Directional over current relays (DOCRs) coordination problem is a nonlinear and nonconvex problem which has many constraints. Optimal coordination of over current relays has been usually solved using linear and nonlinear programming. Also genetic algorithm and particle swarm optimization methods are used to fined the optimal solution of the problem. In this paper a new hybrid approach will be used for optimal coordination of over current relays. In this work, both time setting multiplier (TMS) and current setting (IP) are taken into account in the optimization procedure. The former is consider as a continues parameter and the later is assumed to be discrete parameter. The novelty of this paper is that quantizing IP parameter is taken into account as a part of optimization producer in the particle swarm optimization iterations. After that, we can calculate the optimum TMS with linear programming at the end of each iteration of PSO optimization algorithm. To increase the accuracy of proposed algorithm, first particle positions of swarms was calculated with linear approaches. Both near-end and far-end faults are taken into account in the constraint of the optimal coordination problem and the results are compared with the other methods. The obtained results are quite encouraging and will be useful as an effective method for coordination of over current relays.

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