With the increased loading and exploitation of the power transmission system, the problem of voltage stability and voltage collapse attracts more and more attention. A voltage collapse can take place in systems or subsystems, and can appear quite abruptly. Continuous monitoring of the system state is therefore required. This paper is concerned with the problem of voltage stability, and investigates a proposed voltage collapse proximity indicator applicable to the load points of a power system, based on the optimal impedance solution of a two-bus system. This indicator is generalised and applied to an actual system. The performance of this new indicator is investigated over both the stable and the unstable regions, as the load at a particular node or the system load increases. Tests show that the indicator can provide useful information at any operating point.

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