Abstract
Electric power network stability analysis requires that all system components are represented by their models. Usually, detailed data about system components such as generators, transformers and transmission lines are available. In this case accurate models can be derived from the network devices. However, corresponding data for individual loads are not always available, which makes the modelling of loads an important area of research studies. In this paper it is studied and analysed the effect of the composite load models in the dynamic voltage stability of an electric power network. The composite load models comprise residential, commercial and industrial loads. A severe contingency situation was simulated. The automatic voltage regulators of the generating units, the turbine speed governors, and the under load tap changers models were taken into account. The simulation results were obtained using the EUROSTAG software package. Finally, some conclusions that provide a better understanding of the dynamic voltage stability are pointed out.

Abstract
Actually, the electric energy has a high importance in all activities, namely, the economic activity and on its development. Sectors of activity like, communications, transports, illumination and many others industrial processes are sustained. With the increase of the global energy needs, and considering the Kyoto's protocol, the importance of green power as been increased. Based in that scenario, it's easy to understand the appearance of new low capacity producers, which should satisfy certain operation parameters to be connected to the network. Connections of small electric energy producers lead to some problems related with the voltage stability on transport and distribution systems, being crucial the satisfaction of several parameters with the objective to guarantee the electric system stability. In this article, the state of the art for the stability problems in power systems is made, being specially analyzed the Portuguese case. It's also analyzed lower producers response on voltage collapse and the parameters that they should satisfy in those situations.

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
This work presents a system to manage wind turbines maintenance through a predictive model. The system is based on specific maintenance software and hardware for data acquisition and also on algorithms for prediction based on time series. The maintenance software is called SMIT - (Terology Integrated Modular System). The acquisition system can be interconnected with professional, industrial and low cost acquisition systems. The communication channel is based on IP networks, using clock synchronization for data sampling. A time series prediction algorithm runs on low cost hardware, working as a monitoring trigger of the central system.

Abstract
Lightning caused around 26% of the total number of faults in the Portuguese Transmission system in the analysed period. Based on historical data, this study uses incident data from 2001 until 2008 (2157 faults in overhead lines) and lightning data from 2003 until 2008 (around 1.65 million cloud-to-ground flashes). The performance of the lightning monitoring system is analysed and the statistical analysis of the incidents caused by lightning is made. Therefore, several indicators are presented, such as average failure rate per 100km and the percentage of these faults from the total. In addition, month and hour fault distributions are shown and the overhead lines with the worst performance in terms of lightning are identified. The aim of this analysis is to characterize this type of faults in order to evaluate the impact caused by lightning on an operational level. It will be one of the many inputs of a risk assessment methodology. © 2009 IEEE.

Abstract
Voltage stability has become one of the most important issues in electrical power systems. The power system ability to maintain acceptable bus voltage is a very important characteristic of a network. This paper analyzes the performance of line stability indices. These indices were tested in IEEE 14 and IEEE 57 busbar test systems, with satisfactory results. Simulation results show that using line stability indices the most critical line and the weakest bus of the system can be correctly identified.