VOLTAGE INSTABILITY OF POWER SYSTEMS

Abstract

The research embodied in this Ph.D. project is principally involved with the study of voltage stability estimation and reinforcement for electric power systems. This thesis presents the main outcomes of the author's work in the project. A thorough literature survey has been performed in the field of voltage stability, including mechanism of voltage instability, criteria of voltage stability assessment, effective voltage and reactive power controls, and newly developed technologies in relevant areas. The main research of the project includes the following four aspects: (1) proposing a criterion for a reliable and informative voltage stability assessment; (2) presenting an optimum multi-objective SVC planning scheme; (3) designing an adaptive decentralized fuzzy logic controller for load voltage improvement during tap changing; ( 4) developing a centralized tap changing scheme to enhance voltage stability. The project lays emphasis on a comprehensive solution for voltage stability improvement of transmission networks, containing operating state estimation, var/ voltage control, and consolidation of power systems. As a pre-requisite of the solution, the criterion of voltage stability is proposed to provide guidelines for the project work. Static VAR compensators and load tap changing transformers are two of the most important tools used for voltage stability enhancement of transmission systems. The planning and control of these two devices, therefore, constitute the main body of the project. Since voltage stability is essentially load stability, the characteristics of power system loads have been taken into account in the planning and control algorithms developed by the project. Power system load modeling and parameter identification are also attempted in conjunction with the work listed above. Using techniques of genetic algorithm and Lagrange multiplier method, the developed hybrid algorithm evaluates the worst-case reactive margin of a power system and identifies the busbars most vulnerable to voltage collapse. By doing this, the estimation provides information not only about the global stability margin but also about the local voltage weakness. For an optimal SVC placement, the multi-objective optimization scheme is developed by employing parallel simulated annealing and non-linear programming techniques. A fuzzy performance index representing system reactive margins, real power losses, and voltage depressions at critical points is maximized by the proposed scheme aiming for voltage stability enhancement. In the aspect of load tap-changing transformers, the research of the project contains two parts. The emphasis has been laid respectively on the interaction between LTC transformers and dynamic load responses, and on the coordination of multiple tap changes. By applying the developed control methods, voltage stability of a power system undergoing tap changing is enhanced in terms of preventing reverse action, suppressing excessive voltage dynamics, and coordinating all participating tap changes.