Optimal coordination of directional overcurrent relays using hybrid BBO-LP algorithm with the best extracted time-current characteristic curve

The coordination problem of directional overcurrent relays (DOCRs) is considered as a highly constrained, nonlinear, and non-convex optimization problem. The summation of the operating times of all DOCRs, when they act as primary protective devices, is taken as the objective function that needs to be minimized. This stiff problem is mostly optimized based on IEC standard inverse time-current characteristic curve (TCCC) and using discrete plug setting (PS) to simulate electromechanical DOCRs. From the literature, some few papers have solved this coordination problem by using different TCCCs. However, this approach increases the problem dimension by 250%, which in turn consumes more CPU time and needs more iterations for converging to near-optimal solutions. Moreover, coordinating DOCRs with different TCCCs could violate the selectivity criteria in some unconsidered fault locations, because satisfying the optimality at the near-end 3φ faults does not guarantee the feasibility of other fault locations. This paper solves all these points by heuristically selecting the best TCCC among a large variety of North American and European standard TCCCs. In addition, this paper utilizes the advanced features available in modern numerical relays to obtain new solutions based on continuous PS. The performance of the proposed BBO-LP optimization technique is evaluated using a 15-bus system.