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• Optimal coordination of directional overcurrent relays (DOCRs) in complex distribution network. • Exploring Sine-Cosine algorithm for optimal coordination of DOCRs in distribution system integrated with distributed generation. • Optimization of relay settings and the coordination between primary and backup relay pairs are addressed simultaneously. The modern power distribution networks are very complex due to the growing integration of distributed generators in recent years. Optimal coordination of directional overcurrent relays (DOCRs) used for protection of such complex networks is a highly constrained and nonlinear optimization problem. In this paper, a new optimization algorithm called Sine Cosine Algorithm (SCA) is used first time to solve the optimal coordination problems of DOCRs. SCA is a recently proposed population-based algorithm used for solving highly nonlinear optimization problems. The cyclic pattern of sine and cosine function allows a solution to be re-positioned around another solution. This can guarantee exploitation of the space defined between solutions. The effectiveness of SCA in solving the optimal coordination problems of DOCRs is tested on different fault data generated on 3-bus, 8-bus, 15-bus and 30-bus test systems. The results clearly show that the effectiveness of the proposed algorithm is superior in reducing the overall operating time of primary relays compared to the recently published optimization algorithms for DOCRs coordination. Also, the proposed algorithm is robust in maintaining the coordination between primary and backup relay pairs. Compared to existing optimization algorithms, the proposed algorithm has reduced coordination interval time between primary and backup relay pairs. [ABSTRACT FROM AUTHOR]

• A novel hybrid technique to coordinate DOCRs. • Five well-known IEEE test systems are used to validate the proposed technique. • Simplified implementation, as it combines two well-known algorithms. • Excellent performance and convergence, suitable for online adaptive coordination. • Results are comparable with the best reported in the specialized literature. The coordination of directional overcurrent relays (DOCRs) is a constrained and nonlinear optimization problem which consists in finding suitable plug and time dial settings so that the relay operational times are minimized, keeping selectivity and sensitivity characteristics. Recently, several efforts have been devoted to automate the coordination of DOCRs. This paper proposes a hybrid technique entitled simulated annealing–linear programming (SA-LP) to achieve the optimal coordination of DOCRs. Five test-systems (IEEE-3, IEEE-6, IEEE-8, IEEE-15 and IEEE-30 bus) are used to verify the effectiveness of the proposed technique. Results obtained with the SA-LP are confronted against other optimization techniques reported in specialized literature, under identical conditions. The proposed approach presented good quality solutions, low computational processing times and great convergence towards the optimum solution, presenting an advantage over adaptive coordination tendency by enhancing monitoring, communication capabilities and grid control. [ABSTRACT FROM AUTHOR]