**6. Conclusions and Future Work**

This paper proposed a coordinated frequency and SOC control method for implementing isolated microgrids having a high penetration of RESs with multiple diesel generators and BESSs. The stored energy of all BESSs can be managed with the hierarchical controllers in diesel generators. In addition, frequency can be controlled quickly and individual SOCs of BESSs can be managed at their reference values with the hierarchical controllers and SSCs in BESSs. From Case Study, we validated that the proposed method can give a chance to (1) lower the requirement for the capacity of batteries, (2) utilize BESSs at their highest e fficiency, (3) maintain the active power of BESSs for scheduled or dispatched values, and (4) operate the system robustly even with large fluctuation of RESs.

One of the future works is the magnitude of ac voltage and reactive power control. If e ffective voltage/reactive power control method is coordinated with our proposed method, more stable and efficient operation of islanded microgrids will be implemented.

**Author Contributions:** J.-W.C. organized the manuscript and wrote the manuscript; G.-S.L. conceived the main idea; H.-J.M. performed the simulations; M.B.G. gave some useful information for the manuscript; S.-I.M. provided the overall guidance for the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 20188550000410).

**Conflicts of Interest:** The authors declare no conflict of interest.
