We would like to thank our coaches, Professor Mingguo Hong of Case Western Reserve University and Dr. Yonghong Chen of The Midwest Independent Transmission Systems Operator, the Interfaces special issue editor, R. John Milne, and manuscript editor, Alice B. Mack, for their constructive suggestions on earlier versions of the paper. Their input has significantly improved the readability of the paper. This research is partly supported by State Key Program of National Natural Science Foundation of China [Grant 71032004] and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China [Grant 71321001].