1. Power & Energy Storage System Research Center, College of Mechanical and Electrical Engineering, Qingdao University, No. 308 Ningxia Road, Qingdao 266071, P. R. China
2. National Engineering Research Center for Intelligent Electrical Vehicle Power System (Qingdao), No. 308 Ningxia Road, Qingdao 266071, P. R. China
3. School of Energy and Power, Jiangsu University of Science and Technology, Zhenjiang 212003, P. R. China