1. State Key Lab of Alternate Electrical Power System with Renewable Energy SourcesNorth China Electric Power University Beijing 102206 China
2. Department of Electrical EngineeringTsinghua University Beijing 100084 China
3. Power System DepartmentChina Electric Power Research Institute Beijing 100192 China
4. School of Electrical EngineeringShandong University Jinan 250061 China
5. Department of Electrical Engineering and Electronicsthe University of Liverpool Liverpool L69 3GJ UK