News · July 2017
Undersea HVDC cables: some of the world's top power interconnections
Submarine power cables were taking a larger role in sharing electricity between regions and countries. Efficient transmission saves energy, cuts pollution and makes grids more secure.
At 1,600 metres below sea level in the Tyrrhenian Sea lies the deepest submarine power cable in the world. It links Sardinia with the Italian mainland: the SA.PE.I project, running between the converter stations of Fiume Santo, near Sassari, and Latina.
The system runs 15 km over land before joining a 420 km submarine cable. Where the seabed is rocky the cable is held by collars fixed by hand; in sediment it is protected by burial to a depth of about one metre.
It was inaugurated in March 2011 after years of scientific studies, geophysical seabed surveys and cable laying. The €730m investment lets its owner Terna transmit a peak of 1,000 MW through a cable 12 cm in diameter.
SACOI
Terna was also working with the European project BEST PATHS, which aimed to improve electricity infrastructure so that larger amounts of renewable energy could enter the grid. Its Italian case study was the SACOI line, which links Sardinia to Tuscany by way of Corsica. The international team was looking at ways to revamp the connection, including small- and large-scale digital simulations and prototypes and full-scale components for parts of the system, such as land and submarine cables with special insulation compounds.
SACOI was first completed in January 1968. It has two submarine sections: 105 km between Italy and Corsica and a 16 km cable between Sardinia and Corsica. The original scheme was built to export power from coal plants on Sardinia to the mainland, and in 1988 a third converter station was added at Lucciana on Corsica.
NorNed
Inaugurated in 2008 at a total cost of €600m, NorNed links the grids of Norway and the Netherlands. At 580 km it was the longest submarine power cable in the world when it was built, with a maximum power of 700 MW.
The cable runs from the Norwegian village of Feda to the port of Eemshaven in the Netherlands. Sharing energy has made supply more secure for both countries. Thanks to its technical design losses are very low, efficiency has come close to 96%, and the link has cut CO2 emissions.
NordBalt
NordBalt is a submarine HVDC cable between the Lithuanian city of Klaipėda and Nybro in Sweden. Laying began in April 2013 and took more than two years, partly because, according to the Lithuanian prime minister, the Russian navy interrupted the work several times. Russia was accused of using warships to disrupt deliberately the construction of undersea cables that would reduce the Baltic states’ energy dependence on Moscow.
About 90% of the 450 km link runs under water, and it carries more than 6 TWh of power a year. It cost €580m. The two cables on the seabed and the converter stations are designed to connect to a possible future pan-European grid. On land the line runs for 10 km in Lithuania and 40 km in Sweden.
BritNed
The BritNed cable connects the Isle of Grain in the United Kingdom with Maasvlakte near Rotterdam. Installation began in April 2011. Costing €600m, it consists of two cables bundled together over 260 km, and it was seen as a possible hub for a future continental supergrid.
Laid on the North Sea bed, the cable weighs 44 kg per metre, 23,000 tonnes in all. More than thirty cable-laying ships and a fleet of support vessels bundled and buried the copper cables in water between 30 and 50 metres deep.
Baltic Cable
The 250 km Baltic Cable entered service in December 1994. It was then the second-longest high voltage cable in the world, with a nominal maximum of 600 MW, and it opened a new era of power transmission in Europe.
It connects Lübeck-Herrenwyk in northern Germany with the Swedish grid at Trelleborg. Besides the 233 km section on the seabed, it runs underground for several kilometres and crosses the river Trave in a channel six metres below the riverbed.
The cable weighs 55 kg per metre and has a 1,600 mm² copper conductor made of concentric layers of keystone-shaped strands. No cable ship could carry the full 13,000 tonnes, so it was laid in two sections and joined afterwards.
Skagerrak
The Skagerrak HVDC link connects the Danish grid, based on wind and thermal power, with Norway’s hydroelectric grid, running from Tjele in Denmark to Kristiansand in Norway.
The scheme has four main parts, each partly overhead line and partly submarine cable. It was commissioned in 1977 and crossed the Aggersund strait on 70 m pylons, then the tallest in Europe; underground cables later replaced that crossing. The newest part, Skagerrak 4, was fully operational by late 2014 and inaugurated in March 2015.
In 2017 the link had about 100 km of land cable and 137 km of subsea cable, and is designed to make better use of the renewable energy produced in both countries.
Current figures for every link above are in the directory, which gives the Baltic Cable a route length of 262 km and Skagerrak a combined 1,700 MW; how cables like these are built and laid is explained in the subsea power cables guide.