Efficient non-standard tripping characteristic-based coordination method for overcurrent relays in meshed power networks

The inverse time tripping characteristic of overcurrent relays (OCRs) has been one of the most common schemes utilized in protecting interconnected power systems. Due to the continuous growth of the multi-looped structure of modern power systems, the protection task based on existing OCR’s will become more difficult. In this case, decreasing the relay operating time while maintaining selectivity seems necessary. In this paper, a novel methodology based on Piece-wise Linear Characteristic (PWLC) is developed to coordinate OCRs in meshed networks. A mathematical model of the PWLC is first developed. Then, unknown slopes are assigned to each of the line segments which by determining them optimally, the relay characteristic can be directly extracted. By specifying the relevant decision variables of the problem, the OCRs coordination problem is simply converted to an optimization problem where the main objective is to minimize the overall time delays of the relays as well as operating time differences between the main/backup relay pairs. The proposed technique is applied by simulation to the IEEE 14-bus test system to evaluate its effectiveness and accuracy. The simulation results show faster protection, robust coordination, and improvement in the sensitivity of the relays by using the novel coordination approach.