Abstract :
This report deals with two problems of primary importance in connection with the transmission of three-phase power by-high voltage lines, viz.:? (a) The effects of transposing the line conductors on the operation characteristics and power losses.? (b) The impedance of a line conductor and earth return when one phase is accidentally earthed. Item (a) affects the design and construction of the line and its normal operation, while item (b) is the principal factor in determining the magnitude of the ?failure? current returning through the earth (under fault conditions) upon which the amount of interference with communication circuits is dependent. Transposition makes the power lines electrically balanced, equalizes the phase impedances and reduces the induction from normal operating currents and voltages. On single circuit lines transposition entails practically no additional cost and ought to be adopted as standard in all cases. On double circuit lines, however, special ?transposition towers? (costing in the case of the ?grid? lines several hundred pounds extra per tower) have to be employed at each transposition point (to prevent the conductors touching in the span), and this factor renders it desirable to avoid transposition whenever practicable. On long lines (fed entirely from one end) transposition appears essential both on account of inductive interference and to avoid large differences between the phase impedances. On short lines (say up to 50 miles long, which covers most practical cases in this country), the latter effect is negligible compared with the transformer impedances, and transposition is unnecessary when, in addition, interference with communication circuits arising from normal operating currents and voltages is unimportant. Thus on short double circuit lines no general recommendation regarding transposition can be made and each case must be judged on its merits. On all double circuit lines, whether transposed or not, the time-phase sequence of the c- onductor currents in the two circuits must be co-ordinated to obtain the best results from the standpoint of operation of the system. For transposed lines the maximum reduction in phase impedance occurs when the top conductor of each circuit conveys a current of the same time phase as the bottom conductor of the other, the two circuits being transposed at the same frequency and in the same direction of spiral. This gives a small reduction in phase impedance (3 per cent for the ?grid? lines) compared with that of a single circuit at similar spacings, but allows a small interchange of power (by transformer action) between the circuits which, however is negligible in most practical cases (0-28 kW per mile at 220 amperes on the ?grid? system). For non-transposed lines there are two practicable current sequences. In the first method, which gives minimum phase impedance, the current sequence is the same as that given above for transposed circuits. In the second method, conductors in the same horizontal plane convey currents of the same time phase and have equal impedances, thus giving equality of loading of the two circuits when operated in parallel. The ultimate choice depends on the magnitudes of the power transference between the circuits, the phase impedances and the differences between them. These in turn depend upon the particular conductor spacings, etc., employed, but a few simple calculations by the formulae herein soon indicate which is the better method in a given case. For the ?grid? lines the same phase sequence already given for transposed circuits is recommended. As regards power losses, the effects of transposition are negligible except on ?long? lines fed entirely from one end, a condition which will seldom, if ever, arise in this country. If one phase is accidentally short circuited to earth the interference caused to communication circuits is proportional to the magnitude of the current returning through the earth. The percentage of the total short-ci
