Optimal Directional Overcurrent Relay Coordination in Interconnected Networks Considering User-Defined PWL Characteristic Curve

This paper develops a novel methodology to apply the piece-wise linear characteristic (PWLC) to coordinate the digital overcurrent relays (DOCRs) in meshed power systems regardless of their scales. In this approach, first, a table based on maximum short-circuit fault currents obtained using DIgSILENT software is produced in which the current seen by each relay is recorded by implementing a three-phase fault in front of other relays. Then, based on the fault currents achieved, the PWLC of each relay begins from the maximum short-circuit fault current in front of it and extends toward the lower fault currents. The proposed approach is tested on three different networks, namely the distribution portion of the IEEE 14-bus, the IEEE 8-bus, and the IEEE 39-bus test systems. The simulation results show that the proposed method can be easily implemented on any power network irrespective of the network size. The viability of the proposed scheme in terms of operating time, miscoordination cases, and flexibility is compared with the other standard and nonstandard characteristic curves available in the literature. The comparative study demonstrates a notable reduction in total relays’ operating time. The operating time difference between main and backup relays is also remarkably decreased. Since the number of line segments of the PWLC characteristic and their relevant slope and time delay can be defined arbitrarily, more flexible and desirable approach for the overcurrent protection coordination problem is achieved compared to other NSCs.