Protection coordination for microgrids with grid-connected and islanded capabilities using communication assisted dual setting directional overcurrent relays

This paper proposes a communication assisted dual setting relay protection scheme for micro-grids with grid connected and islanded capability. Previous work on dual setting relays has been applied to grid connected distributed generation systems only, but one major shortcoming is the failure of the backup scheme to operate in a coordinated manner. The proposed scheme relies on the use of dual setting directional overcurrent relays that are capable of operating in both forward and reverse directions, with different settings, and with a low bandwidth communication to maintain proper protection coordination. The problem is formulated as a non-linear constrained programming problem where the relay settings are optimally determined to minimize the overall relay operating time for primary and backup operation. The scheme is tested on a modified version of the IEEE 30 bus test system equipped with distributed generation and with capability of operating in an islanded mode. The proposed scheme is compared against the conventional scheme and the results show that it is capable of overcoming the problem of infeasibility, experienced by the conventional method, without the need of installing fault current limiters. Furthermore, it achieves remarkable reduction in both the primary and backup relay operating times, which was reflected in a reduction in the total relay operating time.