Balduino Rabelo

Modeling and Ride-Through Control of Doubly Fed Induction Generators During Symmetrical Voltage Sags

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.

Control of an optimized power flow in wind power plants with doubly-fed induction generators

The doubly-fed induction generator machine is one of the most widely used generator in the MW-class windmills producing a considerable amount of energy from wind. Its control through the rotor slip-rings with a back-to-back converter enables, together with the maximum active power flow, the distribution of the required reactive power between stator, rotor, and mains. This splitting can be chosen in order to minimise the system losses and avoid overloading the system components. Hence an optimised efficiency of high power windmills using this kind of generator brings a significant improvement on the yearly energy production.

Loss reduction methods for doubly-fed induction generator drives for wind turbines

This paper deals with control techniques for reduction of the losses in doubly-fed induction generator drives. This type of machine is one of the most used in wind power. The proposed method achieves its goal through controlling the decoupled active and reactive power flow on the machine and on the converters. Depending on the operating point and on the desired power factor optimal reactive currents are imposed and the electrical losses are reduced. The theoretical base as well as experimental results are presented

Reactive power control in doubly-fed induction generators for wind turbines

High penetration level of wind power generation in the interconnected electrical network and the increasing rated power of the wind energy converter units imposes new technical challenges to engineers. The first one is to guarantee grid stability despite the stochastic nature of wind while the second one is to limit the power losses since electrical machines efficiency do not augment proportionally to the rated power increase in the MW-class. Attending to the new requirements of the power companies on wind turbines concerning the contribution to network stability through reactive power exchange with the mains supply, this work proposes a reactive power control scheme for a doubly-fed induction generator drive that minimises the system power losses. This drive topology enables besides the control of active power the independent control of reactive power with both inverters. This degree of freedom is used in order to share the reactive currents resulting on the reactive power required on the network and increasing the efficiency. The steady-state power losses are modeled and the optimal reactive power share working points are found by an iterative procedure. The control structure design is presented and discussed as well as implementation issues. Experimental results are presented in order to prove the theoretical assumptions.

On the Emulation of an Isolated Wind Energy Conversion System: Experimental Results

This paper presents the emulation of an isolated wind energy conversion system, which is composed by a doubly-fed induction generator, a back-to-back converter connected to its rotor, a LC filter to minimize the harmonic pollution in the generated voltage and an isolated three-phase load. In first instance, the test bench is described and its operational capabilities are introduced. Afterwards, the control system design is presented. Next, some associated experimental results are shown as well. A special mention must be made to an experimental study which considers the possibility of using the self-excitation of the doubly fed induction generator to achieve the black-start of the isolated wind energy conversion system.

Behavior of doubly-fed induction generator during symmetrical voltage dips — Experimental results

Wind energy conversion systems using the doubly-fed induction generator (DFIG) are the most commercialized topology worldwide, because this technology uses converters dimensioned for ±30% of the system rated power, thus reducing the equipment cost. The direct connection of the DFIG stator to the electrical system increases its sensitivity to grid disturbances. In this context, this paper analyzes the DFIG behavior during balanced voltage sags using the machine classical model for the theoretical development and analyzing experimental results of a 2.2 kW test bench equipped with a crowbar device. The obtained results are useful in the development of strategies to improve the DFIG supportability during grid disturbances.

Doubly-fed Induction Generator Drives for Wind Power Plants

This chapter presents the theoretical basics of the electric power generation using doublyfed induction drives in wind turbines. This type of drive is the most utilized in wind power plants nowadays. This is mainly due to its fast dynamic response and reduced power electronics on the rotor side. This latter aspect is responsible for less harmonic pollution in the network and reduced acquisition costs. To begin with, the mathematical background required for comprehending the formulation used on electrical drives theory is repeated. Then the electromechanical energy conversion process in an electrical generator is explained, followed by a steady-state analysis of the induction machine. Subsequently, the doubly-fed induction generator and the network side circuit are introduced. Finally, active and power flow issues are discussed. The feasibility of reactive power production is in accordance with the new requirements from the power companies on wind energy converters and the degrees of freedom in its production allow for optimisation of the power losses.