Savoie–Piémont

Drivers in the Fréjus road tunnel between Modane and Bardonecchia pass, without knowing it, over 6.5 km of power cable. The Savoie–Piémont interconnection uses the tunnel to cross the Alps, and the rest of its 190 km route is buried too, mostly under existing roads.

RTE’s project page gives the French half in detail. From the Grande-Île substation at Sainte-Hélène-du-Lac, near Chambéry, the line runs 95 km to the Italian border: 66 km along the motorway, 18 km on minor roads, across six viaducts, through three tunnels and one underground conduit. On the Italian side Terna built the rest to the Piossasco substation outside Turin. Keeping the cable in road corridors meant no new pylons anywhere on the route. The public consultation ran from 3 January to 6 February 2012.

The case for the link was exchange capacity. RTE said the new link would raise the maximum capacity between them to 4,450 MW, an increase of 40%. The investment was about €1 billion, split evenly between the two countries.

Construction started in April 2015, after public utility declarations in June and August 2012. RTE’s page still spoke of commissioning at the end of 2021 at the earliest; the first of the two 600 MW links in fact entered service on 7 November 2022, and the second on 4 August 2023. Each has its own converter station at either end, with voltage-source units from General Electric.

Length is the record here. At 190 km it became the longest underground line of this type in the world, beating the 65 km Baixas–Santa Llogaia link between France and Spain, which had been inaugurated in February 2015. Both carry direct current underground for their whole length, with no subsea section at all. For a comparison with the physics of AC cables underground, see the HVDC explainer.

RTE also counted €35 million of local spending during the works, and up to 150 people on the civil engineering sites across a six-year build.