Direct current
Long DC lines, and the seams between the grids. A story in 7 stops on the Wattlas map of the North American grid.
The Pacific DC Intertie
When the builders at Niagara chose alternating current in the 1890s, direct current seemed finished as a way to move power, and for most of the next seventy years almost nobody sent it far. Then the Pacific Northwest found a problem the old loser was better at solving. The Columbia River dams made more power each spring than the region could use, while Southern California's demand kept climbing, and between them lay 846 miles of mountain and desert. Over that distance a high-voltage DC line would lose less energy than an AC one, and its operators could set its flow and hold it there. General Electric and Sweden's ASEA built the converters at each end around mercury-arc valves, steel tanks of about seven tons apiece that turned alternating current into direct and back again. Built to run from BPA's Celilo station near The Dalles to Sylmar, north of Los Angeles, the line was in service in 1970 at ±400 kV, carrying 1,440 MW. Within months the San Fernando earthquake of February 1971 wrecked the Sylmar converter station, and it had to be rebuilt. Its voltage and capacity have been raised since, to ±500 kV and more than 3,000 MW, and the mercury valves are long gone. Each spring and early summer the snowmelt of the Northwest still rides it south to Los Angeles.
On the map: The map records the Pacific DC Intertie as one Bonneville Power Administration DC line at 500 kV, about 1,360 km through Oregon, Nevada and California.
Sources: BPA, Hitachi Energy, DOE, Wikipedia
The Nelson River bipoles
Far down the Nelson River, which carries water gathered from as far west as the Rockies out of Lake Winnipeg toward Hudson Bay, Manitoba Hydro began building the 1,272 MW Kettle Rapids station in 1966, hundreds of kilometers of spruce, muskeg and permafrost from Winnipeg and the south, where the customers were. Ottawa lent the money for a DC line, and on June 17, 1972 Bipole I began delivering power south. It ran at the world's highest operating voltage of its day through the largest mercury-arc valves ever developed for the job, and crews raised more than 3,900 guyed towers to carry it, some on permafrost that let their foundations sink by up to a meter. Bipole II was strung beside it in one shared corridor, and Manitoba Hydro gives the pair 895 and 937 km. Between them they carried most of the province's power, and in 1996 an extreme wind event near Grosse Isle, just outside Winnipeg, knocked out both at once, bringing down 19 towers. For the four days of repairs Manitobans were asked, for the first time, to cut back their power use, and they cut it by 8%, while imports over the 500 kV line from Dorsey to Forbes, Minnesota held up the Winnipeg system. Had the storm struck a few kilometers farther south, it could have taken that line down too. Bipole III, built on a western route well away from the first two, runs 1,400 km down the west side of Lake Manitoba to a new converter station at Riel, east of Winnipeg, and when it was completed in 2018 the power of the north finally had a second way home.
On the map: Three Manitoba Hydro bipoles run south from the Nelson River: Bipole 1 at 450 kV and Bipoles 2 and 3 at 500 kV. Bipole 3 takes its own route, west of the other two.
Sources: Manitoba Hydro newsletter, Manitoba Hydro EIS, Manitoba Hydro, Wikipedia
The CU line
On June 8, 1976 a farmer in west central Minnesota drove his tractor into a surveyor's tripod and smashed it. The crew was marking the route of a line that would carry power from a new lignite plant at Coal Creek, North Dakota, to a converter near Buffalo, Minnesota, and like the Nelson River bipoles going up to the north it would run on direct current, at ±400 kV. Two cooperatives, Cooperative Power and United Power, had planned the mine, the plant and the line after the oil shocks, and it became the largest project the Rural Electrification Administration had ever financed. The only comparable line in the country was the Pacific Intertie. In Minnesota alone, 659 of its towers would stand on the land of 476 owners across nine counties, cutting diagonally across fields that a consultant's route study had scored as worth nothing. Farmers blocked the surveyors with signs and chainsaws, gathering each morning at the town hall in Lowry. In January 1978 Governor Rudy Perpich sent in more than 200 state troopers, nearly half the Highway Patrol, and on March 5 more than 8,000 people marched from Lowry to Glenwood in freezing weather. From August 1978 saboteurs who called themselves the Bolt Weevils went after the line itself, and by the end they had toppled 20 towers and shot out nearly 10,000 insulators. The line entered commercial service on August 1, 1979, carrying 1,000 MW over 436 miles, and in 1980 the REA took ownership so that an attack on it would be a federal offense. In 1981 Paul Wellstone, years before he went to the Senate, and Barry Casper published its history under the subtitle The First Battle of America's Energy War. Southeast of Wing, North Dakota, the line crosses Square Butte, the only place in the Western Hemisphere where two overhead DC lines cross. Rainbow Energy Center bought Coal Creek and the line in 2022, and the power still flows east.
On the map: The map marks the CU line as a historic grid site at Coal Creek, dated 1979, and draws the line at 400 kV DC, about 700 km from UNDERWOOD in North Dakota to DICKINSON in Minnesota. Coal Creek is 1,148 MW on the map.
Sources: Wikipedia, CU project controversy, Wikipedia, CU (power line), Wikipedia, Coal Creek Station
Québec to New England
By the 1980s Hydro-Québec had its own giants in the north, the La Grande dams on the rivers draining into James Bay, and more power than Québec could use. New England, which had leaned on oil for much of its electricity through the price shocks of the 1970s, was eager to buy it. The two grids could not simply be wired together, because Québec runs as an interconnection of its own, out of step with the Eastern grid around it, so the power had to cross the border as direct current. Phase I entered service in October 1986, running from Des Cantons in the Eastern Townships to Comerford on the Connecticut River in New Hampshire. Phase II, from Radisson in the James Bay country to Sandy Pond in Massachusetts, was inaugurated in 1990. Hydro-Québec gives it 1,480 km, and it runs at ±450 kV. It was built as a multiterminal system, with converter stations along the line where power could get on and off, five of them at one time, and for decades it was one of only two such systems in the world. Where the line meets the St. Lawrence, between Grondines and Lotbinière, it passes under the river through a tunnel instead of spanning it on towers.
On the map: The Québec–New England line is drawn at 450 kV on the map, running south from northern Québec through Vermont and New Hampshire to Massachusetts.
Sources: Hitachi Energy, Hydro-Québec, Wikipedia
The seam
Somewhere on the high plains, in country where the fence lines run straight for miles, the grid splits in two and nothing on the ground marks the place. Every generator in the Eastern Interconnection turns in step with every other, sixty times a second, and so does every generator in the Western, but the two grew up apart, and today they do not run in step with each other. That is why EIA describes the Lower 48's three main interconnections as operating largely independently from each other, with limited transfers of power between them. What does cross the seam goes through a handful of back-to-back converter stations, strung from Montana down to New Mexico, where alternating current is turned into direct and straight back again inside a single yard, so that two grids can trade power without ever being synchronized. Québec trades with its neighbors through converters like these. One of them, Eel River in New Brunswick, went into service in 1972 as the first converter station in the world built entirely with thyristors, the solid-state switches that would retire the mercury valves of the first long lines, and it gave Hydro-Québec its first direct-current tie to the rest of eastern North America. For its first thirteen years it was the most heavily used DC station anywhere.
On the map: The map outlines four interconnections: Eastern, Western and ERCOT, drawn here, and Québec, which appears as you zoom in. The seam between East and West runs down the Great Plains.
Texas on its own
Around midnight on May 4, 1976, the lights in Altus, Oklahoma were running on Texas power, and that was the point. The utilities of Texas had kept their grid inside the state for decades, beyond the reach of the federal regulators who oversaw power sold across state lines, and by closing a switch at Vernon and sending power north across the Red River, West Texas Utilities meant to force that arrangement into court. The fight that followed, remembered as the Midnight Connection, was settled in the end by the same technology that runs through this story. FERC determined that ties built with direct current, which pass power between grids without locking them into step, would leave the Texas grid outside federal regulation, and ERCOT has run as an island ever since. ERCOT lists four DC ties linking it to power systems outside ERCOT, to the Eastern grid and across the Rio Grande to Mexico, and they are small beside the load they serve. In February 2021, when Winter Storm Uri forced out nearly half of ERCOT's generation, there was little the rest of the continent could send across them, and millions of Texans went days without power.
On the map: Most of Texas is its own interconnection on the map, apart from the Eastern and Western grids. The map draws no DC line in Texas.
Sources: ERCOT, KUT, Baker Institute, Rice University, FERC and NERC
SunZia
The high grasslands of central New Mexico hold some of the steadiest wind in the country and almost nobody to use it. The customers were in Arizona and California, hundreds of miles west, and for a generation the United States had built almost no long direct current lines to reach anyone that far away. SunZia's planners started in the mid 2000s with an alternating current line. Arizona approved it in 2016 after ten years of planning, and New Mexico sent it back in 2018 for a fuller application. Pattern Energy bought the project in 2022, New Mexico approved it that December, and it went ahead as a point-to-point DC line. Ground was broken in September 2023 on a ±525 kV line able to carry 3,000 MW, backed by $11 billion in financing, while crews raised 916 turbines across the Estancia Valley. The Tohono O'odham Nation and the San Carlos Apache Tribe went to court over its route through the San Pedro Valley, where they said it would damage religious and cultural sites, and in May 2025 the Ninth Circuit sided with them and sent the case back to the district court. The work went on. On June 18, 2026 Pattern announced that SunZia was fully operational, about 3,650 MW of wind feeding a 550 mile line from central New Mexico to Pinal County, Arizona, able to deliver more power than Hoover Dam. More than half a century after the Pacific Intertie first carried Columbia River power south, a new long DC line in the West was carrying the wind of New Mexico to the cities.
On the map: The map draws SunZia as a 525 kV DC line of about 890 km through New Mexico and Arizona, owned by Pattern Energy. At its eastern end, SunZia Wind North and South, 1,089 and 2,561 MW, are listed as operating from 2026.
Sources: Pattern Energy, Pattern Energy, SunZia, Wikipedia, SunZia Wind and Transmission