Detecting and solving the coordination curve intersection problem of overcurrent relays in subtransmission systems with a new method

This study presents a practical and effective method for inspecting and repairing curve intersection situations that occur in upstream and downstream coordinated overcurrent relay pairs in subtransmission networks. This type of miscoordination can cause backup relays to trip before their paired primary when faults occur, resulting in unexpected expansions of blackout areas. The proposed method is based on the IEEE and IEC standard characteristic curve equations of digital overcurrent relays and a customized curve equation for electromechanical overcurrent relays. Consequently, the proposed method is applicable to both digital and electromechanical relays. Two subtransmission network cases for the Taiwan Power Company were simulated to validate the proposed method.

► The approach not only detects but also repairs the curve intersection.
► The approach can be used as an auxiliary tool or a stand-alone utility.
► The approach is applicable to both digital and electromechanical overcurrent relays.
► The original coordination time interval (CTI) remains unaltered after adjustment.
► Multiples of pickup current stays within 2–20 following adjustment.