Optimal-Probabilistic Coordination of Directional Overcurrent Relays Considering Network Topological Uncertainties

The probability of each network topology occurring in an actual power system is entirely different. The relays coordination considering overall possible topology variations without corresponding probabilities tends to cause unsatisfactory performance of relay settings in operation and even difficulty in determining settings. In this paper, a novel method is proposed for optimal coordination of directional overcurrent relays (DOCRs) considering network topological uncertainties through its probabilistic model. The relays coordination problem is formulated as a multi-objective optimization problem with two objective functions. One is the conventional function that minimizes overall operating time of the relays, and the other is to minimize the probabilities of loss of selectivity under network topology variations. With the proposed method, the settings are satisfactorily determined with the minimum probability of losing selectivity. The method proposed in this paper has been applied to an 8-bus power system, and the results demonstrated the better robustness towards the network topology variations over the prevailing approaches.