1. State Key Laboratory of Power System; Department of Electrical Engineering; Tsinghua University; Beijing 100084 P. R. China
2. Department of Polymer Science and Engineering; School of Chemistry and Biological Engineering; University of Science and Technology Beijing; Beijing 100083 P. R. China
3. Key Laboratory of Engineering Dielectrics and Its Application; Harbin University of Science and Technology; Harbin 150080 P. R. China