1. Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, 52 Xuefu Road, Harbin, Heilongjiang 150080, P. R. China
2. School of Electrical and Electronic Engineering, Harbin University of Science and Technology, 52 Xuefu Road, Harbin, Heilongjiang 150080, P. R. China
3. Department of Electrical Engineering and Applied Electronics, Tsinghua University, 30 Shuangqing Road, Haidian District, Beijing 100084, P. R. China