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Mendes V.F.,Federal University of Itajuba | Mendes V.F.,Federal University of Minas Gerais | De Sousa C.V.,Federal University of Itajuba | De Sousa C.V.,Federal University of Minas Gerais | And 3 more authors.
IEEE Transactions on Energy Conversion | Year: 2011

Modern grid codes determine that wind generation plants must not be disconnected from the grid during some levels of voltage sags and contribute to network stabilization. Wind energy conversion systems equipped with the doubly fed induction generator (DFIG) are one of the most frequently used topologies, but they are sensitive to grid disturbances due to the stator direct connection to the grid. Therefore, many efforts have been done in the last few years in order to improve their low-voltage ride-through capability. This paper analyzes the behavior of the DFIG during symmetrical voltage sags using models in the frequency domain. A new strategy, the machine magnetizing current control, is proposed in order to enhance the system response during balanced dips. The method is derived on a theoretical basis and numerically investigated by means of simulation. Experimental results are presented and validate the proposed strategy. Finally, the practical aspects of the use of this strategy are discussed. © 2011 IEEE. Source


Starzmann R.,University of Siegen | Carolus T.H.,University of Siegen | Tease K.,Voith Hydro Wavegen Ltd | Arlitt R.,Voith Hydro Ocean Current Technologies
9th European Conference on Turbomachinery: Fluid Dynamics and Thermodynamics, ETC 2011 - Conference Proceedings | Year: 2011

In an oscillating water column (OWC) power plant the Wells turbine is operated along its complete characteristic curve from no load to overload and back. However, the 'design' point as a distinct point of operation may serve to assess the influence of primary design parameters on the turbine's overall performance more clearly. The design method employed here is based on an analytical free vortex and blade element theory. For a given load, hub solidity and hub-tip ratio the model yields a set of dimensionless performance coefficients at design point as well as the blade geometry. In a second step the performance of several turbine rotor designs are predicted by a 3D numerical RANS method. Selected rotors were manufactured and the measured and predicted performance compared. The results indicate how the turbine's performance characteristics are affected by the choice of hub-tip ratio and solidity. This will identify candidates for optimal turbine designs for a given OWC system. Source


Saltara F.,University of Sao Paulo | Neto A.D.,Voith Hydro Ocean Current Technologies | Lopez J.I.H.,Mackenzie Presbyterian University
International Journal of Offshore and Polar Engineering | Year: 2011

The 3D flow around a circular cylinder free to oscillate transversely to the free stream was simulated using ComputationalFluid Dynamics (CFD) and the Spalart-Allmaras Detached Eddy Simulation (DES) turbulence model for a Reynolds numberReD10 4. Simulations were carried out for a small mass-damping parameter m *ζ=0000858, where m *=3.3 and ζ=000026.We found good agreement between the numerical results and experimental data. The simulations predicted the high observedamplitudes of the upper branch of vortex-induced vibrations for low mass-damping parameters. © by The International Society of Offshore and Polar Engineers. Source


Mota D.D.S.,Voith Hydro Ocean Current Technologies | Goldemberg C.,University of Sao Paulo
IEEE Latin America Transactions | Year: 2010

This paper compares the behaviour of two different control structures of automatic voltage regulators of synchronous machines equipped with static excitation systems. These systems have a fully controlled thyristor bridge that supplies DC current to the rotor winding. The rectifier bridge is fed by the stator terminals through a step-down transformer. The first control structure, named "Direct Control", has a single proportional-integral (PI) regulator that compares stator voltage setpoint with measured voltage and acts directly on the thyristor bridges firing angle. This control structure is usually employed in commercial excitation systems for hydrogenerators. The second structure, named "Cascade Control", was inspired on control loops of commercial DC motor drives. Such drives employ two PIs in a cascade arrangement, the external PI deals with the motor speed while the internal one regulates the armature current. In the adaptation proposed, the external PI compares setpoint with the actual stator voltage and produces the setpoint to the internal PI-loop which controls the field current. © 2005 IEEE. Source


Main directions and velocity distributions over time are essential for evaluation of economic efficiency of tidal current turbines. In addition, numerical analyses help finding suitable spots and can identify and locate eddies or unfavorable flow patterns. This can support the preselection of measurement campaigns. For this, an in-house code is utilized. A description about the preprocessing methods, the simulation and the data assessment of a high resolution ocean modeling case is given. Source

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