Computation of Power Systems Minimum Voltage Stability Security Margins

This paper presents an accurate and efficient method to compute the minimum voltage stability security margin (VSSM) of electrical power systems. Many network planning and operation procedures set forth by Independent System Operators and followed by many utilities consider the computation of the VSSM for a predefined load increase direction. Usually, this direction corresponds to increasing all loads with constant power factor and the MW generated at generation buses by using one loading factor only. However, situations may occur where small variations in this direction, as for example, an unexpected load increase at any bus or set of buses, may result in smaller VSSM, thus leading the system to an insecure operating state. The goal of this paper is to propose a combination of a fast method to estimate the VSSM with an efficient method to calculate the appropriate direction of load increase for which the margin is minimum. This information, along with the usual VSSM, can provide system operators with relevant information regarding the system´s security. More complete information allow operators to take decisions like preventive control actions to move the system to a securer operating point. The procedure is divided into two main stages. The first one consists of estimating the maximum loading point (MLP) and consequently the VSSM, by using nonlinear programming techniques. The second stage consists of computing new directions of load increase. These new directions are based on eigenvector handling. Also, the use of a special load flow method with step size optimization results into a more efficient computation process. Simulation results for IEEE standard and realistic power systems are shown to evaluate the method.