**6. Conclusions**

This paper proposes an on-line pre-decision-making scheme, including security assessment and an emergency control strategy decision-making, for power systems with multi-infeed HVDCs. The security assessment method is based on the EMT-TS hybrid simulation, and can generate accurate

assessment results while maintaining the high computational e fficiency. The emergency control strategy decision-making method can make full use of HVDCs, pumped storages, and interruptible loads to maintain the security and stability of receiving-end systems. The case studies showed that the proposed scheme is reliable. In addition, the results also indicate that it is essential to describe interactions between AC and DC subsystems in security assessment to identify HVDC-related security and stability issues.

In future work, the dynamic average-value modeling method can be used in HVDC modeling to further improve the computational e fficiency. In addition, more attention will be paid to the detailed design of the emergency control strategy, such as the coordination of HVDC controllers and the classification of interruptible loads.

**Author Contributions:** All authors contributed to the research in the paper. Conceptualization, Y.W. and Y.X.; Data curation, Z.S.; Formal analysis, Q.Z. and Z.S.; Funding acquisition, J.H.; Investigation, Q.Z.; Methodology, Q.Z. and Z.S.; Project administration, Y.W., J.H. and Y.X.; Resources, M.L.; Software, Q.Z. and Z.S.; Supervision, J.H., Y.X. and M.L.; Validation, Y.W.; Visualization, M.L.; Writing—original draft, Q.Z. and Z.S.; Writing—review & editing, Y.W. and Y.X. and all the authors have read and approved the final manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the Fundamental Research Funds for the Central Universities, Grant No. 2018YJS155, and the National Key R&D Program of China, Grant No. 2017YFB0902600.

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