Secondary Sections of the British Grid System

Abstract

NEARLY all the British grid operates at a pressure of 132 kilovolts. Two sections operate at 66 kv. and 33 kv. respectively, and there are a few relatively unimportant sections which operate at smaller pressures. In a paper read to the Institution of Electrical Engineers on November 9, Mr. C. W. Marshall describes the 66 and 33 kv. sections. In the scheme of the Central Electricity Board, there are 221 miles of 66 kv. lines and 1,319 miles of 33 kv. lines. Unlike the main grid, these subsidiary lines are mainly used for transmission purposes. The standard conductor material from which they are made is steel-cored aluminium. The minimum clearance between any line conductor and the earth in still air under maximum temperature conditions (50° C.) is 20 ft. If they have to cross Post Office lines then, whenever possible, the P.O. lines are interrupted and cables substituted for them at the crossing. If this is not possible, a guard is provided under the power lines. In this case the minimum clearance between guard and Post Office lines is 3 ft. and between guard and power lines 4 ft. When the power lines cross a railway, the minimum clearance is 24 ft. above rail level. Lattice steel towers are the standard for all the Board's 66 kv. lines and, with one exception, for all the 33 kv. lines also. During the period of the activities of the Board, the development of cable technique has been very rapid. The first cables were made with solid dielectrics, the single core oil duct type came next, then the three core oil duct and finally the ‘gas pressure’ three phase cable. Operating experience is still too scanty to allow definite conclusions to be drawn as to their relative merits. Hitherto they have only been used in situations like central London which preclude the use of overhead lines.