1. Hunan Provincial Key Laboratory of Wind Generator and Its Control, School of Electrical and Information Engineering, Hunan Institute of Engineering, No. 17, Shuyuan Road, Yuetang District, Xiangtan 411101, P. R. China
2. Shanghai Institute of Technical Physics, Chinese Academy of Sciences, No. 500, Yutian Road, Hongkou District, Shanghai 200083, P. R. China