1. Maintenance & Test Center, Extra High Voltage Power Transmission Company, China Southern Power Grid Co Ltd, Guangzhou 510620, China
2. Joint Laboratory for Safe Operation of DC Transmission Equipment and Submarine Cables, China Southern Power Grid Co Ltd, Guangzhou 510620, China
3. Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, China
4. Wuhan Zhilian Xinneng Electric Power Technology Co Ltd, Wuhan 430223, China
5. School of Electrical Engineering and Automation, Wuhan University, Wuhan 430072, China