Undersea HVDC cables, discovering some of the world’s top power interconnections

The deepest submarine power cable in the world runs in the Tyrrhenian sea, 1,600 metres below sea level. It connects Sardinia with the Italian mainland. The project is called SA.PE.I and links the converter stations of Fiume Santo, near Sassari, and Latina.

This system consists of cables running over land for 15km before they connect with a 420km-long submarine cable. Where the seabed is rocky, the cable is anchored using manually fixed collars. In sediment zones it is protected through silting up to a depth of about one metre.

The plant was inaugurated in March 2011 following a number of years of scientific studies, geophysical seabed surveys and cable-laying. The €730m investment now allows the owner Terna to operate with a peak transmission capacity of a thousand kilowatts through a 12cm-diameter cable.

Terna is also collaborating with the European Best Paths project, which is aimed at improving electricity infrastructure to feed larger capacities of renewable energy into the grid. The Italian case study is the SACOI line, which connects Sardinia to Tuscany via the island of Corsica. The project’s international team is looking at ways to revamp the existing connection. Possible solutions include small and large-scale digital simulations and producing prototypes and full scale components for some sections of the plant. These components include, for example, land and submarine cables with special insulation compounds.

The SACOI line was originally completed in January 1968. There are two submarine cable sections: the first (105 kilometres) connects Italy and Corsica, while the second consists of a 16-kilometres long cable between Sardinia and Corsica. The initial scheme was designed to export power from coal stations on Sardinia to the Italian mainland, but the project was expanded in 1988 with a third converter station at Lucciana on Corsica.

Inaugurated in 2008 with a total investment of €600m, NorNed connects the electricity grids of Norway and The Netherlands. With a length of 580km, it was the longest submarine power cable in the world when it was set up. Its maximum power is 700 megawatts.

The cable connects the Norwegian village of Feda with the seaport of Eemshaven in The Netherlands. The ability to share energy has made supply more secure for both countries. Thanks to a series of technical features, energy loss is very low and efficiency has come close to 96%. Furthermore, the intervention has reduced CO2 emissions.

NordBalt is a submarine high voltage direct current cable that connects the city of Klaipėda in Lithuania with Nybro in Sweden. The laying of the link, one of the longest power cables in the world, started in April 2013 but it took more than two years to complete because, according to Lithuanian Prime minister, the Russian navy interrupted the operation several times. In particular, “Russia has been accused of using its warships to deliberately disrupt the construction of undersea power cables that would reduce the energy reliance of Baltic states on Moscow”.

Around 90% of the 450km link runs underwater and carries over 6 terawatt/hour of power a year. It cost €580m to set up. The two cables already laid on the seabed along with the converter stations are designed to connect to a possible future pan-European electricity grid. Part of the line runs over land for 10km in Lithuania and for 40km in Sweden.

The BritNed submarine power cable connects the Isle of Grain in the United Kingdom to the harbour of Maasvlakte near Rotterdam in The Netherlands. The set-up of the interconnector started in April 2011. Costing €600m, the connection consists of two cables bundled together, with a length of 260 kilometres. The BritNed cable will be a crucial hub for a possible continental super grid in the future.

CORDIS