**5. Conclusions**

In this paper, three di fferent types of polycyclic compounds were chosen to prepare the XLPE/polycyclic compounds composites. The e ffects of these polycyclic compounds on the tree structure and electrical treeing characteristics with DC-impulse voltage were studied at 30, 60, and 90 ◦C. Their trap distribution and carrier mobility behaviors were also investigated. The experimental results reveal that polycyclic compound A has grea<sup>t</sup> application prospects in HVDC cables. The following are our main conclusions:


**Author Contributions:** L.Z. and B.D. came up with the idea and designed the structure of the paper. L.Z. finished the experiments and wrote the manuscript. H.L. and K.H. analyzed the data and provided analytical theory for electrical breakdown.

**Acknowledgments:** This work is supported by the National Key Research and Development Program of China (Grant 2016YFB0900701); the Chinese National Natural Science Foundation under the Grant 51537008.

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