An accurate method for overcurrent-distance relays coordination in the presence of transient states of fault currents

Regarding to dynamic nature of power system, short-circuit current also includes transient currents in addition to steady current. Transient components of fault currents may disturb the coordination between overcurrent (OC) and distance (Dis) relays because of the dependence of the operating time of inverse time overcurrent (OC) relays to the value of the short-circuit current. In this paper, a method for OC-OC and OC-Dis relay coordination, considering transient states of short-circuit currents, is presented. The dynamic model of the inverse time OC relay is used. A new formulation based on the dynamic model is presented to incorporate the pickup current (Ip) into the coordination algorithm, in addition to the time setting multiplier (TSM). To improve OC-OC and OC-Dis relay coordination, the selection of various curves types for OC relays is considered. Adapted particle swarm optimization (PSO) algorithm is used to obtain optimal results for the coordination of relays. The proposed OC-OC coordination and OC-Dis coordination methods are implemented on a radial and a meshed network, and the results are compared with the conventional method of coordination.