Enhanced Two-Fault Point Multi-Objective Coordination Scheme for Directional Overcurrent Relays in Meshed Distribution Networks Considering Fault Severity

In this paper, an enhanced directional overcurrent relays (DOCRs) coordination scheme is introduced. The proposed scheme considers the fault severity by applying weighting factors, calculated based on system fault currents at different locations in each relay zone of protection. This enhanced coordination reduces the operating time of the relays for severe fault currents avoiding the greater thermal and mechanical stresses that reduce equipment lifetime. The DOCRs coordination problem is formulated as a multi-objective optimization problem to accomplish better results than the single-objective methodology with respect to relays selectivity and speed. DOCRs coordination problem is solved using Goal Attainment method in MATLAB optimization Toolbox. A two-fault point coordination scheme is applied to IEEE 14 bus system for both the conventional and the proposed scheme. The results are analyzed according to several system performance indices including: the mean of the system's relays operating times, the maximum primary relay operating time, the maximum backup relay operating time and maximum coordination time interval (CTl) between primary/backup pairs. The achieved results indicated that the proposed scheme considering the severity weighting factors has reduced the mean operating time of relays especially for the relays with faults with high severity. It means more reliable, faster protection system with less thermal and mechanical stresses for system equipment.