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This paper reveals that the impedance match (or the Thevenin circuit) based voltage stability monitoring techniques have problems to predict voltage stability limits when applied to multi-load power systems. Power system loads are nonlinear and dynamic. They cannot be simply represented as Thevenin circuit parameters for impedance match analysis. To overcome these difficulties, a new concept called "coupled single-port circuit" is proposed in this paper. The concept decouples a meshed network into individual single generator versus single bus network and, as a result, a modified version of the impedance match theorem can be used. This leads to a real-time voltage stability monitoring scheme without the need to estimate Thevenin parameters. The scheme can estimate voltage stability margin and identify weak areas in a system based on the SCADA and PMU data. Case studies conducted on several test systems have verified the validity of the proposed method.

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