Cruachan pumped hydro plant becomes first facility to provide new stability service

The Cruachan pumped hydro plant in Scotland has become the first facility in Great Britain to provide the new stability service being trialled by National Grid Electricity System Operator (ESO).

Although the service also incorporates voltage control and short circuit level contribution, the main purpose of the pathfinder project is to find a low-carbon source of inertia.

Inertia refers to the resistance of the electricity system to changes in frequency and has traditionally been provided by conventional thermal generators with large turbines spinning in harmony with the frequency of the power grid.

The physical inertia of the turbines creates a similar inertia in frequency; if there is a loss of load and the frequency drops, kinetic energy stored within the turbines is automatically transferred to the power grid, arresting its fall.

This capability is not inherent to asynchronous renewables such as wind and solar, although the ESO is looking to mimic its effects with artificial or synthetic inertia – essentially a form of fast-acting frequency response.

Towards the end of January, the ESO awarded contracts worth £328 million to seven sites as part of the first tender for the new stability service. They included the Cruachan power station operated by Drax – a hydroelectric pumped storage plant built within a hollowed-out mountain in Argyll – which secured a nearly six-year contract ending in March 2026.

The tender sought to procure inertia without any accompanying generation. Drax will effectively operate one of the four units at Cruachan as a flywheel, with power from the grid being used to keep it spinning at a rate of 600 revolutions per minute.

“Cruachan has played a critical role in the growth of renewables over the last decade and this new contract will enable more wind and solar power to come onto the grid in the years ahead,” said Drax Group chief executive Will Gardiner.

ESO head of networks Julian Leslie said: “The GB electricity system is one of the most advanced in the world, both in terms of reliability and the levels of renewable power, and we’re really excited to be adding to that with this new approach to operating the grid.

“Our contracts for stability services with providers such as Drax are cheaper and greener, reducing emissions and saving money for electricity consumers. This approach – creating a market for inertia and other stability services – is the first of its kind anywhere in the world and is a huge step forward in our ambition to be able to operate the GB electricity system carbon free by 2025.”

Statkraft recently signed a deal with GE to install several of its “rotating stabilisers” – again effectively flywheels – at a site at Keith in Moray to enable it to fulfil two of the four total contracts it secured in the inaugural tender.

Last month, the ESO launched its second tender for the new stability service, and the month before announced that it was considering buying inertia as part of its next reactive power tender.