Interconnectors Explained

An interconnector is a high voltage cable or overhead line that joins two power grids, usually across a national or regional border. It lets electricity flow in whichever direction is more useful at the time, so a country short of power can draw on its neighbours, and a country with a surplus can sell it instead of wasting it. In systems with a lot of wind and solar, that flexibility is one of the cheapest ways to handle weather.

What a border link actually does

Power grids are not really separate machines. Across continental Europe, hundreds of generators spin in step at the same frequency, and an interconnector is simply another piece of the same network, sized and metered so the flow across it can be measured and traded. Where two grids are not synchronised, the link has to convert the power in and out, which is a more involved piece of engineering.

Three things make border links valuable.

Cost. At any hour, generators in one area are cheaper to run than in another. Flowing power from the cheaper area to the dearer one lowers the total cost of meeting demand on both sides.

Reliability. Every grid must be able to survive the sudden loss of its largest generator. A link to a neighbour widens the pool of plants and reserves that can respond, so each system needs less spare capacity of its own.

Balancing weather. Wind conditions differ across a continent, and sunset moves west across time zones. A link lets a windy region supply a still one, and lets an evening peak in one country be met by solar still shining an hour to the west.

AC and HVDC

There are two ways to build the link, and the choice comes down to distance and whether the two grids are in step.

AC interconnectors are ordinary alternating current lines, the same technology as the transmission network behind them. They are cheap and simple, and they are the normal choice for overhead links between neighbouring countries inside the same synchronous area, for example between the countries of continental Europe. The catch is that an AC link ties the two systems tightly together. Power flows across it according to physics rather than instruction, following the path of least resistance through the whole network, so a trade between two countries can push unplanned flows through a third one.

HVDC interconnectors convert the power to high voltage direct current at one end and back to alternating current at the other. The converter stations are expensive, but the cable itself is cheaper per kilometre, loses less over long distances, and works under water where AC does not. HVDC also lets operators set the flow directly: the converters are told how many megawatts to move and in which direction, which keeps the transfer predictable and stops it wandering through neighbouring networks.

HVDC has one more property that matters a lot. It can join grids that are not synchronised with each other, because the direct current stage breaks the frequency link between them. That is why almost every subsea connection and every link between separate synchronous areas uses direct current, including the short back to back converter stations that join grids meeting on land without a long cable at all.

Imports, exports and what the figures mean

Over a year, the flows in each direction add up to a net import or net export. Ember’s 2025 statistics show the two patterns clearly. France, with a large nuclear fleet, was a substantial net exporter, sending out about 94 TWh more than it took in. Norway, running almost entirely on hydro, was a net exporter of about 23 TWh. Germany was on the other side of the ledger that year, a net importer of about 19 TWh.

Two cautions when reading numbers like these.

A net figure hides the traffic underneath it. A country can be a net exporter over the year while importing heavily on winter evenings and exporting on windy afternoons. The annual total tells you about the average balance, not about the hours when the link mattered most.

And a net import is not a sign of failure. Importing when a neighbour’s power is cheaper, and exporting when your own is, is the whole point of building the cable. The alternative is running more expensive plants at home.

For the US grid zones we cover, operators publish the flow on each tie with each neighbour, so you can see exchanges hour by hour rather than as an annual total. Those are on the individual grid zone pages.

Why they help with renewables

A wind farm produces when the wind blows, which does not always line up with when people want power. Within one country that mismatch has to be absorbed by storage, by flexible plants or by turning the wind farm down. Across a group of connected countries there are more ways out.

The weather helps. Wind speeds in places a few hundred kilometres apart are only loosely related, so the combined output of a wide area is steadier than any one part of it. Sunset arrives at different clock times across a continent. Rainfall varies by region and by year. Spread across a large connected area, all of that smooths out to some degree.

Storage helps too, and links let one country’s storage serve another. A country with large hydro reservoirs can hold back water while its neighbours’ wind farms are running hard, then release it when the wind drops. In effect the reservoir behaves like a shared battery, without anyone having to build one.

There are limits. A continental weather system can becalm a very large area at once, so links are not a complete answer to a still, dark winter week. Cables have finite capacity, and once a link is full it cannot carry another megawatt however useful that would be. Capacity is also shared with domestic flows, so congestion inside a country can block a trade across its border. And because interconnectors depend on neighbours having something spare, every system still needs enough firm capacity of its own to cope when nobody does.

What to look for

When you read about a new link, the useful facts are its capacity in megawatts, its length, whether it is AC or HVDC, which two systems it joins, and whether those systems tend to be short of power at the same times. A cable joining two grids that peak together on the same cold evenings adds less than one joining grids with different weather, different demand patterns or different fuel mixes. The value of an interconnector is really the value of difference.