1. Key Laboratory of Control of Power Transmission and Conversion (Shanghai Jiao Tong University), Ministry of Education; No. 800, Dongchuan Road Shanghai 200240 P.R. China
2. Electric Power Research Institute; Shanghai Municipal Electric Power Company; No. 171, Handan Road Hongkou District Shanghai 200437 P.R. China
3. Department of Electrical Engineering; Shanghai Jiao Tong University; No. 800, Dongchuan Road Shanghai 200240 P.R. China