Optimal Protection Coordination of Nonstandard Overcurrent Relays Using Hybrid QCQP Method

The optimal protection coordination (OPC) problem of directional overcurrent relays (OcR) in power system networks is a complex, highly constrained nonconvex optimization problem because of the nonlinear inverse nature of the OcR characteristics. Finding the best OcR settings by solving OPC is a challenging task because of its nonconvex nature. Although various heuristic, hybrid heuristic, and traditional optimization-based solution methods are suggested in the literature, such methods are just far enough to provide better optimal solutions to the OPC problem. Besides, the standard inverse characteristics of the OcRs with fixed characteristic coefficients limit the OPC problem from finding the best settings. As digital relaying technology has the feasibility to embed more relay settings, nonstandard relay characteristics can be developed to achieve minimum relay operating times. To find the best OcR operating times, a nonstandard characteristic based OPC problem is formulated in this work by parameterizing the OcR characteristic coefficients along with the OcR settings. By setting two variables of the proposed OPC using particle swarm optimization (PSO), the rest of OPC is formulated as a quadratic programming problem with quadratic constraints (QCQP). The proposed method is developed in MATLAB programming platform, and results are found to be satisfactory.