1. State Key Lab of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China
2. State Grid Taizhou Power Supply Branch Company, Jiangsu province, Taizhou 225300, P. R. China
3. College of Electrical Information Engineering, North Minzu University, Yinchuan 750021, P. R. China