**Abstract**

The paper presents an alternative approach to voltage stability boundaries evaluation. Voltage stability boundaries evaluation is necessary for assessment of minimum security margin. Minimum security margin is a basic criterion applied by System Operators to provide corrective actions for maintaining power system security. The proposed method is direct but has advantages over conventional direct methods. It is based on optimization technique and makes it possible to evaluate the distance to power system voltage stability boundary. The described approach is based on the use of the Newton's method and sum of square roots of power flow equations. The paper demonstrates the concept of the approach based on simple power system models (PV-node – swing-bus, 2PV-node – swing-bus). The issue of the proposed method convergence is given special attention to in the article.

**Keywords**

Power systems stability, stability criteria, voltage stability, stability margin, emergency control.

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