Abstract
Nowadays when utilities are faced with increasing requirements regarding Quality of Service, it is important to be aware that severe contingencies can happen, in an unpredictable way which far exceeds all the security principles used for planning and system operation. In any electricity transmission system, a security condition must be assured - either in steady or in transient state - otherwise the essential service provided to costumers could be severely jeopardized. Present day Quality of Service requirements that are imposed on the electrical utility companies force them to develop efficient processes to analyze and react to incidents that can happen in an unpredictable way and far exceed all the security principles used. The simulation and analysis of these scenarios allow us to minimize the risk of system unbalance and voltage collapse. The steady state simulations of the Portuguese Transmission System were made in PSS/E software from Siemens Power Transmission Distribution, Inc., using savecases that represent pictures of the real system. The simulation results presented focus on aspects like energy not supplied, generation lost and topological changes resulting from an incident, which will affect mainly the power flow distribution within the grid elements that will remain in service (connected) and the voltage level in the system bus-bars. These results allow us to be better prepared to face these severe disturbances, defining procedures for emergency situation and training the control room operators.

Abstract
Performance indices are used to rank the impact of contingencies in power system. security studies. In this paper it is proposed a new approach to the use of performance indices to contingencies screening. The developed methodology produces a screening and classification of the system outages. The developed technique is applied to a test power system 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
This paper presents a study of the application of artificial neural network (ANN) to the evaluation of the distance to the voltage collapse point. Voltage stability has been of the major concern in power system operation. To prevent these problems, technical staff evaluates frequently the distance of the operation state to the voltage collapse point. This distance normally is calculated with power flow equations. This classic technique is very slow for electric power systems with large dimension. In abnormal exploration situations it may introduce serious limitation in the voltage stability analysis process. So, the application of a fast and reliable evaluation technique is very important to diminish the evaluation time. This paper also presents the method FSQV (Full sum dQ/dV) for the detection of the collapse point.

Abstract
The paper presents a study of the convergence characteristics and an assessment of the performance of the algorithm used to solve power system state estimation problem. The algorithm is studied to determine and compare the convergence patterns using several measurement sets consisting of line flows, bus injections, voltage magnitudes and line currents. The algorithm was tested on different IEEE test systems. Results related to the test with IEEE 14 bus bar system are presented.

Abstract
Voltage stability has become a very importante issue of power systems analysis. This paper discusses some important aspects related to voltage stability indices in electric power systems. Some techniques previously studied in the literature are analyzed and a comparison of the performance of several indices is presented. The effectiveness of the analyzed methods are demonstrated through numerical studies in IEEE 14 busbar test system, using several different scenarios of load increase. © 2006 IEEE.

Abstract
The allocation of the system losses to suppliers and consumers is a challenging issue for the restructured electricity business. Meaningful loss allocation techniques have to be adopted to set up appropriate economic penalties or rewards. The allocation factors should depend on size, location, and time evolution of the resources connected to the system. In the presence of distributed generation, the variety of the power flows in distribution systems calls for adopting mechanisms able to discriminate among the contributions that increase or reduce the total losses. Some loss allocation techniques already developed in the literature have shown consistent behavior. However, their application requires computing a set of additional quantities with respect to those provided by the distribution system power flow solved with the backward/forward sweep approach. This paper presents a new circuit-based loss allocation technique, based on the decomposition of the branch currents, specifically developed for radial distribution systems with distributed generation. The proposed technique is simple and effective and is only based on the information provided by the network data and by the power flow solution. Examples of application are shown to confirm its effectiveness.