A growing portion of RTE’s lines are underground


More than 348 km of new lines were added to the public transmission network in 2018 (including overhead lines that were undergrounded).

Similarly to 2013 (with the 400 kV Cotentin – Maine line) and 2016 (400 kV Lonny – Vesle line), 2018 stood out from other years thanks to the completion of a very high voltage overhead project (the “2 Loires” project and the commissioning of the 225 kV Rivière – Sanssac and Pratclaux – Trevas lines, together spanning 75 km). Besides this project, nearly all the new lines built in 2018 used underground technology, as has been the case for about ten years.

RTE also replaced more than 447 km of overhead and underground lines on its network.

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Overhead and underground lines: Complementary technologies

A variety of solutions are leveraged to expand the transmission grid, taking into account technical, economic, environmental and societal factors. Two techniques are used: overhead and underground.

As part of its public service contract, RTE has committed to not increase the total length of overhead lines (removals offset additions) and to place at least 50% of new HV lines underground.

Investment costs for overhead and underground lines depend on voltage levels: costs are the same for 63 and 90 kV, but underground lines cost about twice as much as overhead lines for 225 kV and eight times as much for 400 kV (*).

Underground lines currently represent:

  • 8.3% of all 63/90 kV lines,
  • 5.5% of all 225 kV lines,
  • a negligible share of 400 kV lines (0.02%).

(*) Underground cables for 400 kV AC are quite expensive and substations must be installed every 20 km to offset the capacitive effect of the cables. At this voltage level, buried DC lines can be an option. The cost is the same as for an overhead AC line but transmission capacity is three to five times lower.

Source: Ten-Year Network Development Plan

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400 kV and 225 kV


Underground

A total 30 km of new 400 and 225 kV underground lines were brought into service in 2018. The main completions were:

  • The 225 kV Château-Gombert – Enco-de-Botte double-circuit line in the Bouches-du-Rhône, as part of the plan to connect the Château-Gombert customer distribution substation, notably to enhance security of electricity supply to the Marseille area,
  • A portion (nearly 8 km) of the 225 kV La Rivière – Sanssac overhead/underground line as part of the “2 Loires” project (see Highlights).

Overhead

RTE replaced conductors on nearly 224 km of overhead lines operated at voltages of 400 kV and 225 kV during the year. Key completions included:

  • The upgrading of the 225 kV St-Auban – Ste-Tulle and Oraison – Sisteron lines in Alpes-de-Haute-Provence to increase their transmission capacity so that they can accommodate more renewable generation going forward,
  • The replacement of the towers and conductors on the 225 kV Niort – Val-de-Sèvre line in the Deux-Sèvres,

The replacement of some conductors on the 400 kV double-circuit Le Havre – Rougemontier line.

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63 kV and 90 kV


Underground

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).

Overhead

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.

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