First Hydro Company

Ffestiniog Power Station

Commissioned in 1963, Ffestiniog Power Station was the UK's first major pumped storage power facility. Although of an older generation to those at Dinorwig, Ffestiniog's four generating units are still capable of achieving a combined output of 360MW of electricity - enough to supply the entire power needs of North Wales for several hours.

The Generation Cycle begins at Llyn Stwlan - Ffestiniog's upper reservoir. Large screens inside the intake towers are opened to activate the high-pressure downflow.

27 cubic metres of water per second are discharged through two high-pressure shafts (each 200 metres in depth), which are connected to four concrete-lined tunnels. Steel penstocks then direct the discharge into the station via inlet pipes and valves to start generation.

Water is captured in Tan-y-Grisiau and pumped back to Llyn Stwlan, usually overnight, to complete the cycle.

FFESTINIOG FACTS & FIGURES (STWLAN DAM & RESERVIOR)

Dam:

TypeMassive buttress gravity
MaterialMass concrete Total
Length including spillway373.4m
Maximum height of dam above foundations34m
Maximum thickness4m

Reservoir:

Designed live storage2 million cu metres
Designed maximum operating above water level502.4m ordnance datum
Designed minimum operating above water level482.5m ordnance datum
Average annual rainfall3m
Catchment area65 hectares

Pressure Shafts, Tunnels and Pipelines Intakes:

Number of intakesTwo
Number of intake towers/intakeTwo

Vertical Shafts and Tunnels:

Number of shaftsTwo
Depth195m
Internal diameter4.4m
LiningConcrete
Number of tunnelsFour
Internal diameter of steel pipe linings2.8m
Gradient of tunnels1 in 40

Pipelines:

Number of pipelinesFour
Length of each pipeline213m
Thickness and materialFrom 22m to 26m
Coltuf 32 steel internal diameter2.3m

Tan-y-Grisiau and Reservoir:

Dam typeGravity
MaterialMass concrete
Length including spillway549m
Length of spillway79m
Maximum height of dam above foundations12.2m

Reservoir:

Designed live storage2 million cu metres
Designed maximum operating above water level187.9m ordnance datum
Designed minimum operating above water level182.3m ordnance datum
Average annual rainfall over catchment area2.2m
Catchment area including that for Stwlan reservoir951 hectares

Power Station Plant Generator/Motors:

NumberFour
Rating as generator90 MW at 0.95 PF
Rating as motor75 MW at unity PF
Output as motor at 50 cycles/second104,000 bhp
Speed428 rpm
Rated voltage16 kV
Cooling systemClosed circuit air cooled with air/water heat exchangers

Excitor Main:

Speed428 rpm
DriveDirect
Rated Voltage181
Rated current1050

Turbines:

NumberFour
TypeVertical shaft reaction
Design Net head296m
Discharge at design net head and maximum efficiency29 cu metres/sec
Speed428 rpm

Turbine Inlet Valve:

OperationHydraulic servomotor
Inlet diameter1.8m
Opening time60 seconds
Closing time60 seconds
Weight86.3 tonnes
Spiral casing constructionWelded plate
Number of guide vanes24

Runner:

ConstructionCast stainless steel
Exit diameter1.8m
Number of vanes13 Storage

Pumps:

NumberFour
TypeVertical shaft centrifugal
Number of stagesTwo
Number of inletsTwo
Design head305m
Discharge at design head22 cu m/sec
Power required at coupling at rated discharge93,600 bhp
Speed428 rpm

Pump Discharge Valve:

TypeStraightflow
OperationDouble hydraulic servometer
Outlet diameter2m

Turbines:

Opening time60 seconds
Closing time (normal)60 seconds
Spiral casing constructionWelded plate

Diameter of Impellers:

First stage2.4m
Second stage2.5m
Number of impeller blades10 each stage
Maximum diameter of shaft0.7m

Generator Switchgear:

TypeSF6 circuit breaker
Breaking capacity174 MVA
Voltage between phases16 kV
Current rating6200 amps

Generator Transformers:

NumberTwo
Rating160 MVA
Ratio16 kV/275 kV
Cooling systemWater circulation from lower reservoir
Weight as delivered122 tonnes
Oil capacity27 cu m

Auxiliary Transformers:

Number of station transformersTwo
Rating1 MVA
Ratio16 kV/415 V
Number of local supplies transformersTwo
Rating1 MVA
Ratio11 kV/415 V