply point is converted to DC for transmission. Just as semiconductors have shrunk to enable moon-mission-level data crunching on a device as small as a smartphone, converter-station space requirements have become significantly less, even as capabilities have climbed.
“You see the same general developments you see in all power electronics,” Rosenqvist said. “That drives down the cost because you drive down the footprint and the power losses.”
ABB was the primary contractor on the upgrade of the BPA’s Celilo converter station at the supply end of the Pacific DC Intertie, and this project serves as an example of how today’s advanced control systems enable far greater power transfer in a much smaller space.
“Historically, any kind of substation has had a lot of cable coming in from the yard,” Rosenqvist said.
Today, however, the communications infrastructure is based on fiber optic busses.
“The new station is just a small part of the footprint the original equipment had,” he said.
Solid-state control devices called thyristors have been the technology standard for converter station controls since the late 1970s and form the basis of systems both ABB and Siemens, the world’s two leading HVDC technology suppliers, brand as “HVDC Classic.” In the last 15 years or so, new systems based on voltage source converters (VSCs) have come to market, enabling the capacities and distances now being considered at the cutting edge of HVDC technology, according to Wayne Galli, executive vice president for transmission and technical services at Clean Line Energy Partners. The Houston company is behind the most ambitious U.S. projects now in planning stages. ABB calls its version of VSC-based systems “HVDC Light,” while Siemens dubs it “HVDC Plus.”
While technologists may have addressed many of the operational challenges of bringing DC back into the transmission business, they have not been quite as successful at meeting the political roadblocks raised by the kind of long-distance, interstate projects for which HVDC is well-suited, as Galli’s company knows only too well. Recently, though, interest in boosting the presence of renewable energy in the U.S. generation mix has prompted federal intervention in a fight over one Clean Line effort in a manner that could set a new precedent favoring long-haul HVDC transmission development as a matter of national interest.
The Plains & Eastern Line is proposed to run approximately 700 miles, from the wind-rich Texas and Oklahoma panhandle region, through Arkansas, to Memphis, Tenn., carrying 3,500 MW of wind-generated electricity to the mid-South and Southeast. A mid-Arkansas converter station could supply an additional 500 MW to the state’s customers. The project is estimated to cost $2.5 billion and could mean $140 million in voluntary payments to Arkansas coffers over the course of 40 years. However, a strong opposition movement has threatened to derail development—that is, until the U.S. Department of Energy (DOE) stepped in this spring.
For the first time, the DOE exercised authority granted under Section 1222 of the Energy Policy Act of 2005 to recognize the transmission line as a necessary means for reducing congestion and meeting electricity demand. In essence, this could allow Clean Line to acquire property for its required rights of way through eminent domain, though it would be required to compensate property owners at a fair market value.
“That is a very groundbreaking decision, because it’s the first time the federal government has been involved in a transmission project,” said Peter Kohnstam, HVDC business development manager for Siemens.
He also noted exceptions for federally owned grids, such as the BPA and the Tennessee Valley Authority.
“It’s quite a contentious decision as well,” he said. “It’s a fascinating precedent.”
Clean Line still has plenty of work to do signing up customers at the line’s receiving end, and the actual right-of-way acquisition process hasn’t yet begun. As this is the first use of a provision passed by Congress 11 years ago, legal battles are almost certain to ensue. However, the move could strengthen the company’s position in regard to two other HVDC projects also aimed at bringing wind power from west to east.
Favorable economics for the future
While Clean Line Energy Partners is focused on HVDC over long distances, others say the technology is becoming more affordable for shorter runs, especially in challenging settings. It performs well in underground applications, for example, where AC transmission can be limited to 30–50 miles. It’s often the only feasible option for underwater cables of any significant distance because AC cables have extremely high line losses in such installations. Additionally, the falling cost of HVDC’s required power electronics along means the added expense of control stations is becoming less of a concern in larger financial decisions.
HVDC systems offer advantages that run beyond more efficient transmission over long distances. For example, HVDC offers the opportunity to isolate connected grids from each other, in the case of a failure on either end, which could prevent a regional outage from cascading into multistate event. In dense urban environments, HVDC’s added carrying capacity could allow for distribution upgrades requiring significantly less room than equivalent AC designs would demand.
“It allows us to cross thresholds,” Kohnstam said of the intangible benefits HVDC technology can offer to system planners looking beyond first-cost calculations and toward a more efficient and resilient grid. “When that changes the economics, that’s when it will change the break-even point.”