A massive expansion leads to the first ultrahigh-voltage AC-DC power grid
Wind rips across an isolated utility station in northwestern China’s desolate Gansu Corridor. More than 2,000 years ago, Silk Road traders from Central Asia and Europe crossed this arid, narrow plain, threading between forbidding mountains to the south and the Gobi Desert to the north, bearing precious cargo bound for Imperial Beijing. Today the corridor carries a distinctly modern commodity: gigawatts of electricity destined for the megacities of eastern China. One waypoint on that journey is this ultrahigh-voltage (UHV) converter station outside the city of Jiuquan, in Gansu province.
Electricity from the region’s wind turbines, solar farms, and coal-fired power plants arrives at the station as alternating current. Two dozen 500-metric-ton transformers feed the AC into a cavernous hall, where AC-DC converter circuits hang from the 28-meter-high ceiling, emitting a penetrating, incessant buzz. Within each circuit, solid-state switches known as thyristors chew up the AC and spit it out as DC flowing at 800 kilovolts.
From here, the transmission line traverses three more provinces before terminating at a sister station in Hunan province, more than 2,300 kilometers away. There, the DC is converted back to AC, to be fed onto the regional power grid. Since it opened in mid-2017, the 26.2 billion yuan (US $3.9 billion) Gansu–Hunan transmission line has moved about 24 terawatt-hours.
The sheer scale of the new line and the advanced grid technology that’s been developed to support it dwarf anything going on in pretty much any other country. And yet, here in China, it’s just one of 22 such ultrahigh-voltage megaprojects that grid operators have built over the past decade. In the northwestern region of Xinjiang, China recently switched on its largest UHV link: a 1,100-kV DC circuit that cost over 40.7 billion yuan. The new line’s taller transmission towers and beefier wires parallel the Gansu–Hunan line through the Gansu Corridor, before diverting to Anhui province in the east.
Power Shift: This station in Zhejiang province imports hydropower from Sichuan province as direct current and converts it to alternating current.Photo: Xu Yu/Xinhua/Redux
The result of all this effort is an emerging nationwide supergrid that will interconnect China’s six regional grids and rectify the huge geographic mismatch between where China produces its cleanest power (in the north and west) and where power is consumed (in the densely populated east). By using higher voltages of direct current, which flows through conductors more uniformly than does alternating current, the new transmission lines dramatically reduce the amount of power that’s lost along the way.
But even as China celebrates the completion of more than 30,000 km of UHV lines, power engineers are struggling to master the resulting hybrid AC-DC transmission system. They must ensure that the new long-haul DC lines don’t destabilize China’s regional AC grids. For example, if the 8-gigawatt DC line from Gansu were to unexpectedly go off line, the power shock could cause widespread blackouts in Hunan and beyond.
To minimize the threat, the State Grid Corp. of China, a state-owned company that runs most of China’s transmission and distribution grids, intentionally limits the line’s throughput to no more than 4.5 GW. In practice, the line has carried less than one-quarter of its design capacity on average. That’s one reason why over one-third of Gansu province’s theoretical wind output and one-fifth of its solar potential went unused in 2017. Other UHV lines in neighboring regions have similarly operated below capacity. And eastern provinces don’t have sufficient incentive to import the cleaner power that the UHV lines offer.
The ultimate solution to both issues, according to State Grid engineers, is to double down on UHV. They argue that the country must move far more energy via UHV DC to maximize the use of renewable energy while slashing reliance on coal. State Grid is also building a world-leading set of ultrahigh-voltage AC lines, to help eastern China’s regional AC grids absorb the output from those massive lines.
“The UHV AC power grid is like a deep-water port, and the UHV DC is like a 10,000-ton ship. Only the deep-water port can support the 10,000-ton ship,” says Qin Xiaohui, vice director of power system planning with State Grid’s China Electric Power Research Institute, in Beijing.
China’s Hybrid AC-DC Grids
Ultrahigh-voltage DC lines move coal-fired and renewable generation thousands of kilometers to China’s megacities. UHV AC helps distribute the imported electricity.
Meanwhile, power authorities everywhere are watching. Gregory Reed, a DC transmission expert who runs the University of Pittsburgh’s Center for Energy, says China’s UHV grid puts it far ahead of the rest of the world. “They’re investing significantly, and they’ve gone right to the highest levels of technology capability from day one. There’s no comparison anywhere else in the world. It’s like we’re all still pedaling our bicycles, while the Formula 1 race car goes flying by.”
China’s UHV movement was born of a limo ride. It was late 2004, and Liu Zhenya, then president of State Grid, was sharing a car with Ma Kai, minister of the National Development and Reform Commission (NDRC), the powerful state body that regulates China’s growth and major investments. As Chinese policy expert Yi-chong Xu describes in her 2017 book Sinews of Power (Oxford University Press), Ma complained of the crippling power shortages of the day. Liu blamed “weak and fragmented” grids, ones ill-equipped to exchange bulk power. And he proposed a bold solution: massive cross-country power lines utilizing the most advanced UHV technologies.
Within a year, Ma’s NDRC had approved an ambitious and comprehensive plan that embraced Liu’s vision. It combined UHV DC lines, which excel at moving bulk power from one spot to another over long distances, and a UHV AC backbone to reliably distribute that power to consumers. State Grid would lead the engineering and ensure that domestic suppliers would manufacture 90 percent of the UHV equipment, thus building up a new high-tech export sector for China.
Over the next decade, Liu delivered. He put some 2,000 State Grid engineers on the project and funded more than 300 professors and 1,000 graduate students at Chinese universities to conduct power-grid-related R&D. State Grid expanded and refocused its research centers to attack specific UHV issues, including how to safely handle the higher electromagnetic fields and the more potent impulses during switching and faults.
In January 2009, State Grid energized its first UHV demonstration line—a 650-km, 1,000-kV UHV AC transmission line that linked the North China and Central China regional grids. Ten years on, State Grid has completed 19 of 30 proposed UHV lines.
Nimby: Coal plants in Inner Mongolia feed this station near Shanghai, reducing the megacity’s air pollution.Photo: Imaginechina/AP
That aggressive build-out has helped fast-growing urban centers such as Shanghai stave off power shortages despite delays in the expansion of China’s nuclear power capacity and constraints on local coal power due to air-quality concerns. The new UHV grid is also helping the country lead the global transition to renewable generation, moving 161.5 terawatt-hours of hydro, wind, and solar energy in 2017 alone.
ABB, Siemens, and other international power-technology companies have been instrumental in developing and validating key components of the Chinese UHV grid. But State Grid has insisted on sharing the intellectual property for the technologies developed at its behest.
In a 2014 interview, Executive Vice President Liu Zehong described one tense episode in 2006 when State Grid asked international suppliers to help develop 6-inch-diameter thyristors capable of handling more current than 5-inch thyristors could. The suppliers initially balked, said Liu, but ultimately relented because of State Grid’s “determined attitude” and the “huge market opportunities” of the Chinese market. Two years later, Chinese firms were manufacturing the resulting 6-inch switches.
For all of State Grid’s progress, its UHV deployment remains uneven and incomplete. China could end up with just half of the 89,000 km of UHV lines that its plans called for by 2020 and none of the anticipated UHV links to Kazakhstan, Mongolia, and Russia. Many proposed projects—particularly for the UHV AC backbone—have failed to gain the NDRC’s blessing. As a result, many areas still have no UHV AC lines, and both types of UHV are delivering well below expectations.
What has blocked full implementation is an intense debate over the future of UHV. Some Chinese grid experts question the hundreds of billions of yuan spent on UHV projects and what they see as State Grid’s monopolization of grid engineering and manufacturing. Provincial officials have chafed at the centralization of grid planning and operation that UHV requires.
Supersized: Pushing UHV technology to 1,100 kilovolts requires upscaled components like this 800-metric-ton transformer.Photo: ABB
Some experts have also criticized Liu’s ultimate goal for the UHV AC backbone—linking up and synchronizing China’s regional grids—as far too risky. Han Yingduo, a member of the prestigious Chinese Academy of Engineering and a professor at Tsinghua University, in Beijing, has warned that unifying China’s grid would make it far more vulnerable to cascading blackouts, like the one in 2003 that knocked out power in the northeastern United States and Canada.
Because no other country has ever built a hybrid UHV AC-DC grid, State Grid engineers are having to feel their way along. In a traditional lower-voltage network, the grid operator typically reserves emergency power to cover the sudden loss of the grid’s largest asset. That may mean keeping a gigawatt or two of extra power generation at the ready.
Now add multiple UHV lines to your network, each carrying 8 to 12 GW, and your requirements for reserve power rise dramatically.
Maintaining the ideal voltage on a UHV grid is also enormously challenging. Thyristor-based UHV converters consume what’s known as reactive power—found in AC systems in which the current and voltage are out of phase. (By contrast, active, or real, power is the power that’s actually consumed by the grid’s loads; its current and voltage waves are aligned.) By consuming reactive power, the UHV converters tend to pull down the voltage of surrounding AC lines, so converter stations have equipment to supply reactive power and prop up the AC voltage.
But if an AC line’s voltage sags, nearby converters will consume even more reactive power, pulling voltage down further. A voltage sag can also disrupt the thyristors’ ability to switch from one current path to another, a process known as commutation. A severe commutation failure [PDF] will cause the converter to shut down, deepening the AC voltage drop and starting a potentially destructive feedback loop that could end in a blackout. “Successive DC commutation failures will trigger a chain reaction,” says Qin, the system planning expert at State Grid’s Beijing research institute.
Crushing It: China’s newest UHV line from Xinjiang to Anhui has set world records for transmission distance, power, and voltage.Photo: VCG/Getty Images
The resulting blackout could travel far and fast, notes Zhang Fang, a system operator in State Grid’s National Electric Power Dispatching and Control Center, in Beijing. When a UHV DC circuit goes off line unexpectedly, it creates a power surge hundreds or thousands of kilometers away, on the AC grid that feeds it. “The UHV DC line is actually acting as an amplifier. A small AC disturbance in the receiving end can become a large AC disturbance in the sending-end grid,” says Zhang.
To minimize the risk of multiple converter failures and cascading blackouts, engineers for State Grid’s East China regional grid have deployed a fiber-optic control network that automatically rebalances supply and demand. If necessary, it can boost line voltage within 200 milliseconds of a voltage drop, using a set of fault responses that have been built into the East China grid’s AC-DC converters. As soon as the fiber-optic network flags an outage on a UHV DC line, the converters pull up to 10 percent more power over the remaining DC lines to keep the grid operational. The optical control scheme can also restore balance by releasing power from pumped hydro plants, which store energy by pushing water uphill. And it can trigger small controlled blackouts, shutting off some distribution feeders to reduce demand while sparing hospitals and other essential loads.
These measures have enabled a trio of UHV DC lines that deliver hydropower from the Southwest China grid to operate continuously at their combined 21.6-GW design capacity. The result is an electrical trifecta: Greater Shanghai, China’s most densely urbanized and industrialized region, gets more clean power; the Yangtze River Delta’s megadams spill less excess water during flood season; and State Grid earns more revenue from its UHV investment. Even so, Shanghai still runs short of power for several weeks each summer, forcing State Grid to pay big customers to idle their factories. Keeping pace with growth may require tripling Shanghai’s electricity imports within a decade.
At the national control center, in Beijing, mounting pressure to push more clean power through State Grid’s UHV lines is hard to miss. The main screen displays the status of the AC and DC trunk lines, providing a real-time view of the entire system. Dominating the left wall are warning lights tracking renewable energy curtailment in each of 25 provinces—and who should be fixing it. Green lights mean that all of the potential solar and wind power is being used. Blue, yellow, and orange lights indicate renewable energy waste, which State Grid’s provincial, regional, or national controllers, respectively, must try to stop.
“We are determined to consume the renewable energy to the maximum extent. That’s our job,” says Zhang. Controllers may reroute power from a province with low electricity demand to another where demand is higher. Or they may steer electricity to one of State Grid’s 21 pumped hydro plants, which collectively can soak up 19 GW.
Modern Imports: A trio of UHV DC lines traces the Silk Road in China’s Gansu province.Photo: Peter Fairley
In theory, Chinese law has long required grid operators to prioritize renewable energy. But in practice, each province has its own plans and priorities, which tend to favor electricity generated locally. For instance, in Zhejiang province, south of Shanghai, significant opposition to importing electricity has hampered the operation of an 8-GW UHV DC line from Ningxia province, according to analysts at Bloomberg New Energy Finance.
On the windy, sunny day when I visited Gansu’s DC converter station last year, its UHV line was carrying just 3 GW of its 8-GW capacity. That was the cumulative output from several renewable plants. But the province also has an additional 15 GW of solar and wind that’s connected to the new line but not yet authorized to feed power into it.
Change is coming. Two months after my visit, power companies in coastal Jiangsu province struck a deal to buy power from Gansu’s largest wind farm via another UHV DC line. And last November, State Grid began building a UHV DC line from Qinghai province to move even more of Gansu’s renewable generation. Meanwhile, the NDRC is stoking demand by mandating minimum rates of renewable energy use by each region.
State Grid’s long-term goal to interconnect its regional grids should also reduce curtailment, experts say. Zhang Ning, an authority on renewables integration at Tsinghua University, points out that the Southwest grid’s hydropower can balance the fluctuations in the Northwest’s wind and solar output. “If we interconnect the West, curtailment of wind power there can be reduced from more than 20 percent to 5 percent,” he estimates, and both regions’ use of coal can also be cut.
Even as State Grid irons out the kinks in its UHV grids, the company is pushing its equipment and expertise abroad. It has led the creation of nine UHV standards through the International Electrotechnical Commission and the IEEE—a move that researchers at Argonne National Laboratory, in Illinois, warned would help Chinese suppliers “crowd others out of the global market” [PDF].
State Grid is already working on its first international UHV DC project: a pair of 800-kV lines to move power from Brazil’s Belo Monte megadam. But subsequent UHV sales have been slow to materialize. That may be because most countries do not yet need, or cannot afford, a 1,000-kV AC or DC line.
Undaunted, former State Grid chairman Liu is now crusading to build transcontinental and intercontinental UHV grids. The same technology that went into building the 1,100-kV line from Xinjiang to Anhui could efficiently move power up to 5,000 kilometers. “If we just turn that line around to point west, we are getting close to Europe. So the technology is available,” says Magnus Callavik, general manager of ABB Sifang Power System, a Beijing-based joint venture between Swiss power-engineering giant ABB and China’s Sifang Automation.
Callavik says he is convinced that continental-scale UHV DC will happen, sooner or later. In a world that must decarbonize, figuring out how to balance variable energy supplies such as solar and wind generation with regional loads is a growing concern. “Transmission is a very cost-efficient way of doing that,” says Callavik.
In China the question is how quickly State Grid will overcome the technical and political obstacles that are holding back UHV’s carbon-slashing potential. If the country continues to rely heavily on coal power, importing that power over thousands of kilometers will help clear the air in China’s eastern megacities. But the country’s carbon footprint will remain unchanged, and the benefits for the global climate will be nil. Mobilizing gigawatts of renewable power over a UHV grid, on the other hand, promises a real change, for China and the world.
This article appears in the March 2019 print issue as “A Grid as Big as China.”