1. State Key Laboratory of Power Transmission Equipment and System Security and New TechnologyCollege of Electrical EngineeringChongqing UniversityChongqing400030People's Republic of China
2. British Columbia Hydro, 9th floor, Four Bentall Center1055 Dunsmuir Street, P.O. Box 49260VancouverV7X 1V5BCCanada
3. Department of Electrical and Computer EngineeringIowa State UniversityAmes50011‐3060IAUSA