1. State Key Laboratory of Reliability and Intelligence of Electrical Equipment, Hebei University of Technology, Tianjin 300130, China and Hebei Key Laboratory of Electromagnetic Field and Electrical Apparatus Reliability, Hebei University of Technology, Tianjin 300401, China.
2. Hebei University of Science and Technology, Shijiazhuang 050018, China.
3. Department of Electrical Engineering and Electronics, University of Liverpool, L69 3BX Liverpool, U.K.