Consideration of the transient configurations in the optimal coordination problem of directional overcurrent relays

An iterative methodology is presented to coordinate the operation of the directional overcurrent relays of a meshed system, considering the changes in the system topology, and therefore, the changes in the currents detected by the relays, that occur during the total fault clearing time, when only one of the main relays has operated (transient configurations). A linear programming algorithm is used to calculate the time dial settings that minimize the sum of the operation times of the relays of the system for the faults under study, subject to the time coordination constraints associated to each of the system configurations that take place during the fault clearing process, considering the limits on the relay settings allowed by the corresponding devices and the time-current characteristic associated to each of the relays. Two real cases are presented to illustrate the application of the methodology: an industrial 5 bus system, with two 115/13.8 kV power transformers, and a realistic transmission and sub transmission network consisting of an 18 bus system with 63 kV and 230 kV transmission lines, 13 power transformers and 96 directional overcurrent relays.