1. a Department of Electrical Engineering , North China Electric Power University , Baoding City , Hebei Province , 071003 , China
2. b State Grid Electric Power Research Institute , Wuhan City , Hubei Province , 430074 , China
3. c Jiangsu Electric Power Test & Research Institute , Nanjing City , Jiangsu Province , 211103 , China
4. d School of Electrical Engineering, Wuhan University , Wuhan City , Hubei Province , 430072 , China