1. Graduate School at Shenzhen, Tsinghua University, Shenzhen; Guangdong 518055 China
2. School of Electrical and Mechanical Engineering, North China University of Technology; Beijing 100144 China
3. School of Science, Beijing University of Chemical Technology; Beijing 100029 China
4. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University; Beijing 102206 China