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
Wind energy is the renewable energy source with a higher growth rate in the last decades. The huge proliferation of wind farms across the world has arisen as an alternative to the traditional power generation and also as a result of economic issues which necessitate monitoring systems in order to optimize availability and profits. Tools to detect the onset of mechanical and electrical faults in wind turbines at a sufficiently early stage are very important for maintenance actions to be well planned, because these actions can reduce the outage time and can prevent bigger faults that may lead to machine stoppage. The set of measurements obtained from the wind turbines are enormous and as such the use of neural networks may be beneficial in understanding if there is any important information that may help the prevention of big failures. © 2011 IEEE.

Abstract
This article covers a rather innovative and powerful technology that enables the creation of a 3D interactive laboratory for WEB applications based on open standard VRML format for 3D multimedia and the subsequent distribution of this virtual laboratory on the Internet. The project develops an environment, called "interactive technology of a 3D Internet Laboratory" to create various types of 3D laboratory experiments as well as their application via the Internet. The application produced is the creation of a 3D virtual laboratory for measurements and instrumentation. The description of the laboratory, fonts and data are also provided. The various steps for creating a 3D virtual laboratory are presented. Lastly, the application of the "interactive technology laboratory 3D" is displayed and can be considered as a step forward in the development of innovative technologies for distance learning. © 2011 University of Maribor.

Abstract
Electrical utilities are confronted daily with unpredictable events in their grids, which may lead to severe security level repercussions in the system, far exceeding all the security principles used for operation and consequently jeopardizing the essential service to the consumers. Incidents are unpredictable disturbances and recent experiences prove that severe contingencies happen. Given these facts, the need to evaluate harsher contingencies arose and such analysis must be rigorous and exhaustive. The security principles used for planning and system operation determine that, given an incident in the Electrical Power System (EPS) which involves the breakdown of any grid element or the simultaneous failure of a double circuit overhead line or the failure of the largest generator in service the supply interruption shall never take place (excluding single-feeding points without alternative) or permanent overloads. To analyse the steady-state security of an EPS it is required too, accurately and efficiently identify the critical contingencies set, i.e., those that when occur may endanger the system's security. The steady-state simulations of the Portuguese Transmission System (PTS) were made in PSS/E software from Siemens PTI using snapshots that represent pictures of the real system. All these security studies produce a large amount of data and information. Recently, the Rough Sets Theory (RST) has been used successfully to handle efficiently problems where large amounts of data are produced. RST constitute a framework for inducing minimal decision rules. These rules in turn can be used to perform a classification task. The main goal of the rough set analysis is to search large databases for meaningful decision rules and finally acquire new knowledge. This approach is based on four main topics: Indiscernibility, Approximation, Reducts and Decision rules. A reduct is a minimal set of attributes from the whole attributes set that preserves the partitioning of the finite set of objects and therefore the original classes. It means that if the redundant attributes are eliminated the reducts are found. Decision rules extracted knowledge, can be used when classifying new objects not in the original information system. In this paper it is proposed an efficient study and contingency analysis in the PTS using the RST. The developed methodology produces a system operation classification, distinguishing in four possible states: normal, alert, emergency I and emergency II. These different operating states correspond to a four levels of security. The four states can be classified horizontally as secure, in normal state and insecure for the remaining ones. The computer programs SecurMining2.0 developed, were applied to the Portuguese test power network.