1. Department of Electrical and Computer Engineering, University of Alberta, Edmonton, AB, Canada
2. College of Automation Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, China
3. Department of Electrical Engineering, Tsinghua University, Beijing, China
4. E. ON Energy Research Center, Institute for Power Generation and Storage Systems, RWTH Aachen University, Aachen, Germany