1. School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044, China
2. Key Lab. of Vehicular Multi-Energy Drive Systems (Beijing Jiaotong Univ.), Ministry of Education, Beijing 100044, China
3. Locomotive and Car Research Institute, China Academy of Railway Sciences Co., Ltd., Beijing 100081, China
4. Beijing Zongheng Electro-Mechanical Technology Co., Ltd., Beijing 100094, China