How HVDC Moves Power Across Continents — The Grid's Long-Distance Super-Links

Most of the power grid is a web. The high-voltage lines that carry electricity from the power stations to the cities branch and cross and loop back on themselves, so that power can find many paths and the failure of any one line is absorbed by the rest. That web runs on alternating current — the voltage and current swinging back and forth fifty or sixty times a second — for one overriding reason: AC is trivially easy to step up to a high voltage for the long haul and back down again for use, using nothing but a transformer.

But a handful of the most important links on Earth break that rule. The single longest power lines in existence, the ones that move a whole city's worth of electricity across an entire country, and every power cable that crosses a sea, all run on direct current instead — the steady, one-way kind of current a battery produces. This is HVDC: high-voltage direct current, the grid's long-distance express network. This guide is about what it is, why it exists, and the shape of it on the Grid.

The express lines, not the web

If the AC transmission grid is a road network — every town connected to every other, traffic finding its own way through — then HVDC is the set of long-haul flights laid over the top. An HVDC link doesn't mesh into the grid around it. It runs point to point: from one specific converter station to one other, often a thousand kilometres away, carrying an enormous, precisely controlled block of power in a single straight shot.

That's why this map draws them apart from everything else. Switch on HVDC and you get glowing magenta lines, brighter and heavier the higher their voltage; switch on Transmission as well and the difference is immediate — the dense blue AC web tying whole regions together, and the lonely magenta super-links cutting across it from source to city. The map holds 995 of these links, totalling roughly 111,000 km of conductor — enough to wrap the Earth nearly three times — reaching all the way up to ±1,100 kV.

Why you'd ever switch back to DC

It seems backwards to convert your nice, transformer-friendly AC into DC and back again. The converter stations at each end are big, complex and expensive. But for three jobs, DC wins so decisively that the cost is worth paying.

Long overhead lines. Over distance, an AC line wastes power in ways a DC line doesn't — it loses energy to the constant charging and discharging of the line, and the current crowds toward the surface of the conductor. A DC line has none of that, needs fewer conductors for the same power, and loses less along the way. The converters cost more, but the line costs less and wastes less — so beyond a break-even distance of roughly 600 to 800 km, the whole HVDC system becomes the cheaper way to move bulk power. The further you go, the more it wins.

Cables. This one isn't about cost — it's about physics. Any cable buried under the ground or laid on the seabed behaves like a long capacitor. Run AC through it and an ever-growing share of its capacity is eaten up just charging and discharging that capacitance, until past about 50 to 80 km there's nothing left for actual power. DC has no such limit. So every long undersea power link in the world — without exception — is HVDC. AC simply cannot make the crossing.

Joining grids that aren't in step. Two AC grids can only be wired directly together if they run at exactly the same frequency, perfectly in phase. Often they don't — neighbouring countries, or the two halves of a large country, run as separate synchronised systems. An HVDC link doesn't care: it turns one grid's AC into DC and the DC back into the other grid's AC, so it can bolt together systems that could never be joined with an AC wire. Sometimes the two converters sit in the same building with no line between them at all — a "back-to-back" tie whose only job is to pass controlled power between two grids.

Why China runs away with it

Look at the coverage on the hub and one country dominates: China, which has built more long-distance HVDC than the rest of the world combined. That isn't an accident of mapping — it's geography.

China's biggest power sources are a long way from its biggest demand. Giant hydroelectric dams sit on the rivers of the mountainous southwest; enormous wind and solar farms spread across the northern and western deserts. The cities that consume the electricity are on the eastern and southern coast, a thousand kilometres and more away. That gap is precisely the problem HVDC solves, and China has answered it at a scale no one else has attempted: a fleet of ultra-high-voltage DC lines at ±800 kV, and the first in the world at ±1,100 kV.

The record-holder, Changji–Guquan, runs about 3,300 km across the country and can carry 12 gigawatts in one link — roughly the output of a dozen large power stations, moved from the deserts of Xinjiang to the factories of the east. Brazil tells a smaller version of the same story, hauling Amazon hydropower south to São Paulo and Rio over the ±800 kV Belo Monte lines and the ±600 kV Rio Madeira links; southern Africa moves power from the Cahora Bassa dam in Mozambique down to South Africa the same way. Wherever the power is born far from where it's needed, HVDC is how it travels.

The links under the sea

The other place HVDC shows up is in the water, and here the story is different. Europe's HVDC isn't about distance so much as connection — undersea interconnectors that join one country's grid to another's so they can share electricity.

Because a long undersea AC cable is impossible, every one of these is DC. The North Sea and the Baltic are threaded with them: NorNed between Norway and the Netherlands, NordLink between Norway and Germany, North Sea Link between Norway and Britain, Viking Link between Denmark and Britain, along with BritNed, IFA and COBRAcable; on the other side of the world, Basslink ties Tasmania to the Australian mainland. Their value is in balancing: Norway's hydropower reservoirs act like a giant battery for the region, soaking up surplus wind from Germany and Britain when it's blowing and sending power back when it's calm. On the map these are the HVDC lines that strike out across open water instead of running over land — and they're a close cousin of the other thing crossing the seafloor, the submarine data cables on this same canvas.

Overhead lines and cables

Look closely at the live layer and you'll see two kinds of line. Most HVDC is ordinary overhead line — towers and conductors striding across land, drawn solid. The rest are cables, drawn dashed: mostly short buried stretches feeding converter stations into dense cities, plus the long subsea interconnectors above. By length the network is overwhelmingly overhead — the express super-grids of China, India and Brazil are all pole-and-tower lines — but the cables punch above their weight, because each subsea crossing is long. The single longest mapped cable run is North Sea Link between Norway and Britain, at roughly 700 km under the water. The solid-versus-dashed split is read straight off OpenStreetMap's own power=line and power=cable tags, so at a glance you can tell where the grid takes to the air and where it ducks underground or out to sea.

How it ties into the rest of the grid

This is the Grid canvas — "the world, wired" — and HVDC is the long-distance layer of it. The power plants make the electricity; the meshed AC transmission grid and its substations distribute it across a region; HVDC is what moves it in bulk between regions, across the distances and the seas that AC can't economically cross.

It's also increasingly the grid the energy transition depends on. The best wind and solar resources are often far from the cities — offshore, or in remote deserts and plains — and connecting them to demand over long distances is exactly what HVDC is for. The same is true of the surging electricity demand from data centres: as that load grows, so does the need to move clean power across continents to feed it. The express network is quietly becoming one of the most important parts of the whole system.

An honest map

A few caveats, in keeping with how everything here is built.

These links come from OpenStreetMap, selected as the power lines and cables that volunteers have tagged as carrying direct current. Each link is coloured and weighted by its voltage, which is recorded on about 95% of them — an honest axis the data genuinely supports. Two kinds of line are deliberately left out: HVDC electrode lines (grounding conductors tagged at zero volts, which aren't the power-carrying poles) and low-voltage DC traction and industrial lines below 80 kV, neither of which is the long-distance HVDC this layer is about. Each link's length is a great-circle estimate of its longest single mapped run, so two parallel cables sharing a name can't inflate into one impossible distance; the network total is the true sum of every conductor. As with the other mapped layers on this canvas, coverage follows where OpenStreetMap is best mapped — which for HVDC genuinely leans toward China and Europe — so read this as the publicly mapped network rather than a complete official register.

The solid-versus-dashed build type is taken straight from each way's OSM tag, and it's the mapper's tag rather than a fresh survey. That's reliable enough to draw, with one honest wrinkle worth naming: a few older Nordic subsea interconnectors — Baltic Cable, Konti-Skan, Skagerrak and SwePol — are tagged as lines in OpenStreetMap, so they appear here as overhead even though they run under the sea. Rather than quietly hand-correct them, the map shows what the data says; the finer subsea-versus-buried distinction is too inconsistently tagged to assert, so the layer commits only to the clean overhead/cable two-way.

Open the live Grid map, switch on HVDC alongside Transmission, and the shape of it appears: the dense blue web of the ordinary grid, and the long magenta super-links striking across deserts, mountains and seas to carry power where the web cannot reach.