Directional Overcurrent Relays and the Importance of Relay Coordination

This chapter provides a brief idea about relay coordination. It explains the general concepts of relay grading using different types of overcurrent relays (OCRs); including definite‐time overcurrent relays, definite‐current overcurrent relays, and inverse‐time overcurrent relays. The non‐directional OCRs detect fault currents based on only their magnitudes and the trip signals are sent to the corresponding circuit breaker to clear the faults. The complexity increases when many relays are imposed and both time multiplier setting (TMS) and plug setting (PS) are taken as variables in where TMS is available either in continuous or discrete form, PS is available within a limited set of discretized values, and the minimum and maximum allowable limits of TMS, PS, and T are restricted. There are multiple choices to find minimum break‐point relays or minimum break‐point set. The goal is to find one common acceptable setting for the relays so that their coordination in individual loops is achievable.