**6. Conclusions**

The adaptive OC protection utilizing the LVRT characteristic of DGs and using circuit breaker status signals transmission by IEC 61850 communication standard were presented for the grid-connected and the islanded mode of the radial AC microgrid. Moreover, a new LVRT curve for the islanded mode of operation is proposed. The fixed delays for GOOSE message communication between IEDs are assumed

according to IEC 61850 and the practical values. According to the considered assumptions, the results look promising, as per the evaluation by PSCAD simulations. The general methodology presented in this paper for three-phase faults can also be extended to other types of faults. The e ffectiveness of the considered adaptive OC protection for single-phase and high impedance faults for AC microgrids with di fferent grounding schemes will be an important and interesting topic of the future study. Moreover, HIL simulations using actual IEDs and Ethernet-based IEC 61850 communication will also be performed for the practical implementation of the proposed methods.

**Author Contributions:** Conceptualization, A.A.M.; methodology, A.A.M.; software, A.A.M.; validation, A.A.M.; formal analysis, A.A.M.; investigation, A.A.M.; data curation, A.A.M.; writing—original draft preparation, A.A.M.; writing—review and editing, K.K.; visualization, A.A.M.; supervision, K.K.; project administration, K.K. and funding acquisition, K.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was carried out mainly in the Protect-DG research project with the financial support provided by the European Regional Development Fund (ERDF) through Business Finland with Grant No.4332/31/2014. Some parts of this work were done in the SolarX research project with the financial support provided by the Business Finland with Grant No. 6844/31/2018. The financial support provided through these research projects is highly acknowledged.

**Acknowledgments:** The corresponding author is very thankful to his colleague Sampo Voima for useful discussion and collaboration about the topic during the earlier stage work of the paper in the Protect-DG research project.

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