The Cruachan power station, also known as the Hollow Mountain power station, located in Scotland is one of the four pumped-storage power plants in the UK. Owned and operated by the Drax Group, the facility houses four generating units for a total capacity of 440MW.

Originally owned by the North of Scotland Hydro Electric Board, the power station commenced operations with the commissioning of the first two turbine units in October 1965, while the third and fourth units were brought online in 1966 and 1967 respectively.

Spanish power company Iberdrola acquired the Cruachan pumped-storage hydropower station in April 2007, while the Drax Group acquired the project from Iberdrola in December 2018.

The pumped-storage facility Cruachan power station can store surplus power from intermittent energy sources such as solar and wind during the off-peak period and can achieve full load in 30 seconds and operate at full capacity for a period of approximately 15 hours during the peak demand.

Apart from power generation, the Cruachan facility also provides system support services to the UK energy market.

Location and reservoir details

The Cruachan power station is located in Ben Cruachan Mountain, on the banks of Loch Awe in Argyll and Bute Council, western Scotland, UK.

During the off-peak period, water is pumped by reversible turbines from Loch Awe (lower reservoir) to the upper reservoir located in Ben Cruachan Mountain, while the stored water from the upper reservoir is released to drive the reversible pump-turbine units for electricity generation during the peak hours.

The upper reservoir is formed by a 316m-long, 46.6m-high concrete gravity buttress dam and has a storage capacity of 10,000,000m3. It has a catchment area of approximately 23km2, and it also receives water through tunnels and piped aqueducts, in addition to the water pumped from Loch Awe.

Cruachan power station make-up

The machine hall of the power station is located approximately 400m below the upper reservoir in a cavern inside Ben Cruachan Mountain.

A 1.1km-long, 6.7m-wide, 4m-high entrance tunnel constructed inside the mountain provides road access to the underground powerhouse.

Measuring 91m-long, 23m-wide, and 37m-high, the machine hall consists of four vertical reversible Francis-type pump-turbine units of 110MW capacity each. The transformer hall consists of three oil-filled, water-cooled transformers, including a 230MVA transformer and two 150MVA transformers. The step-up transformers convert the voltage from 16kV to 275kV.

The water flows from the upper reservoir to the machine hall through two penstocks which are 304.8m-long, 5m-diameter concrete-lined shafts inclined at 55 degrees. Each penstock splits into two 152.4m-long, 2.7m to 2.4m-diameter steel-lined pressure shafts fitted with the 1.8m-diameter main inlet valves at their terminals (end of the pipe). The main inlet valve controls the flow of water into the turbine.

After power generation, the water flows to a 21.9m-long, 7.6m-wide, and 30.5m-high tailrace surge chamber, which is connected to Loch Awe via a 975.3m- long, 7m-diameter concrete-lined tailrace tunnel.

Power evacuation

The electricity generated by the power station is transmitted to a terminal tower of a double-circuit 275kV transmission line through six 275kV oil-filled cables passing through a 335.3m-high and 4m-diameter vertical concrete-lined cable shaft.

The 275kV transmission line evacuates power to the Dalmally switching substation and further to Windyhill located north of Glasgow.

Contract from National Grid ESO

The Cruachan power station received a six-year synchronous compensation contract from the National Grid Electricity System Operator (ESO) in January 2020. One of the four turbines of the power station is utilised to provide system support services, including inertia and reactive power to maintain the stability of the national grid.

The contract worth approximately £5m a year was awarded under phase one of National Grid ESO’s System Stability Pathfinder.

Recently awarded contracts

Siemens was contracted to install four vacuum generator circuit breakers to replace the existing sulphur hexafluoride (SF6) gas circuit breakers at the Cruachan power station. The project was completed in April 2021 marking the first installation of Siemens vacuum circuit breakers in a hydroelectric station in the UK.

ITI, formerly Servelec Controls, was awarded a contract to upgrade the PLC control systems for the four turbine-generator units at Cruachan power station by Drax in August 2020. The scope of the contract includes design, documentation, implementation, test, installation, and commissioning of the upgraded control systems for the turbine generator units. Expected to be completed in three years, the value of the contract is estimated to be approximately £1m ($1.3m).