Abstract
In this paper it was studied the performance evaluation of Doubly-Fed Induction Generator (DFIG) in the analysis of dynamic voltage stability during Low Voltage Ride Through (LVRT) in electric power systems. Two models of wind turbines were analyzed. In one of the models the wind turbines are equipped with pitch control coupled with a Fixed Speed Induction Generator (FSIG) and a shunt capacitor bank. In the other model the wind turbines are equipped with DFIG, crowbar and chopper. To protect the rotor side converter from tripping due to overcurrents in the rotor circuit or overvoltage in the DC link during voltage dips a crowbar was installed in DFIG. Nowadays the most efficient plants use technologies that allow them to stay connected during a fault and to produce again normally after the disturbance The model used can take into account these new technologies. The automatic voltage regulators of the generating units, and the turbine speed governors were modelled in detail. Different load models were used and the under load tap changers were also 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 of a system with FSIG and DFIG models during LVRT are pointed out. © VDE VERLAG GMBH.

Abstract
State estimation plays an important role in real time security monitoring and control of power systems. With increasing implementation of PMUs across the electric power grids and the ability of the Phasor Measurement Unit (PMU) to directly measure the system state, the use of these measurements to improve the precision of state estimator becomes imperative. In this paper, a new state estimator including voltage and current phasors and traditional measurements is proposed. The methodology is validated by simulation results on the 14 and 30 IEEE bus test systems. Several comparisons between the use of SCADA measurements and PMU measurements are exposed. Discussion concerning statistical robustness of the implemented state estimators is presented. © VDE VERLAG GMBH.

Abstract
In this paper it is proposed a technique to identify the relevant neighbouring electric power systems using the Horizontal Network methodology. The relevant network of the Spanish system was established, taking into account the actual interconnections and the new cross-border tie-line in Douro International with a voltage level of 400 kV. All simulations of the Portuguese and Spanish transmission systems were performed using the computational software package PSS®E from Siemens/PTI. The savecases used in this study represent pictures of the real systems. From the simulation results, some conclusions that provide a valuable contribution to the understanding of the impact of the new interconnection between the two countries will be pointed out. © VDE VERLAG GMBH.

Abstract
The huge proliferation of wind farms across the world has arisen as an alternative to the traditional power generation and as a result of economic issues which necessitate monitoring systems in order to optimize availability and profits too. Equipments inside a wind turbine are subject to failures which, most of times lead to long downtime periods. When wind turbine is not running due to a failure, no profits are added and operation and maintenance costs increases. The development of advanced techniques to detect the onset of mechanical and electrical faults in wind turbines at a sufficiently early stage is very important for maintenance actions. Neural networks can be used to detect failures in some equipments of wind turbines, but to use them is necessary to create a model to the equipment under surveillance. The training of the neural network represents the big handicap of the developed method that will be presented here. However, after solving this problem, results are very interesting, and failures can be detected with several months in advance. © 2012 IEEE.

Abstract
In the last years, the electric power systems have been restructured, moving from a vertical integrated model to a market oriented environment. The new paradigms associated with the restructuring of the electricity sector made it critical to develop new efficient and reliable methodologies to study and analyze the security of the power networks. In this paper it is proposed a method to evaluate the impact of external elements on the responsibility area of a Transmission System Operator (TSO) using the influence factor method. This approach offers a concrete support in the determination of the observability area, which at the end remains in the responsibility of the single TSO. The influence factor is a numerical value used to quantify the greatest effect of the outage of an external network component on any internal network branch. The developed methodology was applied to study the IEEE 118 bus test power network. All simulations of the transmission systems of the IEEE 118 bus were performed using the PSS®E software package from Siemens PTI, making the division of the network in two countries (areas). From the simulation results, some conclusions that provide a valuable contribution to understanding the influence factor method are pointed out. © 2012 IEEE.

Abstract
Voltage stability is predominantly a load stability phenomenon and solutions to voltage stability can be found by control of the load as seen from the bulk power network. A delay of the load restoration gives time for other corrective actions. The load restoration may be delayed and/or limited by certain countermeasures, such as blocking of Under Load Tap Changers (ULTC). In this paper it was studied the impact of the ULTC on the dynamic voltage collapse of an electric power system with large scale wind generation. It is used the Cigré Electric Power Network with 32 bus and three wind farms equipped with wind turbines, including pitch control coupled with a Fixed Speed Induction Generator (FSIG) and a shunt capacitor bank. The automatic voltage regulators (AVR) of the generating units and the turbine speed governors were modelled. Different load models were used and the ULTC were taken into account. Several significant disturbances were simulated in the test power network, such as the increase of the wind speed, the tripping of an overhead transmission line and three-phase short-circuits. The simulation results were obtained using the EUROSTAG software package. Finally, some conclusions that provide a better understanding of the ULTC effect on the dynamic voltage stability in a system with a large amount of wind power generation are pointed out. © 2012 IEEE.