1. School of Electronic Science and Applied Physics Hefei University of Technology Hefei 230009 P. R. China
2. School of Electrical Engineering and Automation Hefei University of Technology Hefei 230009 P. R. China
3. Anhui Province Key Laboratory of Renewable Energy Utilization and Energy Saving Hefei University of Technology Hefei 230009 P. R. China
4. State Key Laboratory of Electrical Insulation and Power Equipment Xi'an Jiaotong University Xi'an 710049 P. R. China