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The problem of defining and classifying power system stability has been addressed by several previous CIGRE and IEEE Task Force reports. These earlier efforts, however, do not completely reflect current industry needs, experiences and understanding. In particular, the definitions are not precise and the classifications do not encompass all practical instability scenarios.This report developed by a Task Force, set up jointly by the CIGRE Study Committee 38 and the IEEE Power System Dynamic Performance Committee, addresses the issue of stability definition and classification in power systems from a fundamental viewpoint and closely examines the practical ramifications. The report aims to define power system stability more precisely, provide a systematic basis for its classification, and discuss linkages to related issues such as power system reliability and security.

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This paper describes the development of a variable time step algorithm for the numerical simulation of power system phenomena spanning time periods from several seconds to several hours.The algorithm automatically reduces the time step of integration to capture fast transients and increases it when the system variables are varying slowly. The solution algorithm is based on a numerically stable, implicit, low order integration similar to the trapezoidal rule.Simulation results for two scenarios are presented. These scenarios are: a severe system load ramp leading to voltage collapse, and a six-hour operational cycle.

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