A method for local voltage instability assessment

Voltage instability is a nonlinear local phenomenon, which occurs due to insufficient reactive power production or as an inability to transfer it to consumers. In most dangerous cases, it can result in voltage collapse. Therefore early identification of the process is essential. A power system is decomposed into selected radial paths regarding reactive power flows. Next, two-bus equivalents for these radial transmission paths are determined. The analytical proof of maximal stable power transfer for a two-bus equivalent is available. However, each candidate critical load bus may be fed via more transmission paths. It is essential that a set of power sources, feeding the particular candidate critical reactive power drain bus, has sufficient reactive power reserve. Therefore, the proposed voltage collapse assessment procedure combines the voltage collapse proximity index and available reactive power reserve of the involved generators. The method´s properties and numerical results are presented.