This EPRI-sponsored research project deals with the use of the transient stability margin concept as a tool for dynamic security assessment. In this concept, a direct method, namely the energy function method, is used to determine the transient stability of a multimachine power system. The work focused on two major approaches. The first was to develop a normalized transient energy margin profile of a power system operating at a given condition. This permitted the ranking of possible contingent disturbances according to their severity. The second was to predict, for a given operating condition and an initial disturbance, the additional disturbances that the power system could withstand before instability occurs. The work dealt with some basic questions about the transient behavior of a multimachine power system. Among them are several issues that are related to the region of stability, the modes of instability of the system (for a particular disturbance), and the components of the transient energy directly responsible for instability. A better understanding of the transient behavior of a multimachine power system has resulted from this work. The transient energy margin concept was then successfully applied to a 17-generator power network representing a reduced version of the network of themore » state of Iowa.« less