Optimal Coordination of Overcurrent Relays Using Mixed Integer Linear Programming

In distribution systems, the coordination and selectivity of protections are essential for improving reliability and security indicators. In these systems, the overcurrent relays are widely used. During a fault, the heat dissipation is high. Therefore, it is important that the relays act quickly. The problem of coordination is linear, since their pickup currents are known. In this paper, a methodology for an optimal coordination of non-directional overcurrent relays, in radial systems using MILP (Mixed-Integer Linear Programming) is proposed. Its main objective is getting the TDSs (Time Dial Setting) to minimize the operating times of relays, keeping the selectivity. Discrete values of TDS were considered. Thus, the problem becomes discrete and can be solved through MILP. For each relay were considered a range of possible fault currents levels. The proposed methodology was applied successfully in a radial test-system containing five relays.