Optimal Protection Coordination for Microgrids Considering N $-$1 Contingency

Usually, protection coordination problems are solved under the assumption that the network topology is fixed. Yet, in practice, any power system can encounter changes in the network topology due to transient events. These transient events can be in the form of line or generation source outage. Furthermore, in the presence of distributed generation, the network topology can change depending on whether the system is operating in the grid-connected or islanded mode. Thus, it is essential to consider all possible network topologies while designing a protection scheme for distribution systems with distributed generation (DG). In this paper, the protection coordination problem is solved to determine the optimal relay settings considering N$-$ 1 contingency, which can result from a single line, DG unit, or substation outage. In addition, the relays are designed taking into account both grid-connected and islanded operation modes. The problem has been formulated as a mixed integer nonlinear programming problem including coordination constraints corresponding to the various possible outages. The proposed approach is tested on a 9-bus radial distribution system and on the IEEE 14-bus meshed distribution system, both equipped with DG units.