1. Professor, College of Electrical Engineering and Automation, Tianjin Univ., No. 92, Weijin Rd., Nankai District, Tianjin 300072, P.R. China.
2. Ph.D. Student, College of Electrical Engineering and Automation, Tianjin Univ., No. 92, Weijin Rd., Nankai District, Tianjin 300072, P.R. China (corresponding author).
3. Lecturer, College of Electrical Engineering and Automation, Tianjin Univ., No. 92, Weijin Rd., Nankai District, Tianjin 300072, P.R. China.
4. Ph.D. Student, College of Electrical Engineering and Automation, Tianjin Univ., No. 92, Weijin Rd., Nankai District, Tianjin 300072, P.R. China.