The Severn-Wye Cable Tunnel, also known as the Severn Cable Tunnel, carries high-voltage (400 kV) electricity transmission lines beneath the estuaries of the River Severn and River Wye between Newhouse (Mathern), Monmouthshire and Aust, South Gloucestershire.

History

To exploit the generating capacity of new power stations being built in the late 1960s at Pembroke (2,000 MW) and

Aberthaw Aberthaw () is an area containing the villages of East Aberthaw and West Aberthaw, on the coast of South Wales about west of Barry, Vale of Glamorgan, Barry. It is the location of Aberthaw Cement Works, Aberthaw Lime Works, and Aberthaw Power S ...

('A' 600 MW, 'B' 1,400 MW), the

Central Electricity Generating Board The Central Electricity Generating Board (CEGB) was responsible for electricity generation, transmission and bulk sales in England and Wales from 1958 until privatisation of the electricity industry in the 1990s. It was established on 1 Januar ...

(CEGB) planned to construct 400 kV supergrid lines through South Wales and Gloucestershire to National Grid connection points at

Gloucester Gloucester ( ) is a cathedral city, non-metropolitan district and the county town of Gloucestershire in the South West England, South West of England. Gloucester lies on the River Severn, between the Cotswolds to the east and the Forest of Dean ...

and

Melksham Melksham () is a town and civil parish on the Bristol Avon, River Avon in Wiltshire, England, about northeast of Trowbridge and south of Chippenham. The parish population was 18,113 at the 2021 census. History Early history Excavations in ...

, Wiltshire. The CEGB intended the line to Melksham substation to cross the

River Severn The River Severn (, ), at long, is the longest river in Great Britain. It is also the river with the most voluminous flow of water by far in all of England and Wales, with an average flow rate of at Apperley, Gloucestershire. It rises in t ...

at Sharpness using overhead cables. A public inquiry was held in 1967 and objections were raised by the Council for the Preservation of Rural England and several District and Parish Councils. The principal objection was to the impact of the power lines on the scenery of the

Cotswolds The Cotswolds ( ) is a region of central South West England, along a range of rolling hills that rise from the meadows of the upper River Thames to an escarpment above the Severn Valley and the Vale of Evesham. The area is defined by the bedroc ...

and of the Vale of Berkeley. Following the inquiry the CEGB was directed by the Minister of Power to plan a more southerly route. The amended route across the Severn was 11 miles downstream from Sharpness, and close to the existing overhead crossing at Aust, but using a tunnel instead of overhead lines. The additional cost of tunneling was estimated to be about £4 million.

Specification

The Severn-Wye Cable Tunnel carries two 400 kV electricity circuits on the transmission line between the National Grid 400 kV substations at Whitson and Melksham. Each circuit has a rating of 2,600 MVA. The tunnel is 3.1 m (10 ft) in diameter, is 3.6 km (2.25 miles) long, and has a maximum depth of 48.8 m (160 ft). The two principal access shafts are on the East bank of the Severn at Aust, South Gloucestershire (51.608459°N 2.615454°W) and on the West bank of the Wye at Newhouse (Mathern), Monmouthshire, (51.610878°N 2.669090°W) where each of the two cable sealing end compounds are located. There is also an intermediate access shaft at Beachley, Gloucestershire (51.609761°N 2.652498°W).

Construction

The tunnel was engineered by Charles Haswell and Partners and constructed by Charles Brand and Sons Ltd. It was built using three access shafts: at Aust (at a depth of 47.9 m), at Beachley (48.5 m deep) and at Newhouse (34.7 m deep); two tunnel boring machines were used. In December 1970 the machines were boring at a rate of 9.1–15.2 m per week but this increased to 46 m per week in 1971. Difficulties were encountered with the ingress of water from the natural springs in the geology of the area. The civil engineering work cost about £2 million and the electrical installation an additional £1.5 million (1970 prices). The cables were manufactured and installed by British Insulated Callender’s Cables Ltd. In April 1972 a lift at the Aust construction shaft failed and fell 30 m (100 ft), injuring four men.

Operation

Each of the six cables in the tunnel is installed within a PVC pipe though which cooling water is pumped. The circulating water is cooled in air/water

heat exchangers A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...

. At full power (2,600 MVA for each circuit) the cable joints are cooled by circulating oil through the hollow conductors. Water ingress into the tunnel is managed by a pumping system. Initially pumping took place at both ends of the tunnel, but in about 2005 this was reduced to pumping at the Aust end only. There are four 18.5 kW rotary lobe pumps manufactured by Börger. Each pump has a capacity of 40 m³ per hour at a discharge head of 70 m. Normally only two pumps are in operation but if the ingress of water increases all four pumps automatically operate. A powered trolley to move equipment, and in an emergency injured personnel, is provided in the tunnel.

References

{{National Grid, state=autocollapse Electric power infrastructure in England Electric power infrastructure in the United Kingdom High-voltage transmission lines National Grid (Great Britain)

History

Specification

Construction

Operation

See also

References