This IMA Volume in Mathematics and its Applications SYSTEMS AND CONTROL THEORY FOR POWER SYSTEMS is based on the proceedings of a workshop that was an integral part of the 1992-93 IMA program on Control Theory. We thank Joe H. Chow, Petar V. Kokotovic, and Robert J. Thomas for organizing the workshop and editing the proceedings. We also take this opportunity to thank the National Science Foundation and the Army Research Office, whose financial support made the workshop possible. A vner Friedman Willard Miller, Jr. Xl PREFACE Power systems are rich in control and mathematical problems. The presentations given at the Control and Systems Theory in Power Systems Workshop held at IMA in March, 1993, clearly supported that claim. In this volume, we have collected 17 papers from the workshop. For papers with co-authors, the first author was the presenter. These papers deal with several topics of high current interest in power systems: modeling, stability, control, robustness, and computing. Power system modeling is contained in several papers. Sauer's paper presents a time-scale analysis of load models using transient algebraic cir- cuits. Ahmed-Zaid applies the same time-scale method to obtain reduced models of synchrous and induction machines. Chow's paper contains recent algorithms for identifying slow coherent groups of machines and ag- gregating the coherent machines. Vittal's paper develops an uncertainty model for analyzing system stability with respect to variations in loads and power transfer.