**4. Conclusions**

The presented paper illustrated the influence of composite crossarm insulator core diameter, surface hydrophobicity, umbrella structure, arrangement, and insulation distance on the pollution flashover voltage of large diameter composite crossarm insulators through artificial pollution tests. In the range of core diameter from 100 mm to 450 mm, the pollution flashover voltage gradient decreases with the increase of core diameter and then increases with the increase of core diameter. The hydrophobicity of the surface can significantly increase the pollution flashover voltage, and the influence of the hydrophobicity of the surface should be considered in the external insulation configuration of the project. When the CF value is close, the umbrella structure has little effect on the pollution flashover voltage. Under the same test conditions, the pollution flashover voltage of horizontally arranged composite crossarm insulators is higher than that of vertical ones. The pollution flashover voltage of composite crossarm insulators has an approximately linear relationship with the insulation distance. The above factors need to be considered comprehensively in the structural design of composite crossarm insulators. The pollution flashover of composite crossarm insulators is a complex process that is influenced by multiple factors. Thus, tests are performed to study the pollution flashover voltage. In the future, methods of pollution flashover tests, arc development process, and trends of leakage current of composite crossarm insulators with a large diameter shall be further studied to assess the mechanism underlying their flashover performance.

**Author Contributions:** Conceptualization, J.N. and F.L.; methodology, J.N., X.W.; formal analysis, J.N.; investigation, F.H.; data curation, X.W.; writing—original draft preparation, J.N., X.W.; writing— review and editing, H.L.; visualization, J.N.; supervision, F.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Science & Technology Project of State Grid Corporation of China, gran<sup>t</sup> numbers GY71-17-034 and GCB17201700238-01.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors thank all members of UHVAC External Insulation Electrical Characteristics Laboratory of State Grid Corporation of China for their hard work to obtain the experimental data in this paper.

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