1. School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, 11428 Stockholm, Sweden
2. State Key Laboratory of Electrical Insulation and Power Equipment, Xi′an Jiaotong University, Xi′an 710049, China
3. Hitachi Energy Research, 72178 Västerås, Sweden
4. School of Engineering, University of Warwick, Coventry CV4 7AL, UK