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For power system, Thevenin equivalent at a node can represent the aggregation of system's operation conditions, e.g. generator terminal voltages, load powers, and line impedances, seen from a bus. Estimation of Thevenin equivalent can be challenging given that active controls in power systems vary the system's states with time. Traditional Thevenin equivalent method using open-circuit and short-circuit tests at the terminal can not be used for online estimation. A novel Thevenin equivalent calculation based on varying system condition is analyzed. Recursive least square estimation technique is applied to estimate online Thevenin equivalent and track system voltage stability. Case studies under different system operation conditions in New England 39 (NE39) system using PSS/E are provided to illustrate the effectiveness of the proposed method. Index Terms-Thevenin equivalent, recursive least square estimation, on-line estimation, voltage stability critical point.

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