Dissertation•
Analysis, Modeling and Control of Doubly-Fed Induction Generators for Wind Turbines
01 Jan 2005-
TL;DR: In this paper, the energy efficiency of a variable-speed wind turbine system using a doubly-fed induction generator is compared to other wind turbine generator systems, and the sensitivity of different current controllers with respect to grid disturbances is investigated.
Abstract: This thesis deals with the analysis, modeling, and control of the doubly-fed induction machine used as a wind turbine generator. The energy efficiency of wind turbine systems equipped with doubly-fed induction generators are compared to other wind turbine generator systems. Moreover, the current control of the doubly-fed induction generator is analyzed and finally the sensitivity of different current controllers with respect to grid disturbances are investigated.
The energy efficiency of a variable-speed wind turbine system using a doubly-fed induction generator is approximately as for a fixed-speed wind turbine equipped with an induction generator. In comparison to a direct-driven permanent-magnet synchronous generator there might be a small gain in the energy efficiency, depending on the average wind-speed at the site. For a variable-speed wind turbine with an induction generator equipped with a full-power inverter, the energy efficiency can be a few percentage units smaller than for a system with a doubly-fed induction generator.
The flux dynamics of the doubly-fed induction machine consist of two poorly damped poles which influence the current controller. These will cause oscillations, with a frequency close to the line frequency, in the flux and in the rotor currents. It has been found that by utilizing a suggested method combining feed-forward compensation and "active resistance," the low-frequency disturbances as well as the oscillations are suppressed better than the other methods evaluated.
The maximum value of the rotor voltage will increase with the size of a voltage dip. This means that it is necessary to design the inverter so it can handle a desired value of a voltage dip. For the investigated systems the maximum rotor voltage and current, due to a voltage dip, can be reduced if the doubly-fed induction machine is magnetized from the stator circuit instead of the rotor circuit. Further, it has been found that the choice of current control method is of greater importance if the bandwidth of the current control loop is low.
Citations
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TL;DR: In this article, 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 by limiting 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 rotor windings.
Abstract: 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.
879 citations
Cites background from "Analysis, Modeling and Control of D..."
...A large number of papers describe the modeling of DFIGs [11]–[14]....
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...A major drawback of variable-speed wind turbines, especially for turbines with DFIGs, is their operation during grid faults....
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...When it is not possible to keep DFIGs connected to the grid, large-scale introduction of DFIG turbines does not seem feasible....
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...where is an “active damping torque” [11]....
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...When is chosen to be , changes in the mechanical torque are damped with the same time constant as the bandwidth of the speed control loop [11]....
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TL;DR: In this paper, the authors proposed a wind speed estimation based sensorless maximum wind power tracking control for variable-speed wind turbine generators (WTGs), where the aerodynamic characteristics of the wind turbine were approximated by a Gaussian radial basis function network based nonlinear input-output mapping.
Abstract: This paper proposes a wind speed estimation based sensorless maximum wind power tracking control for variable-speed wind turbine generators (WTGs). A specific design of the proposed control algorithm for a wind turbine equipped with a doubly fed induction generator (DFIG) is presented. The aerodynamic characteristics of the wind turbine are approximated by a Gaussian radial basis function network based nonlinear input-output mapping. Based on this nonlinear mapping, the wind speed is estimated from the measured generator electrical output power while taking into account the power losses in the WTG and the dynamics of the WTG shaft system. The estimated wind speed is then used to determine the optimal DFIG rotor speed command for maximum wind power extraction. The DFIG speed controller is suitably designed to effectively damp the low-frequency torsional oscillations. The resulting WTG system delivers maximum electrical power to the grid with high efficiency and high reliability without mechanical anemometers. The validity of the proposed control algorithm is verified by simulation studies on a 3.6MW WTG system. In addition, the effectiveness of the proposed wind speed estimation algorithm is demonstrated by experimental studies on a small emulational WTG system.
347 citations
TL;DR: This paper presents a DFIG control strategy that enhances the standard speed and reactive power control with controllers that can compensate for the problems caused by an unbalanced grid by balancing the stator currents and eliminating torque and reactivePower pulsations.
Abstract: Wind energy is often installed in rural, remote areas characterized by weak, unbalanced power transmission grids. In induction wind generators, unbalanced three-phase stator voltages cause a number of problems, such as overcurrent, unbalanced currents, reactive power pulsations, and stress on the mechanical components from torque pulsations. Therefore, beyond a certain amount of unbalance, induction wind generators are switched out of the network. This can further weaken the grid. In doubly fed induction generators (DFIGs), control of the rotor currents allows for adjustable speed operation and reactive power control. This paper presents a DFIG control strategy that enhances the standard speed and reactive power control with controllers that can compensate for the problems caused by an unbalanced grid by balancing the stator currents and eliminating torque and reactive power pulsations
333 citations
Cites background from "Analysis, Modeling and Control of D..."
...Fast inner current loops have a fast response time, but also tend to push the poorly damped system poles toward the right halfplane [5]....
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...This orientation can be called "grid flux oriented" control [5]....
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TL;DR: In this paper, the authors developed dynamic models in MATLAB/Simulink, validated it through experiments, and investigated the behavior of doubly fed induction generator during unbalanced grid voltage condition, and proposed new controllers in separated positive and negative sequence.
Abstract: Doubly fed induction generator (DFIG) still shares a large part in today's wind power market. It provides the benefits of variable speed operation cost-effectively, and can control its active and reactive power independently. Crowbar protection is often adopted to protect the rotor-side voltage source converter (VSC) from transient overcurrent during grid voltage dip. But under unbalanced grid voltage condition, the severe problems are not the transient overcurrent, but the electric torque pulsation and dc voltage ripple in the back-to-back VSCs. This paper develops dynamic models in MATLAB/Simulink, validates it through experiments, investigates the behavior of DFIG during unbalanced grid voltage condition, and proposes new controllers in separated positive and negative sequence. Methods to separate positive and negative sequence components in real time are also developed, and their responses to unsymmetrical voltage dip are compared. Simulation results prove that the separated positive and negative sequence controllers limit the torque pulsation and dc voltage ripple effectively.
280 citations
Cites background from "Analysis, Modeling and Control of D..."
...Many studies have been conducted on the LVRT capability of DFIG [3]–[6], [9], with most of them focused on the behavior and protection of DFIG under symmetrical fault....
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TL;DR: In this paper, a unified doubly fed induction generation (DFIG) wind turbine architecture which employs a parallel grid side rectifier and series grid side converter is presented, which enables unencumbered power processing and robust voltage disturbance ride through.
Abstract: With steadily increasing wind turbine penetration, regulatory standards for grid interconnection have evolved to require that wind generation systems ride-through disturbances such as faults and support the grid during such events. Conventional modifications to the doubly fed induction generation (DFIG) architecture for providing ride-through result in compromised control of the turbine shaft and grid current during fault events. A DFIG architecture in which the grid side converter is connected in series as opposed to parallel with the grid connection has shown improved low voltage ride through but poor power processing capabilities. In this paper, a unified DFIG wind turbine architecture which employs a parallel grid side rectifier and series grid side converter is presented. The combination of these two converters enables unencumbered power processing and robust voltage disturbance ride through. A dynamic model and control structure for this architecture is developed. The operation of the system is illustrated using computer simulations.
273 citations
Cites background or methods from "Analysis, Modeling and Control of D..."
...34 VA pu each [13], for a total converter VA of approximately 0....
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...4 illustrates the power flow across the stator and rotor circuits in a classical DFIG configuration that uses bidirectional power converters in the rotor circuit [13]....
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...The use of a series connected grid side converter was first considered for the purposes of voltage sag ride briefly in [13], but it properties and...
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References
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TL;DR: Covers in a progressive fashion a number of analysis tools and design techniques directly applicable to nonlinear control problems in high performance systems (in aerospace, robotics and automotive areas).
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Abstract: From the Publisher:
Written by two of the pioneers in the field, this book contains a wealth of practical information unavailable anywhere else. The authors give a comprehensive presentation of the field of adaptive control, carefully bending theory and implementation to provide the reader with insight and understanding. Benefiting from the feedback of users who are familiar with the first edition, the material has been reorganized and rewritten, giving a more balanced and teachable presentation of fundamentals and applications.
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