Novel method for implementing the mho characteristic into distance relays

Traditionally the mho characteristic has been implemented by applying torque-like algorithms using voltage phasors. Presenting and analyzing the behaviour of such algorithms in the impedance plane are not straightforward. Protection engineers, however, need to know the relationship between the measured impedance and the operating characteristic of the protection during power system faults. A tool preferred by the protection engineers is one that plots both the loop impedance trajectory and the operating characteristic in the impedance plane (R-X diagram). This paper describes a novel way of implementing the mho characteristic so that it can be analyzed in the impedance plane in a similar way as the quadrilateral characteristic. The method enables the dynamic expansion of the mho circle as a result of the healthy voltage polarization during fault conditions. An additional advantage of this method is that it reduces the computational burden in the relay terminal as the fault loop impedance can be calculated centrally and utilized in zone boundary comparisons for both the mho and quadrilateral characteristics. This algorithm can be utilized in new distance protection designs applied in power distribution and sub-transmission networks.