Baltic Cable

In April 2017 a fault on Baltic Cable’s electrode line produced hydrogen, which caught fire and sent up a flame three metres high. The incident is a reminder of how this link works: it is a monopole, and the return current flows through the sea between two electrodes instead of through a second cable.

Those electrodes are unusual objects. The anode is 40 titanium nets on the seabed, each with an area of 20 m². The cathode is a bare copper ring 2 km across, lying in the Baltic north of Elmenhorst. The Baltic Cable article describes both.

The main cable runs 250 km from the converter station at Herrenwyk in Lübeck under the Baltic Sea to Sweden, where it turns into an overhead line a few kilometres inland before reaching Kruseberg, near Trelleborg. It was commissioned in December 1994 and cost 2 billion SEK. At 450 kV it has the highest operating voltage used for power transmission in Germany.

On paper the rating is 600 MW. In practice the link has been limited to around 370 MW for most of its life, because the 380 kV line leaving the Lübeck-Herrenwyk converter station ends at the Lübeck-Siems substation, and the network beyond it cannot take more. An additional 220 kV line built in 2004 eased the bottleneck in part, although power moved at lower voltage suffers higher losses. It is a clear case of an interconnector held back by the grid on land; the grid congestion guide explains the wider problem.

Baltic Cable AB, the owning company, belongs to Statkraft, and ABB supplied the equipment. Almost the whole route is submarine: 250 of the 260 km that Baltic Cable AB gives for the route. Its eastern neighbour SwePol, built six years later, chose metallic return cables to avoid sea electrodes altogether.