1. School of Electrical Engineering, Shandong University, Jinan, China
2. Dalian Power Supply Company of State Grid Liaoning Electric Power Company Ltd., Dalian, China
3. Key Laboratory of Electromagnetic Field and Electrical Apparatus Reliability of Hebei Province, Hebei University of Technology, Tianjin, China
4. Shandong Taikai Transformer Company Ltd., Taian, China