SWEB's Pocket Power Stations

by John Gale

In the years between 1959 and 1965, SWEB installed a number of small unmanned power stations at remote points on its system. They fulfilled the dual purposes of peak lopping and system security and came to be known as "Pocket Power Stations". Each unit consisted of a turboprop aero-engine driving an alternator, together with the control gear and switch-gear and associated oil storage tanks. The sets could be started and stopped remotely via a telephone line and could be run up and put on load in minutes. These pioneer power stations led the way to the use of aero-engines for power generation and were the fore-runners of the large gas turbine sets in use today.

The Pocket Power Station arose from an idea of Mr. A.N. (Bill) Irens, the SWEB Chairman (1956-1973). In his previous employment as the Chief Electrical Engineer for the Bristol Aeroplane Company, he had seen generators used to load engines under test (as dynamometers to measure the power output). In such cases the output was usually dissipated in resistance banks. However he realised that aero-engines could be used for power generation.

At the time that Mr. Irens joined SWEB as Chairman, the tariff for electricity levied by the British Electricity Authority (later the Central Electricity Generating Board) incurred a high charge based on peak demand This peak demand occurred for a relatively short time during the winter and Mr. Irens reasoned that if SWEB could reduce this peak a considerable advantage could be gained. For this purpose small, remotely-controlled generators, which could be started and put on load within minutes, would be required- A gas turbine powered generator would be ideal for this purpose. Such a generator would also be ideal to provide an alternative supply at remote parts of the SWEB system, where amenity problems made the provision of additional circuits difficult. Thus the pocket power station was conceived.

However SWEB were not allowed to generate electricity under the rules of nationalisation, and an approach to Parliament was needed to change the legislation. Thanks to the persistence of Mr. Irens, SWEB were successful and the new Electricity Act of 1957 gave the Area Boards the opportunity to operate generating stations in certain circumstances.

The Engine
The engine chosen was the Bristol Siddeley Proteus. This was originally developed for the Bristol Brabazon and the Princess Flying Boat, but both projects were abandoned before the engine had finished its development. The Proteus was then adapted to power the Bristol Britannia, which has had a long and successful career.

The Proteus is a 4,250 SHP (shaft horse power) turbo-prop engine. It uses the free turbine principle, in which the compressor-driving and power turbines are mounted on concentric, but mechanically independent shafts. Tiffs arrangement enables each system to run at its optimum speed under all conditions. It has two valuable advantages in the turbo-generator applications. Only the compressor system needs to be rotated by the starter and flus enables a relatively small battery to suffice. Also the alternator is coupled to a refit of low inertia, which facilitates automatic synchronisation.

Reduction gear, similar to that used on the Proteus aero-engine is used to drive an alternator at 1,000 RPM.