1. State Key Lab of Electrical Insulation and Power Equipment Center of Nanomaterials for Renewable Energy (CNRE) School of Electrical Engineering Xi'an Jiaotong University Xi'an 710049 P. R. China
2. Instrument Analysis Center Xi'an Jiaotong University Xi'an 710049 P. R. China
3. School of Automotive and Traffic Engineering Jiangsu University of Technology Changzhou 213001 P. R. China