1. School of Electrical and Information Engineering, Zhengzhou University, Zhengzhou, P. R. China
2. School of Electrical Engineering and Automation, Henan Institute of Technology, Xinxiang, P. R. China
3. State Key Laboratory of Intelligent Agricultural Power Equipment, Luoyang, P. R. China
4. School of Electronic and Information, Zhongyuan University of Technology, Zhengzhou, P. R. China
5. State Key Laboratory of Power System of Tractor, Luoyang, P. R. China