Abstract
In the last years the importance of the security analysis has been increasing, due to economical and environment constrains. The operation of an electrical power system is a complex task due to the high degree of uncertainty and the large number of parameters involved. This paper is devoted to the presentation of an efficient steady-state contingency classification using the Rough Set Theory. The developed technique produces a classification of the system operating in four possible states. The powerful methodology developed was applied to a test power network in four different scenarios. The numerical results were compared and analysed in order to obtained important conclusions that provide a valuable contribution to the understanding of the power system security problem.

Abstract
This paper presents 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. Performance index to detect and predict stability problems such as the voltage collapse have been a permanent concern for researchers and technical staff in power systems operation. This index can be used on-line or off-line to help operators determine how close the system is to a possible collapse and is a useful analysis tool for planning in power systems too. 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 or line outages. The methodology proposed may be easily implemented in any power flow program.

Abstract
This paper presents and analyses 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. This paper demonstrates that a point of a Newton-Raphson power flow becomes singular when the full sum of the partial derivate of the reactive power equations in relation to the voltage level is approximately constant. 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, line or generator outages. The methodology proposed may be easily implemented in any power flow program.

Abstract

Abstract
Most power systems are nowadays operated very near to their operating limits due to increase in consumption, while economic and environmental constraints have limited construction of new generation facilities and lines. This paper is devoted to the analysis of the ULTC effect in the dynamic voltage stability of an electric power network. Usually substations are provided with transformers with tap-changers facilities. It was long believed that transformers with tap-changers could eliminate or minimize effectively voltage instabilities. In this study it is shown the influence of the ULTC in the voltage instability for some situations in the analysed power network. It was used the BPA test power network. A severe contingency situation was simulated. The automatic voltage regulators of the generating units and the turbine speed governors will be modelled. The simulation results were obtained using the commercial transient software package EUROSTAG. Finally some conclusions that provide a better understanding of the voltage collapse phenomena are pointed out.

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.