How high-voltage DC could rewire the global grid

Posted: July 20, 2026

In 2019, China’s State Grid energized a massive, world-record-setting, 1.1-million-volt direct current (DC) power line. Dubbed the “Power Silk Road,” the Changji-Guquan line is still the longest high-voltage direct current (HVDC) line in the world. It is also the most powerful: 12,000 megawatts of electricity, equivalent to 12 power plants, enough to power 50 million Chinese homes. 

The line begins in Xinjiang, a vast, remote desert region in northwestern China that was once an important stop along the original Silk Road. There, a giant converter station swallows the alternating current pouring out of one of the country's biggest wind and solar belts and converts it into a million-volt river of direct current. That river then flows east for roughly 3,300 kilometers (>2,000 miles), through six provinces, over the Yangtze River, and into a second converter station in Anhui province, in eastern China, where it is converted back to alternating current (AC) and fed into the grid powering China's densely populated east.

The Changji-Guquan line also embodies one of China’s greatest ambitions. In 2016, Liu Zhenya, then Chairman of China's State Grid Corporation, presented his vision for a Global Energy Interconnection: a planet-wide network of clean-energy superhighways linking over 100 countries and 80% of the global population with 180,000 kilometers of ultra-high-voltage power lines. In a presentation at IHS CERAWeek that year, he painted a vivid picture: “Eventually, our world will turn into a peaceful and harmonious global village, a community of common destiny for all mankind with sufficient energy, blue skies and green land.”

Since then, the idea of a global, interconnected clean-energy network propped up by ultra-high-voltage DC lines has garnered a mix of interest, doubt, and concern. But it also represents a striking turnaround.

A little over a century ago, direct current lost the original power war to alternating current. Today, thanks to decades of advances in power electronics, DC has quietly become one of the most important technologies in the global energy transition.

How direct current came back

Most of the world's power grids run on AC. That’s because, in the late 1880s, AC won the War of the Currents, the battle for commercial dominance in which Edison backed direct current while Tesla and Westinghouse backed alternating current. Edison campaigned hard against AC, even publicly electrocuting stray animals to brand it too dangerous for the home. But his real problem wasn’t safety; it was the transformer. This relatively simple device—coils wound around an iron core—can step AC voltage up or down, which is essential for transmitting power efficiently over long distances. DC, at the time, had no equivalent.

Though AC won out as the world’s first power grids were being built, it was not DC’s fate to remain a historical idiosyncrasy. Starting in the late 1920s, Dr. Uno Lamm (later known as the father of HVDC) began work on improving a device known as a mercury arc valve, which could convert high-voltage AC into DC and back. The work bore fruit in 1954 with the world's first commercial HVDC link, a 100-kilovolt undersea cable between mainland Sweden and the island of Gotland.

The United States followed in 1970 with the Pacific DC Intertie, an 846-mile overhead line moving Pacific Northwest hydropower south to Los Angeles. Mercury arc valves gave way to thyristor valves, and newer installations use voltage source converters (VSC) built on high-power transistors known as IGBTs. Each step shrank the converter station, sharpened the control, and reduced power losses. As these technologies evolved, AC was steadily losing the clear advantage that had won it the original war.

What makes HVDC matter now

Three properties of modern HVDC matter for a grid increasingly dominated by wind, solar, and long-distance transmission.

The first is transmission loss: the fraction of electricity lost as heat. Modern HVDC lines lose roughly 3% per 1,000 kilometers compared to the roughly 7% for comparable HVAC lines over the same distance.

The second is the distance itself. The best wind resources are often in remote plains or far offshore; the best solar resources are often found in deserts. But load centers, like cities, are typically somewhere else. The Pacific DC Intertie of the 1970s was built because hydropower in the Pacific Northwest sat more than 1,300 km (>800 miles) from Los Angeles. Today, China is building UHV lines because its sunny and windy northwestern territories are 3,000 kilometers or more from its big eastern cities.

The third is asynchronous interconnection. HVDC can stitch together grids running at different frequencies or out of phase with each other, which AC interconnection cannot do without much more expensive equipment. This feature makes HVDC a natural tool for connecting offshore wind farms to multiple onshore grids, for linking countries across borders, or for bridging the three asynchronous interconnections that divide the United States grid.

Three examples from around the world show how HVDC is growing in reach and influence.

HVDC across the world

The American case: SunZia

In April of this year, Pattern Energy, a San Francisco renewable-energy developer, began testing 916 wind turbines in the high desert of central New Mexico. The wind farm, called SunZia, is the largest in the western hemisphere and is paired with a 550-mile, ±525 kilovolt HVDC transmission line, the largest VSC HVDC installation in the U.S. The line can carry about 3 gigawatts west to a converter station near Casa Grande, in Pinal County, Arizona, where roughly a third is used locally; the remaining 2,131 megawatts continues on to Southern California by way of the Palo Verde substation. That same month, GridStatus reported that California had broken its wind-generation record—the first of several times it would do so in the weeks that followed, each new peak eclipsing the last. And on June 18^th, Pattern Energy announced that SunZia had reached full operational capacity.

“We need more power in the West. The demand is huge and everyone is scrambling to find new resources,” Ric O'Connell, executive director of the consultancy GridLab, told E&E News. “This is a really good story that SunZia is coming online and providing this power.”

The North Sea: an HVDC network

In Europe, the LionLink project, a nearly 2-gigawatt HVDC interconnector planned for the early 2030s, will carry North Sea wind power to the U.K. and the Netherlands. It will also link the U.K.’s National Grid to the Synchronous Grid of Continental Europe. Combining the cross-border link and the wind connection on one offshore platform means fewer cable landings and a more flexible grid.

LionLink is just one piece of a much larger buildout, with the European Commission targeting 64 gigawatts of cross-border HVDC by 2030. Offshore wind in the North Sea is set to grow enormously: under the Hamburg Declaration, signed in January 2026 by the U.K. and nine other European countries, the signatories committed to deliver 100 gigawatts of North Sea wind through joint cross-border projects—part of a longer-standing goal of 300 gigawatts of North Sea offshore wind by 2050. Single-purpose point-to-point links from each wind farm to a single onshore substation would create an unmanageable forest. A multi-terminal HVDC grid would allow grids to share infrastructure: a single Dutch wind farm could be useful to Belgium, Norway, and the U.K. simultaneously.

China's Power Silk Road

China has built out UHV at a scale no other country has come close to. By the close of 2025, 45 lines were in commercial operation across the country, stretching more than 52,000 kilometers and carrying about 300 gigawatts of capacity. That single network now handles more than 70% of the power moved between Chinese grid regions.

But China’s ambitions reach further still. Its Global Energy Interconnection proposal imagines a fifty-year buildout of eighteen ultra-high-voltage backbones linking grids across Asia, Europe, and Africa. “At its core, GEI is a triad of smart grid, plus ultra-high-voltage grid, plus clean energy,” Luo Xi, head of project development at GEIDCO, said at the WIRED and Octopus Energy Tech Summit in 2024. “This is not just about powerlines and substations.[…] It’s about harnessing digital technologies to create smart grids that adapt in real time to our energy needs and building ultra-high-voltage transmission lines that carry massive amounts of clean electricity from source-rich areas to energy-hungry regions,” he said.

The obstacles to a global interconnected HVDC grid

For all these grand visions of a high-tech, interconnected global clean energy grid, plenty of doubts remain.

HVDC, for one, is not a clean-energy tool. The same UHV line that carries remote wind to a coastal city can just as easily carry remote coal. Analysts at Dialogue Earth note that of the nine UHV corridors built under China's 2014 air-pollution-control strategy, eight were designed to transmit coal-fired electricity. Wind and solar still account for only about a fifth of the power moving across China's UHV network.

Permitting and regulatory hurdles add another layer of friction. In the U.S., the "macrogrid" vision—a continental HVDC overlay stitching together the Eastern, Western, and ERCOT interconnections—has been studied repeatedly but never built. SunZia was first proposed in 2006; it took 17 years to permit and roughly three to construct. Phase 1 of the 542-mile Grain Belt Express won its last state approval in 2023 after more than a decade of fights with local opponents. SunZia is "somewhat a cautionary tale," analyst Abby Lestina told E&E News.

The Global Energy Interconnection faces a larger version of the same problem. A 2023 RAND report and analysts at CSIS have raised concerns about the project's industrial-policy aims and the political dependencies it could create. A Columbia Center on Global Energy Policy study of GEI's nearest test case, the Asian Development Bank's Greater Mekong Subregion power-trade scheme, found progress stalled amid weak political trust between China and its southern neighbors. The report concludes that GEI's global grid is better read as "a demonstration of technical potential" than as a blueprint for the future.

Still, skeptics and boosters of a globally interconnected grid agree on some things: decarbonizing at the speed climate targets demand will require thinking about transmission on a grand scale. And however the grid evolves, DC will play a major part in moving power over long distances. HVDC carries greater capital costs, partly because of the converters, but its greater efficiency per mile eventually offsets that upfront premium—and over long distances, AC is simply impractical. A century ago, the War of the Currents ended with a single winner. Whether the power grids of the future look like one global interconnected system or a series of connected microgrids, one thing is almost certain: the AC/DC relationship of the future will look a lot more like partners than rivals.

Resources and further reading:

^{Hitachi Energy — Changji–Guquan UHVDC link
U.S. Department of Energy — The War of the Currents
T&D World — A short history of the mercury arc valve
Video explainer — how HVDC works (YouTube)
POWER Magazine — Offshore wind and HVDC in the North Sea
Hitachi Energy — SunZia
EIA — Today in Energy: transmission
Harvard China Project — Commentary: a global grid
CSIS ReconAsia — Global Energy Interconnection
ScienceDirect — HVDC transmission review
GEIDCO — Global Energy Interconnection planning}