1. College of Electrical Engineering and Information Technology, Sichuan University, Chengdu 610065, China 2. State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710049, China 3. Department of Electronic and Electrical Engineering, University of Bath, Bath BA2 7AY, UK

This work was supported by National Natural Science Foundation of China (No. 51377111), Fundamental Research Funds for the Central Universities (No. YJ201654) and Open Research Subject of Key Laboratory of Sichuan Power Electronics Energy-saving Technology and Devices (No. szjj2017-052).