Shield losses in medium-voltage cables: Final report

Shield circulating current losses and cable ampacities have been calculated for five aluminum conductor sizes of 15kV and 25kV distribution cables, each cable having a concentric copper wire shield/neutral conductor of the size and number of wires corresponding to approximately 1/3, 1/6, 1/8 and 1/10 the circular mil area of the phase conductor adjusted for conductivity of the two metals. Calculations were performed for single conductor cables in close triangular and in flat configurations, in the latter case for 7.5 inch and 12 inch spacings between cable centers. Calculations were also performed for three basic double circuit configurations, one with the cables in close triangular configuration with the two circuits spaced 18 inches apart, the second with the six cables in flat configuration with the cables spaced 7.5 inches and 12 inches on centers, respectively, and 24 inch spacing between the nearest cables of the two circuits, and the third with the six cables in two layers of three, one directly above the other with 7.5 inch spacing between cables and between circuits and 12 inch spacing between cables and circuits, respectively. Circulating current losses were calculated for 100% of rated load based on 75% load factor for the circuit.more » This engineering study clearly demonstrates that shield losses and cable ratings can be optimized by coordination of cable design and the circuit configuration. A guide is presented which provides a step by step procedure for design of metallic shields based on sound engineering principals and for maximum cost effectiveness. 5 refs., 53 figs., 19 tabs.« less