Optimal time-current graded coordination of multistage inverse-time overcurrent relays in distribution networks

This paper proposes a method for optimal coordination of multistage overcurrent relays (OCRs) in radial distribution networks, including industrial, rural, and urban grids. The proposed method optimally coordinates multistage time-current curves (TCCs) using the genetic algorithm (GA). The proposed method is able to determine the proper TCC type (eg, moderately inverse, very inverse, and extremely inverse), pickup current, and time-setting multiplier (TSM) for each stage of TCCs. The proposed method utilizes the dynamic models of OCRs and fuses. This safeguards the relays against the maloperations resulting from the DC decaying part of fault currents and transient overcurrents caused by transformer energization or motor starting. The proposed method is evaluated using DIgSILENT Power Factory for power system analysis and Matlab for implementation of the proposed method. According to the simulation outcomes, the advantages and disadvantages of the proposed method are discussed and compared with the existing methods.