Abstract
This work presents a control strategy applied to a doubly fed induction machine (DFIM) for wind generation in a medium power system. In order to control the active power and reactive power exchange between the machine and the grid, the rotor is fed by a link of AC-DC-AC converters modeled as voltage and controlled current sources, respectively. The control of the rotor-side converter is realized by fuzzy controllers whose performances are compared with that of conventional Proportional-Integral controllers. The control of the grid-side converter is carried out by a control block based on the instantaneous power theory and it is controlled to maintain the DC link voltage constant and to improve the power factor of the system. Some design considerations of the control schemes are discussed. A multimachine model is used in digital simulations to assess the dynamic behavior of the power system control scheme proposed in this work.

Abstract
This paper proposes the utilization of water storage ability to improve wind park operational economic gains and to attenuate the active power output variations due to the intermittence of the wind-energy resource. An hourly-discretized optimization algorithm is proposed to identify the optimum daily operational strategy to be followed by the wind turbines and the hydro generation pumping equipments, provided that a wind-power forecasting is available. The stochastic characteristics of the wind power are exploited in the approach developed in order to identify an envelope of recommended operational conditions. Three operational conditions were analyzed and the obtained results are presented and discussed.

Abstract
Ambitious targets for renewable power production have been defined for the electric power systems in Europe. The accomplishment of these targets requires the increase in renewable energy production, namely from wind power generation. However, the intermittent nature of wind creates several problems to the power system operation and new approaches based on the combined use of wind power and energy storage technologies need to be developed. In this paper, the concept of the combined use of wind power production and hydro storage/production is exploited, through the development of an operational optimisation approach applied to a wind generator park with little water storage ability. The optimisation model defines the operational strategy to be followed for the hours ahead by a pump station and an hydraulic generator embedded in a wind/hydro pumping facility, using the Portuguese energy remuneration rules. The proposed methodology leads to considerable yearly profits for the wind generator production.

Abstract
In the usual operation, wind park production is strongly dependent on the instantaneously available wind power. When energy storage is accessible, the wind park operation can be improved, aiming to a better exploitation of the available wind power resource. The present work proposes an optimization approach to determinate the most probable range of the output production, to be used in the definition of production profiles that will help the participation of wind power in the market. Two production profiles strategies were analyzed and compared. The forecasted wind power is represented as a stochastic variable and real wind power Portuguese conditions have been used in the evaluation of the model.

Abstract
Voltage stability plays a very important role during the planning and design stages of an electric power network as well as during the system operation. In the last years in various countries worldwide, several power network collapses (blackouts) caused by voltage problems have been reported. This can be produced by a lack of sufficient reactive power reserve during heavy load or by the occurrence of severe contingencies. In this paper it is studied and analysed the influence of the load models in the dynamic voltage stability assessment of an electric power system. It was used the BPA test power network. A severe contingency situation was simulated to perform the study. The automatic voltage regulators of the generating units and the turbine speed governors were modelled as well as the transformer taps. 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.