Transmission corridors between interconnected power systems or within countrywide transmission systems are limiting network performance or even market activities. We describe a state-of-the-art wide-area measurement system which is commercially available, and a new method to assess the voltage stability of a transmission corridor. Using only topological information, several lines can be grouped into a so-called virtual transmission corridor, whose parameters can be calculated from wide-area

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... from wide-area measurements. A simple computational procedure for stability analysis, makes it possible to provide approximate stability margins with an update rate of less than a second. Such stability margins can be the basis for on-line dynamic rating of lines. Business cases for such dynamic rating are also discussed.