An Improved Communication-Free Dual Characteristic Over Current-Based Protection Scheme for AC Microgrids

The entry of distributed generation causes the network to become complicated, and as a result, the protection coordination between the network equipment is lost. In previous studies, three decision variables were considered for each directional overcurrent relay: time dial setting in the main mode, time dial setting in the backup mode, and pickup current (I_p). However, the minimum coordination time interval between the main and backup relay was lost. In this paper, communication-free dual characteristics directional overcurrent relays are used, considering six decision variables: I_p, time dial setting of the relays in the main and backup mode, breakpoint current, and the type of curves in main and backup operations. The proposed scheme maintains the minimum time interval between the main and backup relays, which reduces the operation time of the relays compared to previous studies. The proposed scheme is implemented on the modified IEEE 30-bus network distribution section. DIGSILENT software is used for simulation, and load currents and fault currents of protection relays in grid-connected and islanded conditions are extracted based on load flow and short circuit studies. Then, the optimization problem is solved using the genetic algorithm in MATLAB software, and optimal settings are obtained to minimize the operation time of the relays. The simulation results of the proposed method shows that the operation time of the relays can be improved by 17% compared with the previous existing method.