Connection of Distributed Generation (DG) units to a distribution network will result in a local voltage increase. As there will be a maximum on the allowable voltage increase, this will limit the maximum allowable penetration level of DG. By reactive power compensation (by the DG unit itself) a significant increase in allowable penetration level can be achieved. In this paper this and several methods (overrating and generation curtailment) are investigated to achieve a further increase in allowable penetration level.@en

Turbine aerodynamics remains a challenging and crucial research area for wind energy. Blade aerodynamic forces responsible for power production must be augmented to maximize energy capture. At the same time, adverse aerodynamic loads that fatigue turbine components need to be mitigated to extend machine service life. Successful resolution of these conflicting demands and continued cost of energy reduction require accurate blade aerodynamic models. This, in turn, depends on clear physical understanding and reliable numerical modeling of rotational augmentation and dynamic stall, the two phenomena principally responsible for amplified turbine blade aerodynamic loads. The current work examines full-scale turbine blade aerodynamic measurements and current modeling methodologies to better understand the physical and numerical attributes that determine model performance@en

Improved and extended DG capability in voltage regulation by reactive and active power Triggianese, M.; Morren, J.; de Haan, S.W.H.; Marino, P.; Power Engineering, Energy and Electrical Drives, 2007. POWERENG 2007. International Conference on 12-14 April 2007 Page(s):583 - 588 Abstract: Distributed Generation (DG) units are fluctuating power sources that may cause a fluctuating voltage at the point of common coupling (PCC) with the grid. Their connection to distribution networks makes the voltage regulation important since the voltage fluctuations can generate flickers problems. Wind Generation is considered as example of DG. The power electronic converters that connect wind turbines to the grid can contribute to voltage regulation. Reactive power compensation is mostly adopted. Moreover since distribution networks are much more resistive than transmission ones, the conventional technique of voltage regulation by reactive power compensation shows its limits. Therefore new techniques are needed. In this contribution the active power curtailment is investigated and a new controller for voltage control by reactive power compensation is proposed. This controller concerns a new technique in the voltage control loop which improves the dynamical response of the voltage control. It is given by a combination of a feed-back and a feed-forward action.@en

With the request of renewable energy also the size of the wind turbines is growing. The converters used to connect them to the grid have to transfer a big amount of captured power. Since they handle high power, the switching devices work with a low switching frequency. This results in a high harmonic distortion of the injected currents and the need of big passive filters on ac side. In order to improve the Power Quality of the transferred wind energy, an inverter system made up of interlaced Active front-ends is proposed. Its mathematical model is also developed in order to analyze harmonic distortion. It can be introduced in the existing models of wind turbines and grid. The numerical results prove the benefits of the proposed system.@en

The increasing penetration of variable speed wind turbines in the electricity grid results in a reduction of the number of connected conventional power plants. This requires changes in the way the grid frequency is controlled. Together with the wind turbines devices like fuel cells can be installed, which can be used to supply the primary frequency control contribution of the wind turbines. In this paper it is analyzed how large the installed capacity of fuel cells should be to enable a correct response to frequency deviations. When fuel cells have stored hydrogen available, their response to changes in the power setpoint is fast. When the hydrogen has to be obtained from natural gas, or other fuels, their response is much slower. In that case the kinetic energy stored in the rotating mass of wind turbines can be used to support primary frequency control for a limited time period. The proposed control schemes are shown with the simulation of two case studies.@en

The increasing penetration of distributed generation (DG) in the electricity grid will result in a reduction of the number of connected conventional power plants, which are nowadays responsible for control of the electricity network frequency. Currently DG units do not contribute to frequency control. With increasing penetration of DG it will become necessary, however, that they also contribute to frequency control. A significant part of the DG units are connected to the grid by a power electronic converter. It is possible to implement additional control in this converter to let the DG unit contribute to frequency control. In this paper it is investigated how these controllers can be implemented and it is analysed how large the contribution of several types of DG units can be. Copyright © 2006 John Wiley & Sons, Ltd.@en

The increasing penetration of variable speed wind turbines in the electricity grid will result in a reduction of the number of connected conventional power plants. This will lead to a reduction of inertia in the grid, as the rotational speed of a variable speed turbine and the grid are decoupled by a power electronic converter. A lower system inertia will result in larger and faster frequency deviations after occurrence of abrupt variations in generation and load. It is possible to implement an additional control loop in the power electronic converter of the turbine, which connects the turbine inertia directly to the grid. It is even possible, by control, to make a `virtual inertia¿ that is larger than the real inertia. During a frequency drop additional power can be released to the grid with this control. This behaviour will be called inertial response. The additional power is obtained from the kinetic energy that is stored in the rotating mass of the turbine. In this contribution two different control strategies are investigated and compared with each other. A type of control that is equal to the primary frequency control of large power plants shows the best results, both with respect to the power that is needed to limit the frequency drop and with respect to electrical and mechanical stress of the turbine. Keywords: Frequency control; Inertia; Stability; Wind power generation@en

The increasing penetration of variable-speed wind turbines in the electricity grid will result in a reduction of the number of connected conventional power plants. This will require changes in the way the grid frequency is controlled. In this letter, a method is proposed to let variable-speed wind turbines emulate inertia and support primary frequency control. The required power is obtained from the kinetic energy stored in the rotating mass of the turbine blades.@en

Liquid pumpable food is mostly pasteurised by heat treatment. In the last decennia there is an increasing interest in so-called Pulsed Electric Field (PEF) treatment. During this treatment food is pumped between two metal electrodes and exposed to short high electric field pulses, typical 2¿4 kV·mm¿ 1 during 1¿10 ¿s. During this treatment the stainless steel electrodes are in direct contact with the food product. Associated with the charge that has to be exchanged at the interface of electrode and food, electrode material may be transferred from electrode to food. This transfer of material may cause serious electrode corrosion and adds metals to the food. The magnitude of material transfer depends on many factors such as current magnitude, pulse duration, pulse shape and fluid constitution. In this contribution the effect of three different mono-polar pulse shapes on treatment chamber corrosion is investigated. Experiments are carried out in an aqueous sodium chloride solution. The experiments showed that due to PEF treatment dissolved metals are present in the liquid. The four main elements of stainless steel, iron, chromium, nickel and manganese, have been considered in particular. These elements could only be detected in the liquid when it was repeatedly exposed to a Pulsed Electric Field treatment, which was realised by repeatedly circulating. From the experiments a relationship between dissolved metals in sodium chloride and the transferred charge is derived. With these values an upper limit for dissolved metals caused by PEF treatment can be defined.@en

In this paper, a solution is described that makes it possible for wind turbines using doubly-fed induction generators to stay connected to the grid during grid faults. The key of the solution is to limit the high current in the rotor in order to protect the converter and to provide a bypass for this current via a set of resistors that are connected to the rotor windings. With these resistors, it is possible to ride through grid faults without disconnecting the turbine from the grid. Because the generator and converter stay connected, the synchronism of operation remains established during and after the fault and normal operation can be continued immediately after the fault has been cleared. An additional feature is that reactive power can be supplied to the grid during long dips in order to facilitate voltage restoration. A control strategy has been developed that takes care of the transition back to normal operation. Without special control action, large transients would occur.@en