As the United States moves to decarbonize its electricity supplies, transmission planners face a conundrum. The most productive and reliable wind, solar and hydro generating resources are often far from the cities that need power most. Simply building additional high-voltage alternating current (HVAC) transmission lines to connect them is difficult because environmental permitting and local community concerns can tie up such projects for years. Now, developers are investigating novel high-voltage direct current (HVDC) options that take advantage of existing infrastructure to minimize development hurdles.

Finding new ways to ship power over longer distances is a critical issue as a number of states are setting ambitious goals to move their electricity supplies away from fossil fuels such as coal and natural gas. As of 2020, nine states, the District of Columbia and Puerto Rico had set requirements for 100% renewable electricity by 2050 or earlier. Other states have set 50% targets by 2040 or sooner. Numerous investor-owned utilities have set aggressive goals for incorporating carbon-free energy into their portfolios.

However, as Roger Rosenqvist, vice president of business development for ABB, Memphis, Tenn., points out, these new commitments require a rethinking of the transmission paradigm on which a century of experience is based.

“The U.S. has historically been very dependent on fossil fuel and nuclear generation that has been built and located close to major load centers,” he said. “The transmission systems to distribute electricity have not been very long. They’ve been more local systems, which have been interconnected over time.”

These systems have become quite dense in population centers, Rosenqvist said, but are virtually nonexistent in areas where the biggest opportunities to tap renewable resources exist.

“Where we have our best resources for wind is primarily in very remote areas in the Midwest, and in northern Maine and New York state. Solar is in desert areas in the West and Southwest,” he said. “We’re going to need new transmission lines to transmit that power to load centers.”

Rosenqvist also noted that the intermittent nature of both wind and solar production can make backup resources a necessity. Large hydropower is one carbon-free option, and, meanwhile, many states and utilities are looking toward hydro-rich Canada for this power.

“In order for that to be connected to the U.S., you need new connection points,” Rosenqvist said.

Similarly, he added, new transmission connections also will be required to facilitate the nascent offshore wind industry.

“There is really no transmission capacity at the coastline to receive that power and bring it into the existing transmission backbone,” he said.

HVDC technology offers significant efficiency advantages for all these scenarios. It’s been in use for almost half a century in the United States, and it was first introduced as a way to bring hydropower from Portland, Ore., down to the Los Angeles area. That system—the Pacific DC Intertie—was upgraded to a capacity of 3,220 megawatts (MW) several years ago, with the capability to be expanded up to 3,880 MW.

Such enormous capacity is enabled by the nature of DC power. The current encounters less line resistance, which leads to significantly lower line losses in long-distance installations. Plus, HVDC systems can make full use of a conductor’s carrying capacity because AC power is only carried along the conductor’s “skin.” As a result, HVDC designs can accommodate greater loads with similarly sized conductors.

An added feature of HVDC is the ability to connect asynchronous grids together, which is much more challenging with AC transmission. The lines feature converter stations at either end that can convert, say, 50 Hz power from a Canadian hydropower station to DC at one end and then convert it back to AC, this time at 60 Hz, on delivery to the U.S. grid.

In the past, converter stations were potential budget-busters. However, recent advances in converter station engineering have reduced both the required size and cost. These improvements have boosted the economic viability for HVDC proposals.

Despite these advantages, several ambitious HVDC transmission proposals have foundered. Three of the largest were initiated by the company Clean Line Energy Partners, Houston, and these have all struggled to gain approval.

The 4,000 MW Plains and Eastern Clean Line project would have carried wind-generated electricity from Oklahoma 720 miles east through Arkansas to connect with the Tennessee Valley Authority, Knoxville, Tenn. The 3,500 MW Rock Island Clean Line was intended to run 500 miles from Sanborn, Iowa, to Morris, Ill. The Grain Belt Express, with a proposed 4,000 MW capacity, is intended to run 780 miles from southwest Kansas to the eastern edge of the PJM Interconnection in Indiana.

Both the Plains and Eastern and Rock Island projects failed to gain needed approvals after investors poured hundreds of millions of dollars into their development. New owner Invenergy, Chicago, plans to increase the Grain Belt Express capacity to up to 2,500 MW. Clean Line Energy Partners has ceased business operations. In all cases, local opposition to needed rights-of-way acquisition forced expensive legal battles that, in the case of the first two efforts, proved unsuccessful.

The point-to-point nature of HVDC transmission has raised hurdles in these interstate proposals. The communities through which the lines run see no benefit from the renewable energy they carry. With HVAC transmission, it’s easy to offload electricity to local utilities along the way. HVDC systems, however, would require a converter station at every connection, which could increase costs to unacceptable levels. Instead, their financial viability is based on the economics of scale created by carrying bulk power over long distances.

In this way, HVDC transmission is similar to a pipeline that carries oil or natural gas from a collection point to a hub, where smaller lines might send the fuel to local distribution systems. But, unlike federally regulated pipelines, transmission systems are governed at the state level. As a result, HVDC developers can face multiple jurisdictional hurdles that can prove difficult to overcome.

However, HVDC lines—unlike the HVAC counterparts—can be buried over long distances, which is one approach where some proposed projects are seeing success. The underground, underwater Champlain Hudson Power Express has made it through a number of significant approvals for a proposed 1,000 MW line (actually, two 5-inch-diameter cables) connecting New York City to hydropower resources in Montreal. The cables will run under Lake Champlain and the Hudson River, and elsewhere be buried along existing rail and highway rights-of-way.

The SOO Green Renewable Rail project also is going underground. It’s planned to carry 2,100 MW of wind-generated energy from Mason City, Iowa, near the Minnesota border, 349 miles to a substation in Plano, Ill., outside Chicago. This will provide an important new connection between two adjacent transmission systems, the Midcontinent Independent System Operator and the PJM Interconnection, enabling wind power from the Midwest to be sold into Eastern U.S. energy systems for the first time. The lines will be buried within an existing rail corridor, which could minimize the right-of-way issues that have plagued previous HVDC development efforts.

The plan was inspired by a model first used by the fiber optic industry, according to Sarah Lukan, partner with public relations firm LS2group and spokesperson for Direct Connect Development Co., the transmission project’s owner. She said it helps address the approval issues that, so far, have kept Midwestern and Western wind generation from making its way eastward.

“There’s a lot of market demand within PJM for renewable energy,” she said. For example, Illinois has a goal of 25% renewable energy by 2025, and proposed legislation would target 100% renewables by 2050. “This will help to give them the product they’re looking for.”

Converting existing HVAC lines to HVDC is another way to take advantage of existing infrastructure to speed approvals and reduce cost. The first such major project, the 211-mile UltraNet line in Germany, will carry 2,000 MW of wind energy from North Sea wind farms to the country’s industrial south. It’s expected to be operational in 2022.

In the United States, researchers at Carnegie Mellon University (CMU) in Pittsburgh have been studying the limits of this approach. Specifically, they are looking at how it could be used to reduce the distance where HVDC makes financial sense. Building new HVDC lines, whether above-ground or underground, really makes the most sense at longer distances. That’s where the reduction in line losses versus HVAC, and the greater load carrying capability, beat the added expense of converter stations.

As a doctoral student in CMU’s Engineering and Public Policy Department, Liza Reed was the lead author of a 2019 study in the Proceedings of the National Academy of Sciences that explored the economics of HVAC-to-HVDC conversions. Traditionally, 375 miles has been seen as a lower limit for new HVDC development, she said. However, her research supports that projects as short as Germany’s 211-mile UltraNet could also make financial sense in the United States.

New converter stations and insulators would be required under such an approach, according to ABB’s Rosenqvist.

Additionally, depending on the design, transmission towers might need modification, though the right-of-way width requirement would remain the same. But such projects could make use of the existing conductors, Reed noted.

“You can use the same conductors because they just move electrons,” she said. However, the related savings in switching to dedicated HVDC equipment comes with a trade-off in efficiency. “There is an aspect of this that is suboptimal; the losses might be more than with a custom DC system.”

Reed said the paper, co-authored with CMU faculty members M. Granger Morgan, Parth Vaishnav and Daniel Armanios, attracted attention for its findings that repowering in-place HVAC lines for HVDC service can pencil out at shorter spans. For her, though, the work only raises more questions regarding other opportunities for efficiently adding more renewable resources to existing energy portfolios. Two of these queries stand out for her.

“What are we overlooking?” Reed asked, hypothetically, “and what could it mean to us?”