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FFESTINIOG POWER STATION

lake image 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.

LLyn Stwlan image27 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.

This cross-section schematic details the key components of the Ffestiniog pump storage layout.

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FFESTINIOG FACTS & FIGURES (STWLAN DAM & RESERVIOR)


Dam:
Type Massive buttress gravity
Material Mass concrete Total
Length including spillway 373.4m
Maximum height of dam above foundations 34m
Maximum thickness 4m
Reservoir:
Designed live storage 2 million cu metres
Designed maximum operating above water level 502.4m ordnance datum
Designed minimum operating above water level 482.5m ordnance datum
Average annual rainfall 3m
Catchment area 65 hectares
Pressure Shafts, Tunnels and Pipelines Intakes:
Number of intakes Two
Number of intake towers/intake Two
Vertical Shafts and Tunnels:
Number of shafts Two
Depth 195m
Internal diameter 4.4m
Lining Concrete
Number of tunnels Four
Internal diameter of steel pipe linings 2.8m
Gradient of tunnels 1 in 40
Pipelines:
Number of pipelines Four
Length of each pipeline 213m
Thickness and material From 22m to 26m
Coltuf 32 steel internal diameter 2.3m
Tan-y-Grisiau and Reservoir:
Dam type Gravity
Material Mass concrete
Length including spillway 549m
Length of spillway 79m
Maximum height of dam above foundations 12.2m
Reservoir:
Designed live storage 2 million cu metres
Designed maximum operating above water level 187.9m ordnance datum
Designed minimum operating above water level 182.3m ordnance datum
Average annual rainfall over catchment area 2.2m
Catchment area including that for Stwlan reservoir 951 hectares
Power Station Plant Generator/Motors:
Number Four
Rating as generator 90 MW at 0.95 PF
Rating as motor 75 MW at unity PF
Output as motor at 50 cycles/second 104,000 bhp
Speed 428 rpm
Rated voltage 16 kV
Cooling system Closed circuit air cooled with air/water heat exchangers
Excitor Main:
Speed 428 rpm
Drive Direct
Rated Voltage 181
Rated current 1050
Turbines:
Number Four
Type Vertical shaft reaction
Design Net head 296m
Discharge at design net head and maximum efficiency 29 cu metres/sec
Speed 428 rpm
Turbine Inlet Valve:
Operation Hydraulic servomotor
Inlet diameter 1.8m
Opening time 60 seconds
Closing time 60 seconds
Weight 86.3 tonnes
Spiral casing construction Welded plate
Number of guide vanes 24
Runner:
Construction Cast stainless steel
Exit diameter 1.8m
Number of vanes 13 Storage
Pumps:
Number Four
Type Vertical shaft centrifugal
Number of stages Two
Number of inlets Two
Design head 305m
Discharge at design head 22 cu m/sec
Power required at coupling at rated discharge 93,600 bhp
Speed 428 rpm
Pump Discharge Valve:
Type Straightflow
Operation Double hydraulic servometer
Outlet diameter 2m
Turbines:
Opening time 60 seconds
Closing time (normal) 60 seconds
Spiral casing construction Welded plate
Diameter of Impellers:
First stage 2.4m
Second stage 2.5m
Number of impeller blades 10 each stage
Maximum diameter of shaft 0.7m
Generator Switchgear:
Type SF6 circuit breaker
Breaking capacity 174 MVA
Voltage between phases 16 kV
Current rating 6200 amps
Generator Transformers:
Number Two
Rating 160 MVA
Ratio 16 kV/275 kV
Cooling system Water circulation from lower reservoir
Weight as delivered 122 tonnes
Oil capacity 27 cu m
Auxiliary Transformers:
Number of station transformers Two
Rating 1 MVA
Ratio 16 kV/415 V
Number of local supplies transformers Two
Rating 1 MVA
Ratio 11 kV/415 V