First Hydro Company

Dinorwig Power Station

Dinorwig Power Station

When it was fully commissioned in 1984, Dinorwig Power Station was regarded as one of the world's most imaginative engineering and environmental project.

Today, Dinorwig's operational characteristics and dynamic response capability are still acknowledged the world over. Dinorwig is the largest scheme of its kind in Europe.

Dinorwig is comprised of 16km of underground tunnels, deep below Elidir mountain. Its construction required 1 million tonnes of concrete, 200,000 tonnes of cement and 4,500 tonnes of steel.

The station's six powerful generating units stand in Europe's largest man-made cavern. Adjacent to this lies the main inlet valve chamber housing the plant that regulates the flow of water through the turbines.

Dinorwig's reversible pump/turbines are capable of reaching maximum generation in less than 16 seconds. Using off-peak electricity the six units are reversed as pumps to transport water from the lower reservoir, back to Marchlyn Mawr.


Surge Pond Data:

Dimensions of surge pond80x40x14 metres deep
Diameter of surge shaft30 metres
Depth of surge shaft65 metres


TypeVertical shaft, salient pole, air cooled
Generator rating330 MVA
Motor rating312 MVA
Terminal voltage18kV
ExcitationThyristor rectifier
Starting equipmentStatic variable frequency

Generator-Motor Transformer:

Approximate rating340 MVA
Voltage ratio18 kV/420 kV

Underground Caverns:

Distance of power station inside mountain750 metres
Depth of turbine hall below top level of Llyn Peris71 metres

Machine Hall:

Length180 metres
Width23 metres
Height51 metres max

Transformer Hall:

Length160 metres
Width23 metres
Height17 metres
Diversion tunnel length2,208 metres
Width6.5 metres
Height5.5 metres
Maximum flow60 cubic m/s
Normal flow1-8 cubic m/s


TypeReversible Francis
Plant orientationVertical spindle
Average pump power input275 MW
Pumping period (full volume)7 hours
Synchronous speed500 rpm
Average full unit over all heads (declared capacity)288 MW Generation potential at full load
Output5 hours
Station power requirements when generating12 MW
Standby operational mode
Synchronised and spinning-in-air emergency load pick-up rate from standby0 to 1,320 MW in 12 seconds

Transmission Switchgear:

TypeSF6 metal clad
Breaking capacity35,000 MVA
Current rating4,000 A
Voltage420 kV


Main underground excavation1 million cubic metres (approx. 3 million tonnes)
Total scheme excavations12 million tonnes