The Nant de Drance pumped storage power plant in the Swiss canton of Valais is one of the most powerful power plants of its kind in Europe, generating around 2.5bn kWh per year, when it goes fully operational in 2021. The plant has been commissioned successively in sections in recent years. TSCNET shareholder Swissgrid, the Swiss transmission system operator (TSO), is responsible for connecting the power plant to the extra-high voltage grid and for transmitting the hydropower to urban areas in Switzerland. Two of the three sections of the extra-high voltage link to Nant de Drance – an underground cable of 6.5 kilometres between the power station and the Châtelard substation via the access tunnel of the plant and an overhead line of 12.5 kilometres between the Châtelard and La Bâtiaz substations – have been in operation since the end of November 2018.
Tunnel boring has now begun for the third and final section between the substations of La Bâtiaz and Le Verney, which belongs to the municipality of Martigny. The mini tunnel boring machine, weighing around 60 tonnes, was traditionally given a female name on 2 October 2020 at the construction site in Le Verney and will in future be known as Giorgia. Shortly after its christening, it was lowered into the shaft, where it will take around eight months to dig and secure the tunnel between the substations. The tunnel will have a diameter of 2.5m and is dug 12 to 20m deep into the ground. Giorgia is controlled entirely from the control centre at the construction site in Le Verney.
Construction of the underground cable link commenced in June 2020 with the excavation of the shaft at Le Verney and will take two years. The shaft at La Bâtiaz will be dug at the same time as tunnelling work. The finishing work in the tunnel will follow from summer 2021, after which the gantry to connect the Nant de Drance link to the 380kV Chamoson-Romanel overhead line in Le Verney will be built. Finally, the cables will be laid in the tunnel. Commissioning of the 1.2km long underground line is scheduled for 2022. Nant de Drance is already now connected to the transmission grid even before the last section of the link is commissioned: An interim solution was introduced in 2019 by increasing the voltage of one of the two overhead lines between La Bâtiaz and Le Rosel to 380kV.