1. State Key Laboratory of Reliability and Inteligence of Electrical Equipment School of Electrical Engineering, Hebei University of Technology, Tianjin 300130, China
2. Key Laboratory of Electromagnetic Field and Electrical Apparatus Reliability of Hebei Province, Hebei University of Technology, Tianjin 300130, China
3. State Grid Zhoukou Electric Power Supply Company, Zhoukou 477150, China