Abstract
The electrical power production from dispersed energy sources or distributed generation is playing an increasing role in the supply of electricity in the liberalized electricity markets. Distributed generation can have a significant impact on the power flow, voltage profile, stability and the power quality for both customers and electricity suppliers. In this paper it is studied and analysed the influence of a wind farm in the dynamic voltage stability of a power network. A short-circuit in the middle of the transmission line and a significant load demand disturbance situation were simulated. The automatic voltage regulators of the generating units, the turbine speed governors and the double fed induction generator 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
This paper describes an integrated wind farm maintenance system, including software, hardware and some algorithms. The approach will imply the optimization of the maintenance planning among different wind towers owned by a company, considering the management of situations where the distances among them have a considerable weight in the maintenance costs. This reason, associated to the generators accessibility and the on-line maintenance variables acquisition, are very important to justify the necessity of improvement of the maintenance planning. This objective is achieved through the application of integrated maintenance management systems to which are associated new functionalities, in parallel with the usual ones in these systems that fit within the on-condition predictive maintenance, including on-line data acquisition modules. Usually, the manufactures construct, deploy and give the means to the suppliers to perform the wind system's maintenance. In some cases, the owners of wind farms can choose the maintenance company. This is a very competitive area, where companies hide the development details and implementations. Another important factor is that, by the first time in last ten years, the three countries with the largest installed power, together, have fallen to a minimum historic, below 40% of the market for wind power in the EU. Definitively, the other European countries encouraged the appearing of programs for installing wind power, with prominence for Italy, Portugal and Netherlands. Within this scenario, the development of maintenance management models for multiple wind equipments is important, and will allow countries to be more competitive in a growth market.

Abstract
All over the world, energy companies are investing in technologies to make better use of renewable energy to generate electric power. Wind energy is the renewable energy source that had a higher growing in the last decades and can be considered a hope in future based on clean and sustainable energy. Unlike conventional fuels, wind energy is a massive indigenous power source permanently available in virtually every nation in the world. Wind power delivers the energy security benefits of fuel costs, no long term fuel price risk, and avoids the economic and supply risks that can arise with reliance on imported fuels and political dependence on other countries.

Abstract
In this paper it is studied the influence of the transient stability performance indices on a contingency screening and ranking algorithm. The proposed methodology consists of three modules, with different complexity levels. Due to the large number of contingencies scenarios and calculations required, the application of a hybrid method to assess the system stability reduces drastically the computing time. The single dynamic security indices are obtained as a sub product of the hybrid method with a very reduce computing effort. The composed security indices are evaluated combining the single dynamic indices using weights. These indices are used to select, classify and rank the contingencies in accordance with the system security level. The developed methodology was applied to study the transient stability of the IEEE 17 machines test power system.

Abstract
The increasing competitiveness of the power systems' environment has created new challenges to the power system security assessment, demanding the development of new methodologies. Important aspects of this evolution are the unbundling of generation, transmission and distribution, caused by regulatory changes and creating a more competitive environment, the increased environmental concerns making it more difficult to build new assets, and the strong incentives to the generation from renewable energy sources. These factors contribute to the increasing stress on operating conditions and, consequently, cause more vulnerable networks. In recent years, the daily routine of transmission system operators has become more complex. The interconnected network is no longer just for mutual support; nowadays it has become the base platform for trading electricity allowing the shifting of increasing volumes of power across the system. Despite all the benefits that doubtless are many, from the operations perspective, the free electricity market presents more unpredictable and less stable power flows, a market-dependent generation pattern, also more unpredictable and less controllable. In addition to the latter, the significant amount of generation from renewable energy sources also contributes to place the system closer to its security limits. Therefore, and despite the bigger complexity and increased quality of service requirements, the energy supply and the optimization of resources remain a permanent goal, fulfilling all the security principles used for system operation. Incidents are naturally unpredictable and after the large disturbances which occurred in recent years (such as the blackouts on 9th May of 2000 in Portugal, on August 14th 2003 in the United States of America, on September 28th 2003 in Italy and in the European UCTE Network on November 4th 2006), the awareness of the risk of unpredictable disturbances has increased, and it has become essential for the decision-making process to have security indices that allow the operator to react in advance and to be aware of inherent risks, preventing harmful situations. The use of methodologies that take advantage of the actual real-time conditions in combination with past knowledge is extremely interesting in the system operation environment, because it gives a more accurate overview to the control room operators. Since long, the deterministic security assessment has been used to guarantee a high level security in system operations of Transmission Systems, being known as N-1 criterion. It tends to provide a conservative security region for protecting the system against severe contingencies. In the deterministic approach only the consequences of contingencies are assessed, but it does not consider likelihood, or better it does consider likelihood equal to one for all contingencies, therefore it cannot address increased or decreased uncertainty in operating conditions, such as adverse meteorological conditions or risk of forest fires. The advantages of probabilistic methods in comparison with deterministic ones are further developed in [1, 2]. Hence, the probabilistic assessment has become an important topic for researchers and engineers, because by combining consequences and probability it is possible to evaluate risk of contingencies, as in (1): Riskcontingency i = probabilitycontingency i × severitycontingencyi (1) Through the historical data, it is possible to affirm that most incidents originate from overhead lines (OHL). From 2001 until the end of 2008, OHL represent on average 91% of the incidents with origin in the Portuguese transmission system. Overhead lines, due to their dispersion over a wide geographic area and because of the many different ground characteristics where they are deployed, are more exposed to the action of external factors which cause most of the grid incidents. An accurate characterization of each incident's cause, an analysis of their occurrence and of the intrinsic characteristics of the Portuguese territory is presented in reference [3], including geographical and meteorological data, which are a crucial part of the inputs for the risk assessment methodology. This paper aims to describe the key issues of the calculation of the circuit-part of overhead lines contingencies probability, as part of the risk assessment methodology (figure 1), which is being developed and has as main purpose to support control room operators in their continuous task.

Abstract
In this paper it is applied a Rough Set approach that takes into account an incomplete information system to study the steady-state security of an electric power system. The Rough Set Theory has been conceived as a tool to conceptualize, organize and analyze various types of data, in particular, to deal with inexact, uncertain or vague knowledge. The knowledge acquisition process is a complex task, since the experts have difficulty to explain how to solve a specified problem. So, an incomplete set of relevant information may arise. The study presents a systematic approach to transform examples in a reduced set of rules. These rules can be used successfully to avoid security problems and provides a deeper insight into the influence of parameters on the steady-state system performance. ©2009 IEEE.