Abstract
In this paper it is presented a methodology to assess the transient stability of an electric power system using trajectory sensitivity analysis. This approach studies the variations of the system variables with respect to the small variations in initial conditions and parameters. Trajectory sensitivity functions of the post fault system with respect to relevant parameters are evaluated. The developed technique is combined with an accurate, flexible and efficient hybrid method that allows detailed modelling of the different devices of the power network, the simulation of distinct contingency scenarios, as well as the suitable identification of the different modes of instability. Moreover, it indicates unequivocally the set of the critical machines and allows to easily evaluate the transient stability margin and the critical clearing time.

Abstract
The increasing loads, the great distances between the production and consumption centers as well as the state of the transport and distribution networks turns the Electrical Power System operating near its stability limit. When the system gets instable a blackout situation may occur, which is critical to the system operator and consumers. We have recent examples in United States and Italy where the system was without power for several hours, causing serious problems in hospitals and public transportations. One of the causes of a blackout may be the Voltage Instability, which turns this subject widely discussed and studied. The speed of evolution of the Voltage Stability phenomenon allows us to analyze the problem as a static problem rather than a transitory one [1]. This article will show one way to "measure" locally the proximity of the system to the Voltage Stability Limit.

Abstract
Simple performance index for detecting and predicting stability problems such as the voltage collapse are useful analysis tools for planning and operating in power system. A new method for the detection of the point of collapse and a new index for the definition of the distance of the voltage collapse point are proposed. Numerical experiments using standard IEEE 14 and 57 bus systems show that the proposed method can compute the voltage collapse point using several different scenery of load increase. The methodology proposed may be easily implemented in any power flow program.

Abstract
In this paper it is proposed an efficient steady-state contingency classification using the Rough Set Theory. The developed methodology produces a classification of the system operating in four possible states: normal, alert, emergency I, and emergency II. The methodology developed by the authors was applied to a test power network and the results obtained were analyzed. 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 presented a three-step technique for on-line contingency screening, ranking and transient stability assessment based on a hybrid method. 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 computer programs developed by the authors were integrated in the software package TRANsySTEM. In order to validate the established mathematical models as well as the software package, the transient stability of the New England test power network was studied. The results obtained with the hybrid formulation were compared with the solutions produced by a time domain simulation program and show a very close agreement. Consequently, the proposed technique classifies correctly, without any dangerous diagnostic or false alarm, all the analysed situations. In addition, this approach provides the transient stability margins as well as the critical machines of the severe contingencies, therefore enabling further design of corrective security actions. Finally, some conclusions that provide a valuable contribution to the understanding of the transient stability assessment of a multimachine power system are pointed out.

Abstract
In this paper it is presented a combined on-line hybrid transient stability formulation with a parallel processing state estimator. The transient stability assessment uses a time domain integration scheme to compute the initial system trajectory and the equal area criteria to obtain the transient stability margins and the critical clearing times. The state estimator that provides the pre-fault operating conditions uses a parallel processing approach. In this technique the power system is split into K areas, and each subsystem is supervised by a local control centre. The measurements in each area are collected in the individual local control centre that has at least one computer system for data acquisition, data processing and computation. The computer programs developed by the authors were applied to a multimachine power network. The results obtained with the proposed formulation were compared with the solutions produced by other methods. Finally, some conclusions that provide a valuable contribution to the understanding of the multimachine power system preventive control are pointed out.