1. Lecturer, State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power Univ., Beijing 102206, P.R. China (corresponding author).
2. Professor, State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power Univ., Beijing 102206, P.R. China.
3. Professor, School of Renewable Energy, North China Electric Power Univ., Beijing 102206, P.R. China.