Optimal Coordination of DOCRs Considering Transient States of Fault Current in Interconnected Distribution Systems using Two-level User-defined Dynamic Model of Overcurrent Relays

Optimal coordination of directional overcurrent relays (DOCRs) has been studied in researches based on the assumption that the fault current passes through these relays with a fixed value. However, the short circuit (SC) current has no fixed value in real electrical networks. So, to avoid possible miscoordination between the DOCRs, the transient states of fault current should be considered during the coordination study. This paper presents an optimal coordination study for DOCRs using a non-standard two-level user-defined dynamic relay model and considering the transient states of the fault current contributed by the main grid and the distributed generations (DGs) units. Three optimization techniques are used to solve the coordination problem: two hybrid meta-heuristic techniques: gravitational search algorithm-sequential quadratic programming (GSASQP) and particle swarm optimization-gravitational search algorithm (PSOGSA) and one mathematical method: find minimum constraints (FMINCON). The IEEE 14 bus system is used to test the effectiveness of the proposed coordination scheme. The results show the superiority of the meta-heuristic techniques over the mathematical method and show that the presented two-level user-defined method can significantly reduce the relay's operating times while maintaining the coordination among DOCRs. MATLAB program is used to obtain the transient SC current and to apply the proposed techniques.