The length of underground lines operated at voltages of 63 kV and 90 kV increased in 2018, as 193 km of new lines were added. RTE notably brought into service:
- The 90 kV Le Laitier – Rom line in Deux-Sèvres (to address the distribution of load growth across different areas and the elimination of transmission constraints observed on the Poitiers-sud network),
- The 90 kV Herie-la-Vieville – Marle line in the Aisne (strengthening of the network to accommodate new renewable generation),
- The 63 kV Boutre – Cadarache line in the Var (to boost security of electricity supply to the CEA customer site in Cadarache),
- The 63 kV Scheer – Selestat line in the Bas-Rhin (part of the plan to strengthen the grid in the centre of Alsace),
- The 63 kV Athélia – le Castellet line in the Bouches-du-Rhône and Var (to enhance security of electricity supply to the coastal area between Marseille and Toulon),
- The 63 kV Breteuil – Hargicourt – Valescourt line in the Oise (restructuring of power supply to the central part of the Oise).
The undergrounding rate for new 63 kV and 90 kV lines was 93.2% in 2018 and has averaged 97.4% over the past three years (2016-2018).
Conductors were replaced on 138 km of 63 kV and 90 kV overhead lines. Examples included:
- The 90 kV double-circuit Juine – St-Evroult line in the Essonne,
- The 90 kV Cerizay – Mauleon line in the Deux-Sèvres,
- The 90 kV la Farradière – le Thou line in Charente-Maritime,
- The 63 kV Betting – St-Avold line in the Moselle,
- The 63 kV la Chapelle-du-Châtelard – Cize – Servas line in the Ain,
- The 63 kV Bagatelle – Bram – Castelnaudary – Valgros line in the Aude.
Nearly 14 km of new lines were brought into service during the year, including 10 km on the Argentière – Briançon link in the Hautes-Alpes, as part of the plan to boost security of electricity supply to the Haute-Durance.