Multimegawatt wind-turbine systems, often organized in a wind park, are the backbone of the power generation based on renewable-energy systems. This paper reviews the most-adopted wind-turbine systems, the adopted generators, the topologies of the converters, the generator control and grid connection issues, as well as their arrangement in wind parks.

Overview of Multi-MW Wind Turbines and Wind Parks

Doubly fed induction generators (DFIGs), often organized in wind parks, are the most important generators used for variable-speed wind energy generation. This paper reviews the control systems for the operation of DFIGs and brushless DFIGs in wind energy applications. Control systems for stand-alone operation, connection to balanced or unbalanced grids, sensorless control, and frequency support from DFIGs and low-voltage ride-through issues are discussed.

Overview of Control Systems for the Operation of DFIGs in Wind Energy Applications

This paper explores and compares the performance of alternative voltage control strategies applied to doubly fed induction generator (DFIG) Different combinations of reactive power control of rotor- and grid-side converters are investigated for voltage-control purposes Simulations are performed using detailed models built in DIgSILENT PowerFactory in order to illustrate the influence of controllers on transient stability and steady-state operation of the DFIG-based wind plant This paper also proposes appropriate control strategies for different sets of network operating conditions and topologies Operational limits, such as current margins and pulse-width modulation limits, are also taken into account

Reactive Power Control Strategies for DFIG-Based Plants

This paper presents an analysis of a model reference adaptive system (MRAS) observer for the sensorless control of a standalone doubly fed induction generator (DFIG). The analysis allows the formal design of the MRAS observer of given dynamics and further allows the prediction of rotor position estimation errors under parameter mismatch. The MRAS observer analysis is experimentally implemented for the vector control of a standalone DFIG feeding a load at constant voltage and frequency. Experimental results, including speed catching of an already spinning machine, are presented and extensively discussed. Although the method is validated for a standalone generator, the proposed MRAS observer can be extended to other applications of the doubly fed induction machine.

MRAS observer for sensorless control of standalone doubly fed induction generators

Condition monitoring and fault diagnosis are currently considered crucial means to increase the reliability and availability of wind turbines and, consequently, to reduce the wind energy cost. With similar goals, direct-drive wind turbines based on permanent magnet synchronous generators (PMSGs) with full-scale power converters are an emerging and promising technology. Numerous studies show that power converters are a significant contributor to the overall failure rate of modern wind turbines. In this context, open-circuit fault diagnosis in the two power converters of a PMSG drive for wind turbine applications is addressed in this paper. A diagnostic method is proposed for each power converter, allowing real-time detection and localization of multiple open-circuit faults. The proposed methods are suitable for integration into the drive controller and triggering remedial actions. In order to prove the reliability and effectiveness of the proposed fault diagnostic methods, several simulation and experimental results are presented.

Open-Circuit Fault Diagnosis in PMSG Drives for Wind Turbine Applications

The high penetration level of wind power generation in an interconnected electrical network carries new technical challenges regarding system stability. Despite the stochastic nature of the wind, grid reliability has to be guaranteed. In compliance with power companies' new requirements on wind turbines as regards contribution to voltage regulation, this paper proposes a reactive power controller design for doubly fed induction generator drives. This drive topology enables the control of reactive power independently of the active power flow using both rotor and mains-side inverters. This degree of freedom is used in order to share the reactive powers circulating in the drive system and resulting from the reactive power required in the network. The controllers are designed using well-known linear control techniques in order to present the same dynamics. Furthermore, the proper reactive power share can be used to enhance the drive efficiency. Experimental results show the performance of the proposed reactive power control.

Reactive Power Control Design in Doubly Fed Induction Generators for Wind Turbines

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.

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

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.

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

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

Loss reduction methods for doubly-fed induction generator drives 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.

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Papers

Reactive Power Control Design in Doubly Fed Induction Generators for Wind Turbines

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

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

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

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