Fenno-Skan 1

When Fenno-Skan went into commercial operation in November 1989, no submarine power cable anywhere was longer. It was also the first HVDC cable to run at 400 kV. About 200 km of it lies on the floor of the Gulf of Bothnia, between the Finnish and Swedish coasts.

The link is a monopole: one high-voltage conductor, with the return path through electrodes at each end. The Fenno–Skan article on Wikipedia describes the electrode lines as overhead conductors carried on wooden poles in both countries. On the Finnish side a 33 km overhead line brings the power from the coast to the converter station at Rauma; on the Swedish side the station is at Dannebo, north of Stockholm. ABB built the converters and Nexans the cable. The rated power is 500 MW, with 550 MW given as the maximum.

For thirty years it did the job it was built for, letting Finnish and Swedish power flow across the gulf in whichever direction prices pulled it. In 2011 Fenno-Skan 2 joined it at Rauma, sharing the Finnish end.

The question since then has been age. The link’s expected technical lifetime originally ran out in the late 2020s. In March 2021 Fingrid and Svenska kraftnät signed a framework agreement to extend its life until at least 2040, which buys time without pretending the cable is new.

The replacement already has a name. In 2025 the two operators signed a letter of intent to plan Fenno-Skan 3, an 800 MW cable meant to take over from Fenno-Skan 1 at the end of its life, with completion expected by 2038. The Fingrid announcement says the grid study should be done by the end of 2026. If the schedule holds, the 1989 cable will have run for close to half a century before it is switched off.