This paper analyzes the suitability of existing methods for long-term voltage stability assessment for real-time operation. An overview of the relevant methods is followed with a comparison that takes into account the accuracy, computational efficiency and characteristics when used for security assessment. The results enable an evaluation of the run time of each method with respect to the number of inputs. Furthermore, the results assist in identifying which of the methods is most suitable for realtime operation in future power system with production based on fluctuating energy sources.

This article studies the influence of PMU's accuracy in voltage stability assessment, considering the specific case of Thévenin equivalent based methods that include wide-area information in its calculations. The objective was achieved by producing a set of synthesized PMU measurements from a time domain simulation and using the Monte Carlo method to reflect the accuracy for the PMUs. This is given by the maximum value for the Total Vector Error defined in the IEEE standard C37.118. Those measurements allowed to estimate the distribution parameters (mean and standard deviation) of the studied voltage stability indices and grid transformation coefficients which have applications in voltage stability assessment. The obtained distributions have a direct impact in the number of samples needed for estimating system parameters and compromise between timescale and uncertainty in those estimations is shown.

The possibilities offered by the use of Phasor Measurement Units (PMU) in real-time monitoring provide interesting ways to ensure secure operation of power systems. This paper studies the specific case of voltage stability and the possible improvements to the Thevenin equivalent methods, which is applied generally with local measurements. This paper uses the PMU measurements to calculate the grid transformation coefficients to obtain wide-area information. This is achieved by studying the generator's electromotive force estimated using values in the coefficient transformation matrix. The improvements are tested in a small system and a through comparison with traditional Thevenin equivalent methods is carried out.

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