Abstract
This paper analyses the security of an Electric Power System (EPS) using an efficient contingency filtering and ranking algorithm. The proposed technique is divided in three modules, with different complexity levels. The contingencies are ranked in accordance with severity indices that allow to accurately evaluate the impact of the overloads and voltage limit violations in the power network. The last module also includes indices that can detect voltage instability situations. The computer programs developed by the authors were applied to the IEEE 118 busbars test power network and the results obtained were analysed. Finally, some conclusions that provide a valuable contribution to the understanding of the power system security analysis are pointed out.

Abstract
Voltage stability is concerned with the ability of a power system to maintain acceptable voltages at all busbars in the system under normal conditions, and after being subjected to a disturbance. A system enters a state of voltage instability when a disturbance causes a progressive and uncontrollable decline in voltage. In this paper it is studied and analysed the influence of the series capacitor compensation in the dynamic voltage stability assessment of an Electric Power System. It is used the BPA test power network considering different load models. A transmission line outage was simulated and different values of the series capacitor compensation were used to avoid voltage collapse. The automatic voltage regulators of the generating units and the turbine speed governors were modelled. The results were obtained using the software package EUROSTAG. Finally, some conclusions that provide a better understanding of the mechanism of the voltage collapse phenomenon that highlight the importance of the dynamic series compensation are pointed out.

Abstract
In this paper it is proposed an efficient steady-state contingency classification using the Rough Set Theory. This approach allows the evaluation of the importance of particular attributes and the elimination of redundant attributes from the decision domain. Moreover, the construction of a minimal subset of independent attributes ensuring the same quality of classification as the original set. The elimination of redundant attributes can help in identification of strong non-redundant classification rules. The developed methodology produces a classification of the system operation in four possible states. The methodology developed by the authors was applied to a test power network and the results obtained were analysed. Finally, some conclusions that provide a valuable contribution to the understanding of the power system security analysis are pointed out.

Abstract
In this paper it is simulated the effect of the circuit breakers reclosure in the transient stability of an electric power system using a new hybrid approach. This efficient and robust formulation combines both a time domain integration scheme to compute the initial system trajectory with the equal area criterion to obtain the transient stability margins and the critical clearing times, In this hybrid approach it is used a stop criterion to provide automatic and early termination of the simulations as soon as post-fault (un)stability are detected. The early termination algorithm assess the transient stability of the multimachine power network based on the concept of the dot products from the potential energy boundary surface method. The computer programs developed by the authors were applied to a multimachine test power network. The results obtained with this new hybrid formulation were compared with the solutions produced by a time domain simulation program. Finally, some conclusions that provide a valuable contribution to the understanding of the auto-reclosing effect in the transient stability problem are pointed out.

Abstract
In this paper it is used a hybrid approach to study and analyse the tripping effect of one or more generators in the transient stability of a multimachine power system. This efficient and robust approach combines the advantages of the time domain integration schemes to compute the initial system trajectory with the equal area criterion. The transient stability assessment is obtained using stability indices. Consequently, the time domain simulation is only computed during a short integration period. This early termination procedure reduces drastically the computing time. The software package TRANsySTEM developed by the authors was applied to a multimachine test power network. The results obtained with this hybrid formulation were compared with the solutions produced by a time domain simulation program. Finally, some conclusions that provide a valuable contribution to the understanding of the generators tripping effect in the transient stability problem are pointed out.

Abstract
In this paper it is studied and analysed the influence of the selective load shedding in the dynamic voltage stability assessment of the BPA test power network. The outage of a 500 kV overhead transmission line was simulated. Different values of emergency load shedding were used to avoid voltage collapse. The automatic voltage regulators of the generating units and the turbine speed governors were modelled. Different load models were used and the under load tap changers were also taken into account. The simulation results were obtained using the commercial transient software package EUROSTAG, developed by Electricité de France and Tractebel Energy Engineering. Finally some conclusions that provide a better understanding of the voltage collapse phenomena are pointed out.