Multi-terminal HVDC grids to facilitate cross-border energy exchange

Wind energy already covered more than 11% of Europe’s electricity demand, according to WindEurope (formerly EWEA), the association of the wind industry. It projected 20% by 2020, 33% by 2030 and 50% by 2050, alongside a growing share for solar. Growth on that scale requires an upgraded transmission system.

Offshore wind farms produce more energy than onshore ones because the wind at sea is stronger and steadier. Yet the existing offshore farms send their power ashore as high-voltage alternating current (HVAC), and when the turbines stand far from the coast too much energy is lost in the cables.

Operators and manufacturers were therefore turning to high-voltage direct current (HVDC), which has technical advantages over long distances and above certain power levels.

Where the AC limit lies

SINTEF Energy Research in Trondheim, Norway, had been working on HVDC solutions to cut these losses. Salvatore D’Arco, one of its researchers, explained: “HVAC transmission with cables becomes less preferable or even unfeasible when the distance increases. The (distance) threshold depends on many factors, such as the cable construction or the operating voltage, but typically is in the range of 80-100 km. This is the main reason why HVDC is instead applied for interconnecting Norway with Denmark, or in the planned link between Norway and the United Kingdom.”

A DC lab in Trondheim

Working closely with partners in the EU project BEST PATHS, SINTEF researchers built a “DC lab facility” to simulate offshore multi-terminal HVDC grids (MT-HVDC) and how they interact with wind generators. In a meshed MT-HVDC grid, if one cable fails, the power can still reach the mainland by another route.

The project’s demonstrator consisted of three converters installed in the Norwegian National Smart Grid Laboratory, small-scale replicas of the large converters used in offshore wind farms. There the researchers varied power generation and looked for grid integration problems that could arise in a real system, before they could appear in expensive infrastructure.

The work could serve as a benchmark for supergrid projects in Europe. Experts were discussing, for example, a North Sea offshore grid, an interconnected network to replace the one-by-one links between renewable sites and the mainland of ten countries.

“There is a perspective of creating an offshore DC-terminal in the North Sea to facilitate not only the integration of offshore resources but also the power exchange between countries,” D’Arco said. “Energy can be transferred from a country with an excess of production to a country lacking energy or, in general, from a country where the energy costs is lower to a country where this cost is higher.”

The Norway–United Kingdom link D’Arco called “planned” is now North Sea Link: 720 km, 1,400 MW at 515 kV, in operation since 2021. The Norway–Denmark crossing he mentioned is Skagerrak, whose first stage dates from 1977.