Reliability-Driven SOP Siting and Sizing Under Multi-Line Failure Scenarios in Resource-Complementary Island Microgrids

Island microgrids face major challenges arising from the spatial mismatch between renewable generation and load centers as well as their heightened susceptibility to line failures. Traditional strategies such as full-line reinforcement are often impractical for islands due to the extremely high investment cost. This paper proposes a reliability-driven planning framework for microgrids under various failure scenarios, introducing Soft Open Points (SOPs) as flexible interconnection devices. Line failure rates are estimated based on component health indices and submarine cable lengths, and a scenario-based mixed-integer linear programming (MILP) model is developed to jointly optimize distributed energy resource scheduling and SOP siting and sizing. The proposed framework is validated on the Orkney Islands microgrid. Case studies demonstrate that optimal SOP deployment can reduce load shedding and renewable curtailment, improve voltage profiles, and lower total operational costs by over 50%, without the need for large-scale infrastructure upgrades. The results provide practical insights into enhancing the reliability and cost-effectiveness of island microgrids under reliability constraints.