**6. Conclusions**

A wavelet entropy-based protection method is proposed for symmetrical monopole MMC-HVDC overhead lines in this paper. Firstly, the fault characteristics of PPF, PGF, LD, LF, SMF, and AG-AC are analyzed in detail. Then, the definition of wavelet entropy is introduced and the wavelet entropy of different waveforms is analyzed. According to the different wavelet entropy values, the corresponding protection method is designed. Finally, theoretical analysis, simulation results generated by PSCAD/EMTDC, and discussions on the influence of different factors on the proposed protection method demonstrate that this method can not only accurately identify internal faults and external faults, but also effectively distinguish PPF, PGF, LD, and LF. At the same time, the method has strong adaptability and good noise tolerance in different fault locations and different fault ground resistors.

For systems with similar topologies, where same transient post-fault procedure can be produced, the proposed protection method can be used directly. However, for some other HVDC systems, such as asymmetrical monopole transmission system or bipolar asymmetrical transmission systems, the transient characteristics are different. The settings or criterions of the proposed wavelet-entropy-based protection method should be adjusted according to the spectrum distributions of different transients.

The research in this article is not to deny the application of traditional protection methods in MMC-HVDC, but to provide a new idea and perspective. Future work can focus more on improving the noise tolerance of the protection or introduce artificial intelligence methods to classify the characteristics of different faults.

**Author Contributions:** W.H. wrote the paper; G.L. developed the method; M.C. performed the experiments; J.H. provided some suggestions to improve this paper; Z.L. helped improve the expression of this paper; Y.Z. helped improve the performance of the algorithm. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key R&D Program of China, gran<sup>t</sup> number 2018YFB0904600.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.

**Acknowledgments:** This research is funded by the National Key R&D Program of China (No. 2018YFB0904600).

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