A novel strategy for directional overcurrent relays (DOCRs) coordination is proposed. In the proposed method, the objective function is improved during the optimization process and objective function coefficients are changed in optimization problem. The proposed objective function is more flexible than the old objective functions because various coefficients of objective function are set by optimization algorithm. The optimization problem is solved using hybrid genetic algorithm and particle swarm optimization algorithm (HGAPSOA). This method is applied to 6-bus and 30-bus sample networks. 1. Introduction Protection of distribution networks is one of the most important issues in power systems. Overcurrent relay is one of the most commonly used protective relays in these systems. There are two types of settings for these kinds of relays: current and time settings. A proper relay setting plays a crucial role in reducing undesired effects of faults on the power systems [1, 2]. Overcurrent relays commonly have plug setting (PS) ranging from 50 to 200% in steps of 25%. The PS shows the current setting of the overcurrent relays. For a relay installed on a line, PS is defined by two parameters: the minimum fault current and the maximum load current. However, the most important variable in the optimal coordination of overcurrent relays is the time multiplier setting (TMS) [3]. So far, some researches have been carried out on coordination of overcurrent relays [3–7]. Due to the difficulty of nonlinear optimal programming techniques, the usual optimal coordination of overcurrent relays is generally carried out by linear programming techniques, including simplex, two-phase simplex, and dual simplex methods [3]. In these methods, the discrimination time of the main and backup relays ( ) are considered as constraints and then the optimal coordination problem is solved using both objective function and constraints. In [8], a fast method for optimization of the TMSs and current settings by evolutionary algorithm and linear programming has been proposed. In [4], an online technique to estimate the setting of DOCRs is introduced. This technique is based on estimation of parameters of a proper equivalent circuit of the grid. Relay coordination which is very constrained discrete optimization problem is hardly solved by traditional optimization techniques [5]. In [9], the pickup current and the TMS of the relays have been considered the optimization parameters for optimal coordination of directional overcurrent relays. These optimization techniques are started by an
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