Growing electrical demand and insufficient investment sources for suitable expanding of existing electrical networks have led to operating such systems near to their maximum transferring power capability. As the operating point approaches the maximum loading capacity of transmission systems, the voltage stability margin of power systems has been decreased. Therefore, in order to operate a power system with a sufficient security margin, it is necessary to online evaluate the voltage stability using suitable indices. By deploying phasor measurement units (PMUs) in the transmission network, the access to voltage and current phasors with high sampling rate is more feasible especially at the installation point of PMUs. Accordingly, several algorithms for estimating the Thevenin equivalent circuit of a power system seen from each busbar have been proposed to online evaluate the voltage stability by comparing the system Thevenin impedance and the local load impedance. Basic assumption for all of these algorithms is that the system-side of the measuring point is constant enough without high variations and the load-side is sufficiently variable. The major challenge in Thevenin equivalent circuit estimation methods is the lack of this assumption in reality at many times. In this thesis, while introducing some of the algorithms presented in previous works for estimating the Thevenin equivalent circuit of the power system, the practical aspects related to these algorithms, such as the effect of not fulfilling the basic assumption or the effect of changing the frequency of the power system on the results of estimating Thevenin equivalent circuit parameters, are examined. Based on this study, two 1- Voltage stability 2- Thevenin equivalent 3- Maximum transmission power 4- Data measured by the PMU
