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This paper proposes a new voltage instability risk indicator based on local phasor measurements at fast sampling rate. The effectiveness of the indicator is analyzed at EHV load and ldquotransitrdquo buses. The risk criterion is based on the real-time computation of the Thevenin equivalent impedance of the classic electrical circuit given by an equivalent generator connected to an equivalent load impedance through an equivalent connecting impedance. The main contribution of the paper is the innovating algorithm utilized on the real-time adaptive identification of the Thevenin voltage and impedance equivalents. The algorithm performance is shown through a detailed sensitivity analysis. The paper presents important numerical results from the actual Italian EHV network.