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Inside the Robert-Bourassa generating station, in northern Quebec, the largest underground power station in the world with an installed capacity of 5,616 MW.

An underground power station is a type of hydroelectric power station constructed by excavating the major components (e.g. machine hall, penstocks, and tailrace) from rock, rather than the more common surface-based construction methods.

Often underground power stations form part of pumped storage hydroelectricity schemes. Their basic function is to level load. They use cheap or surplus off-peak power to pump water from a lower lake to an upper lake. Then, during peak periods (when electricity prices are often high), the power station generates power from the water held in the upper lake.

Notable examples

Some notable underground power stations are:

• Snoqualmie Falls power station in King County, Washington, United States, built in 1898, was the world's first underground power station and is still used to provide power to the Seattle area. It was designated an ASCE Civil Engineering Landmark in 1981.

• Robert-Bourassa generating station, Quebec, Canada is the largest underground power station in the world. It generates 5,616 MW from 16 turbines with a net rated head of 137.2 m.

• Chaira Hydroelectric power plant, Bulgaria, is the largest underground power station on the Balkans, build in 1980-1998. It generates 864 MW from 4 Toshiba turbines with a net rated head of 701 m, and maximal speed of 600 rpm.

• Churchill Falls Power Station, Newfoundland and Labrador, Canada is the second largest underground power station in the world. It generates 5,428 MW from 11 turbines. The powerhouse is 232 m long, 45 m high, 19 m wide and located 330 m underground. The two tailrace tunnels are 1691.64 m long. The net head is 312.4 m.

• Goldisthal pumped storage plant, in Thuringia, Germany, built in 1991-2004, generates 1,060 MW from 4 turbines. It is unique (for its scale) in Europe, in that two of the four motor generators are designed as variable speed asynchronous machines. The machine hall is 147 m long, 49 m high, 16 m wide, with a separate transformer cavern (120 m long, 15 m high, 16 m wide).

• Kanagawa Hydroelectric Plant is under construction in Japan. When completed, it will be the world's largest pumped storage plant, generating 2,700 MW. The power house is 216 m long, 33 m wide, 52 m high. The effective head is 714 m. The first power is scheduled for 2005.

• Kariba hydro-electric power scheme (1,200 MW) is on the Zambezi river, which forms the border between Zimbabwe and Zambia. The Kariba system comprises two underground power stations. The Kariba South station in Zimbabwe houses six 100 MW generators. The Kariba North station in Zambia houses four 150 MW generators.

• Kazunogawa Power Plant is a 1,600 MW underground pumped storage plant in Japan. Kazunogawa consists of four 400 MW generation units. The cavern for the underground power station is 500 m below the surface. It is 210 m long by 54 m high and 34 m wide. The head is 714 m.

• Manapouri Power Station, Fiordland, New Zealand, built 1963-1972, generates 850 MW from 7 turbines. It is built 200 m underground, and has two 10 km tailrace tunnels. The net head is 170 m. The most notable feature of this station is that the lake and power station are located on the eastern side of the Southern Alps, with the tailrace tunnels travelling under a major mountain range, discharging in Doubtful Sound on the west coast.

• Raccoon Mountain Pumped-Storage Plant, Chattanooga, Tennessee, United States, built in 1970-1978 generates 1,530 MW. It is an early test of the pumped-storage approach.

• Dinorwig Power Station, Llanberis, United Kingdom, built in 1984, is a pumped-storage system, delivering 1,650 MW to Wales and the north-west of England. It stands in Europe's largest man-made cavern.

• Ben Cruachan Power Station, United Kingdom, built in early 1960s, a pumped storage plant generating 440 MW from 4 turbines.

• Edward Hyatt Power Plant inside the Oroville Dam, United States, is in a cavern carved into the bedrock of the Feather River canyon. It houses 3 Generator and 3 Pump/Generator units and their respective Transformers 650 feet below the crest of the dam.