Supergrids: the challenging comeback of direct current transmission

Moving power from one country to another more efficiently, while cutting CO2 through better links between renewable generation and the grid: that is where high-voltage direct current (DC) supergrids come in. They add to, and in places replace, the old high-voltage alternating current (AC) grid.

Direct current can carry electricity over very long distances with little loss, around 3% per 1,000 km, and it opens wider prospects for interconnected, integrated grids across the world. DC lines can run for several hundred kilometres, whereas AC lines reach at most 70 to 100 km.

DC also makes it possible to connect grids with different voltages, frequencies and operating regimes, and to use the DC connection as a generation reserve, since it can deliver the required power quickly and so adds stability, flexibility and security to the grid.

DC cables take up less room than traditional AC lines and can run underground for long distances. That eases public worries about safety or expropriation of land, so the grid can expand faster.

AC is not finished

The advantages of DC do not mean alternating current is gone for good. AC allows the use of transformers, highly efficient machines for changing voltage, and it is cheaper over short distances.

“We will have multifaceted, interconnected grids, with layers of AC and DC, and a two-way flow of energy,” said Thomas Pozzi of General Electric Grid Solutions. “Thanks to the DC meshes intertwined with alternating current, the AC grid will become more stable, more controllable and blackouts will become rare events.”

Projects under way

The European Commission’s energy roadmap foresaw a supergrid for Europe by 2050, and DC interconnections between the countries around the Baltic and North Seas were already developing quickly.

The agreement between the Italian energy agency ENEA and the Japanese energy and technology agency NEDO to build a new DC transmission plant belonged to this trend. Designed by Toshiba, the plant was to be installed at ENEA’s Casaccia research centre near Rome.

The technology is a high-voltage DC system based on modular multilevel converters (MMC) using VSC technology, which gives more control over power flows. “The fact that a new player has come onto the market for direct current energy transmission is an advantage also for the European utilities that will have a wider array of options from which to choose,” said Munehiko Tsuchiya, executive director of NEDO.

Another project was an undersea line between North Africa and Italy. “We have presented the project to the European Commission. It was approved in the first phase and we hope we will obtain financial support to achieve this connection from the south of Sicily to the north of Tunisia,” said Matteo Del Fante, then chief executive of Terna, who had worked on it for years with the Tunisian operator STEG.

Terna was also a partner in BEST PATHS, a project to help integrate large amounts of renewable energy into Europe’s electricity networks. It coordinated a research area developing technologies “to achieve a reliable and efficient direct current grid, which is required for bulk energy transmission even over long distances, and to enable clean sources of energy to be integrated”, as a Terna representative put it. The goal was lower-loss transmission, for example from a solar plant in a sun-baked African desert to an office in cold Scandinavia, or from a North Sea wind farm to a warm flat in Spain.

One of the North Sea DC links built since is Viking Link between Britain and Denmark, 765 km long and in operation since 2023; the others are mapped in the interconnectors directory.