Optimal Coordination of Double-Inverse Overcurrent Relays for Stable Operation of DGs

Due to the increasing penetration of distributed generations (DGs) in distribution networks, the performance of protective relays is becoming more critical to maintain the stability of the DGs especially during fault conditions. Among different types of DGs, squirrel cage induction generators (SCIGs) and synchronous DGs are sensitive to faults and might become unstable even after fault clearance. Since the fault clearing time depends on the overcurrent relays (OCRs) characteristic curves, this paper proposes a double-inverse OCR (DIOCR) characteristic to provide proper coordination while maintaining DGs stability. The proposed method defines the critical clearing time curve for each DG and combines them with DIOCRs curves and coordination time interval constraints for finding optimal DIOCRs settings. The proposed method is simulated on the IEEE 33-bus distribution network equipped with SCIG- and synchronous-based DGs. The obtained results highlight the capability of DIOCRs in protecting distribution system with DGs considering stability constraints.