Fundamental Basis for Distance Relaying on 3-Phase Systems

In the preceding investigation the quantities upon which distance relays must depend for their operation have been determined for all possible types of faults occurring at a single location. It has been shown that the quantities obtained by the use of earlier distance relay connections for both line and ground faults do not give the closest approach to the desired result, a constant indication for all types of faults at a given location. It has been shown further that, by the use of different connections and certain auxiliary apparatus, a constant relaying indication may be obtained by the line relays for all types of line faults and by the ground relays for all types of faults involving ground, if the fault resistance is zero, and, if the fault resistance is not zero, that smaller deviations from the normal value will be obtained for a given fault resistance. The analysis has been based upon the assumptions of sine wave currents and voltages, constant circuit impedances, and negligible distributed capacitance in the faulted sections of line. It has been assumed further that the fault impedance is resistance only and that it is effectively constant throughout each cycle. All of these assumptions, except perhaps the latter, are reasonably valid as far as relaying is concerned, although some consideration must be given to the d-c component of the short-circuit currents with very high-speed relays.