Electric Power Principles, 2nd Edition

7Polyphase Lines and Single‐phase Equivalents

By now, one might suspect that a balanced polyphase system may be regarded simply as three single‐phase systems, even though the three phases are physically interconnected. This feeling is reinforced by the equivalence between wye‐ and delta‐connected sources and impedances. One more step is required to show that single‐phase equivalence is indeed useful, and this concerns situations in which the phases have mutual coupling.

7.1 Polyphase Transmission and Distribution Lines

Lines are system elements as transmission or distribution lines: overhead wires, cables or in‐plant buswork. Such elements have impedance, so that there is some voltage drop between the sending and receiving ends of the line. This impedance is more than just conductor resistance: the conductors have both self and mutual inductance because currents in the conductors make magnetic flux which, in turn, is linked by all conductors of the line. A diagram of one format of overhead transmission line is shown in Figure 7.1. The phase conductors are arranged as parallel conductors in bundles (in this case, bundles of two conductors). These wires are supported by insulators. In many cases these insulators are actually strings of individual plates, often made of ceramic material, arranged to have a very long surface between the conductor and ground. Usually there are also accompanying ground wires that serve to carry the inevitable but usually small neutral currents ...