Utilities distribution # 1974 Southhampton
The 400 kV cables installed by the Central Electricity Generating Board to match their largest 400 kV overhead lines appear to set the limits of conductor size which cable manufacturers produce, Before the Severn installation, with which this Paper in the main deals, the maximum size of conductor available for 400 kV cables was 3 sq.in. copper (2000 mm2). Two such cables per phase were required to match a 400 kV heavy duty overhead line if the cable installation relied on heat radiation to the surrounding ground, even though techniques of controlled low thermal resistivity backfills were employed. The high cost of such a cable installation, which in the late 60’s was costing about £620,000 per km compared to an overhead line cost of £35,000 to £45,000 per km, was the principal argument used by the Generating Board against requests from amenity associations to underground sections of 400 kV overhead lines.
This argument came under severe criticism not only from lay public objectors, but also from their technical advisers. The Board therefore stepped up an already very costly research and development programme to try to contain the cost of cable installations, and the most obvious way to achieve this objective was to try to bring down the number of cables per phase from two to one, within the limits. of the size of conductor which could be manufactured. The rating of a cable is determined by the maximum permissible dielectric temperature at which the cable will safely and continuously operate, and it was appreciated that to achieve the economy aimed at it would be necessary to remove the heat generated in the cables during operation by a separate cooling agent.
The first system adopted for forced cooling of 400 kV cables was in the Fawley tunnel under the Solent, where cables were laid in trefoil(cloverleaf)in an open duct through which water flowed under gravity from.one end of the tunnel to the other. The disadvantage of this system was that the tunnel had to have a slope from one end, and in the case of the Fawley Tunnel because of this tunnelling had to be carried out in difficult strata under compressed air working making for very expensive and dangerous tunnel construction. To be able to tunnel in favourable strata if water was to be the coolant it was necessary to develop a system of pressurised water pipes into which the cable could be pulled.
In 1967 the Generating Board was required, as a result of a Government decision, to cross the Rivers Severn and Wye by a cable tunnel and it was decided to adopt an integral cooled pipe system in which the cable is run in a pipe through which water is circulated in a closed circuit through a heat exchanger. It had been experimentally demonstrated that with this technique one cable per phase could be adopted for a rating equivalent to a heavy duty 400 kV overhead line, and it was hoped that this could be 3 sq.in. (2000 mm”) copper section showing a 50% saving over previous systems in the use of this semi precious metal. In the event, a 4 sq.in. (2600 mm“) cable was considered necessary and cable contractors demonstrated that such a conductor could now be manufactured.