1. College of Electrical Engineering and Automation, Hefei University of Technology, Hefei 230009, P. R. China
2. College of Automation, Chongqing University, Chongqing 400044, P. R. China
3. State Key Laboratory of Power Transmission Equipment and System Security and New Technology, College of Automation, Chongqing University, Chongqing 400044, P. R. China