1. State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi'an Jiaotong University,Xi'an,Shaanxi,China,710049
2. Virginia Tech,Department of Electrical and Computer Engineering,Blacksburg,VA,USA,24061
3. State Key Laboratory of Control and Simulation of Power Systems and Generation Equipment, Tsinghua University,Department of Electrical Engineering,Beijing,China,100084