The 900-megawatt battery in the Valais Alps

A five-minute drive through a foggy tunnel nestled deep inside the mountain brings you to the Nant de Drance machine cavern. It is nearly 200 meters long and 50 meters high – dimensions approaching those of St. Peter’s Basilica in Rome – and is located around 600 meters below the surface.

Six pump turbines with 150 MW capacity each

Six pump turbines are installed here with a capacity of 150 megawatts each. Water can be pumped through the turbines to generate electricity and then pumped back up again. This is done in a largely closed circuit between the higher-lying Vieux Emosson reservoir and the lower, larger Emosson reservoir; Vieux Emosson has only smaller mountain streams as tributaries.

In other words, the Nant de Drance pumped storage power station, which was officially inaugurated by Federal Councilor Simonetta Sommaruga in early September 2022 and is located a good ten kilometers west of Martigny, acts as a giant, continuously rechargeable battery.

Designed to stabilise the power grid

Production and consumption must always be in balance in the power grid. The energy generated by solar and wind power plants is subject to natural fluctuations. As part of the energy transition, more and more of these systems are being integrated into power grids around the world. Switzerland is also gradually incorporating these systems into its power grid. Consumption has always been inconsistent over the course of the day, although peaks in demand are more predictable.

“Nant de Drance was designed to perform this balancing act,” explains Jean-Daniel Dayer, who is responsible for control systems at the pumped storage power station. “We can feed up to 900 megawatts of electricity into the grid any time and reach full-load turbine operation in just a few minutes.” Water from Vieux Emosson is fed to the underground power station via two 441-meter-high vertical shafts.

Theoretically supplying the whole of Switzerland for three hours

If the upper reservoir fills up, this tremendous output can be fed into the grid for a period of up to 20 hours. In purely theoretical terms, the energy generated in the process is roughly the equivalent of all the energy the entire country of Switzerland consumes in a three-hour period. Put another way: This energy could fully charge around 400,000 electric vehicles.

If electricity production exceeds demand, the power station can store this excess electricity. The generators switch to engine operation and pump water from Emosson back up to Vieux Emosson. The system can switch from turbine to pump operation in under eight minutes. Nant de Drance SA’s shareholders – Alpiq, SBB, IWB and FMV – invested over 2 billion francs in this pumped storage power station.

ABB’s system 800xA installed

The enormous plant depends on reliable monitoring and control. ABB Switzerland designed the process control system that was awarded the public tender for this project at the end of 2014. “Our solution is based on the ABB Ability™ System 800xA”, explains ABB Switzerland’s project manager Thomas Benz. ABB is global market leader in distributed control systems, with an installed base of around 35,000 systems across more than 100 countries.

“System 800xA is the higher-level distributed control system that is used to start up or shut down the machine groups – in this case the turbines and their generators,” Benz adds. Normally, this is done remotely from the Alpiq headquarters in Lausanne. There are only around a dozen technicians on site to perform routine maintenance on the expansive power station.

Several interfaces programmed

“Our control system not only provides commands for controlling, regulating and protecting the machine groups, but also data for operating, monitoring, visualizing and alerting all the other main, auxiliary and ancillary systems here, from the generator switch to the cooling water circuit to the reservoir level measuring systems,” explains Benz. Interfaces for data communication with the subordinate process controls and control systems from other batch suppliers had to be defined and programmed specifically for this purpose.

“System 800xA works well as a distributed control system for our power station,” Jean-Daniel Dayer concludes, just a few weeks after all six machine groups began operating at full capacity. “There are still some isolated – non-critical – interface communication issues that are currently being resolved.”

Also medium voltage equipment and synchronisation system from ABB

ABB supplied additional solutions and systems for this pumped storage station, which enabled Nant de Drance SA to procure the necessary medium-voltage switchgear – over 70 panels in total – from our company. Six ABB Synchrotact 5 devices are used to automatically synchronize the six generators with the grid frequency. ABB also supplied the generator switches and generator transformers before selling the Power Grids business to which they belong.

Nant de Drance SA has concluded a three-year service contract with ABB Switzerland for the control system. Ensuring the continued availability of this pumped storage power station is a top priority for the operators – and has been a priority for some time – even before electricity became a potentially scarce commodity and electricity prices became quite volatile and began seeing dramatic upswings. The battery in the Valais mountains will help secure the supply of power in Switzerland for 80 years or more to come, particularly when the giant solar power plants that are currently planned in the alps are connected to the grid.