Skagerrak 1–4

At 530 metres, the deepest point of the Skagerrak crossing made the first cables laid there by Alcatel the world’s longest and deepest underwater HVDC power cables when they went into service in 1977. That record has long since passed to other links, but the route between Kristiansand and Tjele in Jutland still carries four poles built across almost four decades.

The first two, Skagerrak 1 and 2, formed a bipole using thyristor valves, each cable rated at 250 MW and 250 kV. In 1993 Skagerrak 3 was added as a monopole at 350 kV and 440 MW, and the two older poles were reconfigured as monopoles running with the opposite polarity to it. For all three, the 240 km route combines a 113 km overhead line with 127 km of submarine cable, as the Skagerrak article describes.

Skagerrak 4 was a different machine. Agreed in November 2009, it uses voltage-source converters at 500 kV and about 700 MW, runs as cable along its whole length, and can black-start a grid, that is, help restart a network that has gone dark. It entered a test phase on 1 October 2014 and was operational by the end of that year. The crown princes Frederik of Denmark and Haakon of Norway inaugurated it on 12 March 2015. It works alongside the older line-commutated poles; the HVDC explainer sets out why the two converter types behave differently.

The combined capacity is given as 1,700 MW by Hitachi Energy, whose predecessor ABB built the converters, and as 1,632 to 1,700 MW in other sources. Statnett and Energinet operate the link. It pairs Danish wind with Norwegian hydropower, the same logic as NordLink and Viking Link later applied on a larger scale.

The first two poles are now nearly fifty years old. A replacement date for Skagerrak 1 and 2 is the open question hanging over the route.