108. Mutual Impedance in Parallel Lines – Protective Relaying and Fault Location Considerations

When two or more lines are running parallel to each other, mutual impedances between the lines modify the voltage and current profile measured in the protective relays protecting each line. Analysis of transmission line impedance formulas can provide interesting data to the protection engineer. Fault location is an algorithm in protective relays that reports the distance to the fault. Ground fault location is discussed, as is the implication of requiring the measurement of the parallel line 3I0 for a more accurate calculation. Since the algorithm is slow and done after the trip decision, the 3I0 measurement does not necessarily have to be sampled in the same box. Protective relaying considerations for preventing overreach and loss of directionality under certain power system operating conditions are illustrated and discussed. The paper illustrates the benefit of measuring IOp (the parallel line zero-sequence current) for fault location.