1. Xi'an Jiaotong University, Xi'an, China
2. State Key Laboratory of electrical Insulation and Power Equipment, Xi'an Jiaotong University, Xi'an, China
3. School of Electrical Engineering, Xi'an Jiaotong University, Xi'an, China
4. University of Cambridge, Cambridge, U.K.
5. Department of Engineering, Cambridge University, Cambridge, U.K.