A critical study of a three-phase system of unarmoured single-conductor cables, from the standpoint of the power losses, line constants and interference with communication circuits

This paper has been prepared as a guide to the selection of methods of laying three separate unarmoured ? singleconductor cables for the transmission of three-phase power. A wide extension of the use of cables of this type is contemplated for transmission at 66 kilovolts and upwards. There are two general practicable methods which may be adopted for laying such cables. They may be laid in a symmetrical manner, the axes of the cables being equidistant from one another; or they may be laid alongside one another with their axes in a plane. The electromagnetic effects when carrying three-phase alternating current are materially different in the two cases. The principal factors affected are :? (1) The voltage which may be generated in the sheaths. (2) The energy loss in the sheaths. (3) The impedances of the different cables. (4) The influence on communication circuits. The arrangement in a plane leads to asymmetry and inequality in several of the factors involved, such as voltage generated in the sheath, power loss, impedance and temperature-rise; and in addition the effects are materially greater on the whole than when the same current is transmitted by cables arranged symmetrically. The current-carrying capacity of the two systems for a given temperature-rise has been given some consideration in view of the possibly better cooling conditions of the plane arrangement; but it is found that, in general, the increased losses by separation or substitution of the flat arrangement substantially offset the better cooling conditions. The general conclusions are that, as regards the electromagnetic and heating effects, the symmetrical triangular arrangement, with spacing as close as possible and with cross bonds and transposition at every joint, should be adopted as standard The difference between the effects of triangular spacing and of flat spacing with regular transposition is small and the latter is permissible when special circumstances render it desirable. These conclusio- ns apply to cables laid both direct in the ground and in ducts.