Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Electrical and Computer Engineering


Recent efforts to apply direct methods of transient stability analysis to multi-machine power systems have used the so-called "energy functions." These functions describe the system transient energy causing the synchronous generators to depart from the initial equilibrium state, and the power network's ability to absorb this energy so that the synchronous machines may reach a new post-disturbance equilibrium state. In spite of the recent successes it has become increasingly evident that system separation depends not on the total system energy, but rather on the energy of the individual machines or groups of machines tending to separate from the rest. Thus, there is a need for generating transient energy functions for individual machines (or for groups of machines);Using a center of inertia frame of reference, the energy function V(,i) for machine i is derived. V(,i) is composed of kinetic energy and potential energy components. It is shown that the critical value of V(,i) is given by the maximum value of its potential energy component and that this value is fairly constant for any unstable post-fault trajectory. A special computer program has been written to compute the critical value of V(,i) for sustained fault conditions;A procedure for first swing transient stability assessment has been developed using the energy function of individual machines and groups of machines. The method has been tested extensively on two power networks: a 17-generator, 163-bus system which is a reduced version of the network of the state of Iowa; and the IEEE 20-generator, 118-bus system;A theoretical justification for using the critical energy of individual machines in stability assessment is provided using the concept of partial stability. Power system transient stability is analyzed as a partial stability problem with respect to the critical group of machines. It is shown that the transient energy function for the critical group of machines satisfies conditions for partial asymptotic stability.



Digital Repository @ Iowa State University,

Copyright Owner

Vijay Vittal



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78 pages