This paper presents a method for determining Thevenin’s Equivalent (TE) for a node in a power system using local PMU measurements at that node. Three consecutive voltage and current measurements are used to determine the TE. The proposed method recognizes and considers the phase angle drift caused by the slip frequency between the PMU sampling frequency and the power system frequency. The accuracy of the proposed method has been verified through application to the IEEE 30 bus test system as well as to a real system data measured at the terminals of a wind farm. The proposed method is going to be used to determine an online TE, which will be used to monitor the system capability to accommodate the wind power by updating a capability chart of the system at the node where the wind farm is connected.