The Nant de Drance pumped storage power plant in the Swiss canton of Valais is being commissioned successively in sections since 2019. The plant has a capacity of 900MW, making it one of the most powerful pumped storage plants in Europe – when it is fully operational in 2021, it will be able to generate close to 2.5bn KWh a year. TSCNET shareholder Swissgrid, the Swiss transmission system operator (TSO), is responsible for connecting the power plant to the extra-high voltage grid and for to transmitting the hydropower to the urban areas of Switzerland.
Swissgrid is implementing the 380kV connection in three sections, two of which were already commissioned at the end of November 2018: the 6.5km underground cable line connecting the Nant de Drance power plant to the Châtelard substation via the plant’s access tunnel, and the 12.5km overhead line between the Châtelard and La Bâtiaz substations. The last section – from La Bâtiaz to the connecting point to the Chamoson-Romanel overhead line in Le Verney – crosses the Rhône valley over a length of 1.3km. An underground variant for this section was finally approved in January 2020 by the Federal Inspectorate for Heavy Current Installations (“Eidgenössisches Starkstrominspektorat”, ESTI). It provides for a tunnel with a diameter of around 2.5m and a depth of 12 to 20m to be drilled between different groundwater levels. This is the second underground extra-high voltage line built by Swissgrid, but the first to extend at great depth for more than one kilometre.
Preparations for construction work on the two sites in Le Verney and La Bâtiaz have been underway since mid-May. Excavation of the shafts at Le Verney will commence in mid-June. In August, the mini tunnel boring machine will be installed on the main construction site in Le Verney. Tunnel driving will take about eight months and start in September 2020 at the same time as the excavation of the shafts in La Bâtiaz. After around two years of work, the third section is scheduled to be commissioned, until which time a provisional solution has been found: In 2019, the voltage of one of the two overhead lines La Bâtiaz – Le Rosel was increased to 380kV.