A new formulation for coordination of directional overcurrent relays in interconnected networks

In large and interconnected networks, the coordination between all protective devices, without any violation, is very complicated and sometimes impossible. For obtaining reasonable and optimal coordination with suitable operating time (OT) for relays, some constraints are obliged to violation. In order to reduce the related violation and also the relays OTs, some objective functions (OFs) are proposed in literatures in which the discrimination times of the primary and backup relays is added to the OF. In this paper, for obtaining the optimal coordination of directional overcurrent relays, the OF is improved to overcome the drawbacks of the existing OF. In other words, as the main contribution, a new problem formulation is proposed so that not only the optimal settings of relays are obtained but also the OTs of the primary and specially backup relays are reduced considerably. In order to solve the proposed problem formulation, the effects of both near and far-end faults are considered. Also, regarding the protection system flexibility, the program is capable of choosing the best characteristic for each overcurrent relay. The genetic algorithm is used as an optimization tool, and the effectiveness of the proposed method is demonstrated by comparing the coordination results for the two sample networks with those obtained by using the recent developed OFs. Copyright (c) 2013 John Wiley & Sons, Ltd.