Optimal Coordination of Overcurrent Relays Using Mixed Integer Linear Programming

In distribution systems, the coordination and selectivity of protection devices 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 when the pickup currents are previously known. In this paper, a methodology for an optimal coordination of non-directional overcurrent relays in radial systems using Mixed-Integer Linear Programming (MILP) is proposed. The main objective is to compute the Time Dial Settings (TDSs) that minimize the relay operational times, without any loss of sensitivity, selectivity and reliability. The TDSs are considered as discrete variables, so the problem becomes discrete and is solved through MILP. The selectivity is guaranteed for a range of possible fault currents levels. The proposed methodology is successfully applied in a radial test system containing five overcurrent relays.