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Numerical instabilities in power system transient simulations Araújo, António E. A.

Abstract

In power system simulations, control systems act upon the electric power system in a variety of ways. There is, therefore, a necessity of simulating the control system equations together with the power system equations. This is the case not only instability studies, but also in the simulation of electromagnetic transient phenomena. Because of the different structure of the two sets of equations, computational efficiency can be improved if both sets of equations are solved separately, with the result from one set of equations showing up in the other set of equations one or more time steps later. This procedure, however efficient, can lead to numerical instability. In the simulation of the control system itself, the modelling of nonlinearities in closed loops poses some problems as well. The most efficient way of handling a nonlinear block is to open the closed loop and use, instead of the current value of the variable, a predicted value of the feedback path. This can also lead to numerical instabilities. In this thesis a simultaneous solution of the control and power system equations is proposed. For the cases of nonlinearities in closed loops, an iterative solution is suggested. The implementation of the method is carried out in the Electromagnetic Transients Program (EMTP). Several comparisons between the proposed and other methods are performed. Incases in which the analytical solution is not available, the comparison is made against the results of the TAGS package ("Transient Analysis of Control Systems") which is integrated with the EMTP version of DCG/EPRI ("Development Coordination Group/Electric Power Research Institute"). It is shown through simple, but typical, case studies that the proposed method is not only accurate and efficient, but also numerically stable, thus representing a significant improvement to the ability of the EMTP to reliably simulate electromagnetic transients in power systems when control systems are involved.

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