1. School of Chemistry and Biological Engineering, University of Science & Technology Beijing, Beijing 100083, P. R. China
2. Key Laboratory of Engineering Dielectrics and Its Application, Harbin University of Science and Technology, Haerbin 150080, P. R. China
3. School of Electrical Engineering, Guangxi University, Nanning 530004, P. R. China
4. State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing 100084, P. R. China
5. Department of Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK
6. Shunde Graduate School of University of Science and Technology Beijing, Shunde 528399, P. R. China