Robustness of distance relay with Mho characteristic against inter phase faults resistance

This paper discusses the maximum fault resistance, which can be tolerated by a distance relay with Mho characteristic in the case of inter phase, phase to phase and three phase, faults. This is defined as the boundary fault resistance, that in the case of the lower fault resistances, the relay can discriminate between in-zone and out-of-zone faults satisfactorily. The robustness criterion is defined based on the boundary fault resistance. In order to achieve this objective, the related equations are derived and then the boundary fault resistance curve and the robustness criterion are presented.