Correspondence: New approach to analysis of polarised mho relays under close-up-fault conditions

Under close-up-fault conditions the voltage signal from the faulty phase may not be sufficient to operate the mho relay. A voltage signal from one of the healthy phases, external polarisation, is added, and the relay, in this case, may be called an externally polarised mho relay. The main effect of external polarisation is the variation of the relay characteristic with the power-system conditions, which results in an increase of the resistive reach of the relay, under close-up fault condition. The paper introduces a new approach by which the resistive reach of externally polarised mho relays under close-up-fault condition could be evaluated without much analytical complexity and with reasonable accuracy.