The Dinorwig power station, also known as Electric Mountain, is the biggest hydroelectric facility and the fastest power-generating asset in the UK, capable of delivering up to 1,728MW of electricity in just 16 seconds.

Operating since 1984, it is a pumped-storage hydropower facility built in caverns inside the Elidir Fawr mountain in Dinorwig, Llanberis, in north Wales. It comprises six pump-turbine units housed in the main cavern, which is considered to be the biggest man-made cavern in Europe.

The Dinorwig pumped-storage hydropower station is owned and operated by First Hydro Company, a joint venture between Engie (75%) and Brookfield Renewable (25%).

The nearby Electric Mountain visitor centre underwent a refurbishment project to provide an attractive visitor experience. The refurbishment works were started in September 2018 and the renovated centre was opened to public in March 2020.

Dinorwig power station history and background

The pumped storage concept was introduced in Britain through the planning and demonstration of the Blaenau Ffestiniog hydroelectric pumped storage scheme in the 1950s. Although the 360MW facility was opened in 1963, it was deemed to be insufficient in regulating the country’s growing electricity demands.

The project site in the Elidir Fawr mountain was preferred over the other two sites in north Wales because of the presence of two natural lakes at the peak and the bottom of the mountain as well as because of the cavity created through hundreds of years of excavation in the mountain by the slate quarrymen.

The construction works were started in 1974, while the hollowing out the mountain, creating tunnels, enlarging the lakes, and shifting all the heavy machinery into the mountain took ten years. Inaugurated by Prince Charles in 1984, the project generated approximately 2,000 jobs during the construction phase.

It is considered to be one of the most imaginative engineering and environmental projects ever undertaken in the UK.

Dinorwig power station make-up

The pumped storage hydropower station site is located deep inside the Elidir Fawr mountain on the boundary of the Snowdonia National Park. It comprises upper and lower reservoirs and an underground powerhouse.

The upper reservoir is the pre-existing lake of Llyn Marchlyn Mawr, which is formed by a 36m-high rockfill dam. It is located 503m above the lower reservoir Llyn Peris, which is also a pre-existing lake.

The power station is accessible via 16km-long tunnels and its machinery is housed in nine 750m-deep man-made caverns. The turbine hall, which is 180m-long, 23m-wide, and 51m-high, lies 71m below the top level of Llyn Peris. The transformer hall measures 160m in length, 23m in width, and 17m in height.

Water is released from the upper reservoir to the lower reservoir through a 1.7km-long, 10.5m-diameter low-pressure tunnel during peak demand periods. From there, water flows to the six turbines through a 10m-diameter high-pressure tunnel.

A surge pond with dimensions 80mx40m is situated at a depth of 14m halfway between Llyn Marchlyn Mawr and Llyn Peris, with a 30m-diameter and 65m-deep surge shaft positioned in the base of the pond.

Dinorwig power station technical details

The electricity at the Dinorwig pumped storage power station is generated by six reversible, vertical Francis type pump-turbine units of 288MW capacity each. The synchronous speed of each unit is 500rpm.

The power station uses vertical shaft, salient pole, air-cooled type motor generators with a terminal voltage rating of 18kV. It has six motor-generator transformers with an approximate rating of 340MVA and a voltage ratio of 18kV/420kV.

A 420kV SF6 metal-clad transmission switchgear with 35,000MVA of breaking capacity is installed to control the flow of electric power.

Power evacuation

The electricity generated by the Dinorwig pumped-storage power station is fed into the National Grid through 10km of 400kV underground cables connecting a substation at Pentir.

Contractors involved

ABB installed six generator circuit-breakers (GCBs) at the Dinorwig Power Station for protecting generators and transformers against short-circuit faults.

ABB was awarded a $4.5m high-voltage service contract for replacing one of the six GCBs installed at the Dinorwig pumped storage hydropower station in March 2019.

A framework agreement was also executed to replace the remaining GCBs by 2028 based on the success of the first unit.