Today's opération of large povver Systems is continuously increasing in complexity due to size growth, to high ievel of interconnections, to the use of new techniques, and to economical, political and ecological constraints. Thèse factors lead the operators to operate thèse Systems close to their limit of stability and security. The risk of instability is increasing in case of a strongïy growing power consumption. To avoid this phenomenon, the study of stability (voltage and rotor angle) is proposed in this thesis. This study is a very signtficant mean to détermine available transfer capability (ATC) in deregulated envïronment. The work of this thesis has been conducted vvith the use of the static analysis techniques and dynamic simulation vvith Matlab and Eurostag software. Thèse techniques are applied mainly in the Vietnam povver System {220 and 500 kV), the BPA test System 500 kV in USA and the Massif Central power System in France. In this work, several static analysis techniques are presented in order to estimate the stability margin and to identify the entical areas for the voltage stability. The results obtained are in conformity with the real exploitation of Vietnam povver system. Then, dynamic simulation allows studying ail the behaviors of the system for any opérations. With thèse simulations, the response of each variable of the éléments of the system is observed; the results of measurements and simulations are compared: the components most sensitive to the voltage stability are identified as well as the origins of instability (voltage or rotor angle). In the end, the eombination between the static analysis and dynamic simulation are carried out in order to take advantage from the two analyses. Under certain critical conditions, the povver system can be affected by a phenomenon of instability. This phenomenon can be avoided by applying préventive and corrective measures. In this thesis, three methods of préventive measures are presented: two classical methods and a new method. They are used to decrease the risks of voltage instability in the steady state before or after a disturbance, by using the optimal power flow techniques (OPF). A new method of corrective measure is also presented in this thesis, it is used to avoid the voltage instability by using the sensitivity of the active and reactive power to the voltage. The critical nodes, where the load must be shed, are determined by the static analysis. In the end, several solutions, to improve stability (voltage and rotor angle) and to avoid instability. are given by using modem reguîatory devices (FACTS, load compensation, PSS).
