ABSTRACT
Inductive coupling between lines that are paralleled for part or all their length must be considered primarily for unbalanced faults. This applies to power lines of the same or different voltages and to communication wire lines that parallel power lines. The basic fundamentals are reviewed with applications to typical power and communication lines. The use of fiber optics in communication circuits eliminates the problem of extraneous voltages in communications circuits caused by the power circuits. Thus the induced voltage is the resistance drop along the sheath. This can vary considerably, depending on the sheath. With the line transposed, the reactance of the line will be equal to one-third of the sum of the values for the three sections, where the distance of each section will be one-third of the total line length. Mutual coupling between lines paralleled in the available right-of-ways even for relative short distances can cause protection problems. Most of the problems result from zero-sequence mutual.
