Some observations for distance protection of power transmission lines

This paper presents some observations related to distance protection for power system transmission lines. Classical distance protection relays adopt the line terminal voltage and line current to calculate the distance between the fault and the relay location and then make a decision on whether a tripping signal should be issued and, if so, what the time delay should be. An interesting question is: is it possible for two faults at different points of the system to result in the same bus voltage and the same line current at a relay location such that the relay cannot distinguish between them? This paper presents some of these scenarios that may impose challenges on distance relay settings and operation.