1. Professor, Dept. of Electrical Engineering, Northeast Electric Power Univ., Jilin 132012, China.
2. Research Assistant, Dept. of Electrical Engineering, Northeast Electric Power Univ., Jilin 132012, China.
3. Research Assistant, Dept. of Electrical Engineering and Computer Science, Univ. of Tennessee, Knoxville, TN 37996.
4. Assistant Professor, Dept. of Electrical Engineering, Northeast Electric Power Univ., Jilin 132012, China (corresponding author).
5. Professor, School of Electrical Engineering and Automation, Tianjin Univ., Tianjin 300072, China.
6. Assistant Professor, Dept. of Electrical Engineering, Northeast Electric Power Univ., Jilin 132012, China.