Most power systems are nowadays operated very near to their operating limits due to increase in consumption, while economic and environmental constraints have limited construction of new generation facilities and lines. This paper is devoted to the analysis of the ULTC effect in the dynamic voltage stability of an electric power network. Usually substations are provided with transformers with tap-changers facilities. It was long believed that transformers with tap-changers could eliminate or minimize effectively voltage instabilities. In this study it is shown the influence of the ULTC in the voltage instability for some situations in the analysed power network. It was used the BPA test power network. A severe contingency situation was simulated. The automatic voltage regulators of the generating units and the turbine speed governors will be modelled. The simulation results were obtained using the commercial transient software package EUROSTAG. Finally some conclusions that provide a better understanding of the voltage collapse phenomena are pointed out.
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