Each day the operation of electric power system is more complex. Maintain a correct supply of energy empirically is difficult and unreliable task, this work was realized to comply with more reliably this power supply. This dissertation presents the modeling of power flow to get different steady state. The load response to frequency deviation, the primary and the secondary frequency control were models. The models are extended to the evaluation of voltage stability, even considering the frequency influence. The evaluation is done in the state before and after the response of load-frequency control. In each case, the generators units absorb the variation (infinitesimal) in the load or in the generation (bus in analysis) in different ways: according to the proportionality factor in the secondary control, the speed regulations in the primary control or the proportionality of the swing bus generation in the proportional dispatch. According to the presented in this dissertation was obtained a state in permanent regime, in this state was evaluated the voltage stability in all bus of the system. The tested system had 6 and 39 bus.