1. College of Electrical Engineering and Automation, Shandong University of Science and Technology, Qingdao 266590, China
2. Department of Mathematics and Statistics, Curtin University, Perth, WA, Australia
3. Department of Mathematics, Bohai University, Jinzhou, Liaoning 121000, China
4. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China