A dual-comparator mho-type distance relay utilizing transistors

The paper describes further work on a mho-type relay using transistors, which depends for its operation on direct phase-comparison. The relay is rendered free from transient overreach by the use of a dual comparator circuit configuration. Relay accuracy is discussed and the methods which have been employed to improve the accuracy of the relay during operation without increasing its sensitivity are described. The tests applied to the relay and the reasons for their adoption are given briefly. Accuracy/range charts and dynamic mho-curves are presented for the relay connected as a phase-fault element with normal polarization, with polarization from an unfaulted phase and with the use of memory. It is concluded that the relay is a marked improvement over proto-types described earlier by the authors, particularly in respect of accuracy and absence of transient overreach. Speed of operation is discussed, with particular regard to the use of an alternative criterion of operation in which only one of the two comparators must detect a fault before a tripping signal is produced. Probable improvements in the relay design with a view to economy in the number of transistors are considered. No transistor failures have been experienced.