Doubly Fed Induction Generator Rotor Research Articles

Impact of PAJ with varying POW in voltage sag on rotor over-voltage in DFIG-based wind generator

By the help of the vector analysis, this study studied the impact of voltage sag with different characteristics on the rotor over-voltage of doubly fed induction generator (DFIG) using the stator flux analysis. The influence principle of phase angle jump (PAJ) with varying point on wave (POW) on DFIG is presented. Vector analysis method is used to relate voltage sag features, stator flux, and rotor voltage for symmetrical and unsymmetrical voltage sags. Considering the practical constraints of PAJ, the general variation rules of DFIG rotor over-voltage caused by PAJ with varying POW in voltages sag are validated by simulation.

Short Circuit Current Calculation Method of Doubly-Fed Induction Generator with Crowbar Protection Based on Flux Transient Characteristics

With the wide application of doubly-fed induction generator (DFIG), accurate calculation and analysis of the short circuit currents of DFIG with crowbar protection has become very important key to realizing the low voltage ride through (LVRT) capability of DFIG and establishing the corresponding grid protection scheme. In view of this, a method to calculate the short circuit currents of DFIG with crowbar protection when different types of fault occur in the grid is proposed in this paper. First, the DFIG stator and rotor 2nd-order differential equations in the cases of grid symmetrical fault and asymmetrical fault are established respectively. And the short circuit current calculation model when crowbar resistance remains unchanged in the fault duration is obtained. On this basis, the short circuit current calculation model when crowbar resistance varies in the fault duration is derived. And then, the influence of crowbar resistance and DFIG rotor speed on the short circuit current calculation model is analyzed, including the variation pattern of flux eigenvalues with the crowbar resistance, and the variation feature of flux amplitude and phase angle with the rotor speed. Besides, the transient variation characteristics of different flux components are studied. Finally, time-domain simulation tests verify that the proposed calculation method is correct and applicable to different types of crowbar circuits in the market. Besides, the influence of crowbar resistance and DFIG rotor speed on the stator short circuit current peak, peak time and current frequency in different fault cases is revealed.

Current-Based Fault Detection and Identification for Wind Turbine Drivetrain Gearboxes

This paper proposes a new fault detection and identification framework for drivetrain gearboxes of wind turbines equipped with doubly-fed induction generators (DFIGs) based on the fusion of DFIG stator and rotor current signals. First, the characteristic frequencies of gearbox faults in DFIG stator and rotor currents are analyzed. Different time- and frequency-domain features of gearbox faults in DFIG stator and rotor current signals are then defined, and the methods to extract these features are introduced. These features are used as the inputs of multiclass support vector machines with probabilistic outputs for fault mode identification. Different schemes that use a single stator or rotor current signal or both stator and rotor current signals for the feature- or decision-level information fusion are designed. Experimental results obtained from a DFIG wind turbine drivetrain test rig are provided to validate the proposed current-based fault detection and identification framework.

DC grid/bus tied DFIG based wind energy system

This paper presents a novel configuration of Doubly Fed Induction Generator (DFIG) based wind energy systems (WESs) to improve the reliability of power systems. In the proposed configuration, rotor and stator sides of DFIG are connected to Direct Current (DC) grid/bus through converter and 12-pulse diode bridge rectifier, respectively, while loads are connected with the Alternating Current (AC) bus. Furthermore, the rotor current amplitude and frequency have been utilized to regulate the voltage of stator AC bus. The RMS detection of load/stator voltage method is used to calculate the reference rotor currents. The control pulses for rotor side converter are supplied by the hysteresis current controller, operated on the error signal, calculated between actual and reference rotor currents. Also, a comparative study between the speed-sensorless control and speed-sensor control techniques is presented for the rotor side converter of the proposed configuration. Furthermore, three possible cases, namely, ‘step change in reference input voltage’, ‘step change in wind speed or wind gust’, and load switching are simulated in MATLAB® for2 MVA DFIG based WESs connected to DC grid/bus to evaluate the proficiency of the proposed configuration. Also, proposed configuration is tested on experimental set-up to reveal its efficacy in real environment.

A Proposal of Project of PI controller gains used on the Control of Doubly-Fed Induction Generators

This paper groups basic information about the control of doubly-fed induction generators (DFIG) for wind power generation. The DFIG stator is connected directly to the power grid and its rotor is connected by a double converter called back-to-back. To carry out the control of the generated power, are needed some calculations with complexity equivalent to the machine model, making it difficult to obtain controllers' initial parameters of a part of the system. In this sense, this paper has presented a proposal for the design of the gains of the proportional-integral controller (PI) used in the power vector control for the DFIG rotor drive side using the pole compensation technique. It has also presented the current mesh control of the converter connected to the main grid, the design of the DC link capacitance and the inductance of the harmonic filter. With the above proposals, satisfactory results were obtained both in simulation and in experiment, through an analysis of a simplified model of the machine model. The control derived from the simplified analysis of the model showed similar results compared to the complete and complex model of the machine proposed by other authors.

Enhanced Control of DFIG Wind Turbine Based on Stator Flux Decay Compensation

For the doubly fed induction generator (DFIG)-based wind energy conversion system, the decaying flux and negative flux are the main reasons to cause the DFIG rotor overcurrent, during grid faults. The stator decaying flux characteristics versus the depth and instant of the stator voltage variation are analyzed first. On the basis of the stator flux performances, the enhanced control strategy is proposed to limit the decaying flux approximately into zero in half fundamental period by changing the stator voltage drop and recovery mode, and consequently the rotor transient current pulse is significantly reduced during grid faults. The experimental results based on the 7.5 kW DFIG setup are carried to validate the correctness and feasibility of the proposed strategy. Dynamic voltage restorer (DVR) can be one of the applications. With the proposed strategy, the DVR only works in a half fundamental period and its output voltage amplitude is half of the stator voltage variation, during the grid voltage drop and recovery, respectively. As a consequence, the DVR can be rated for lower power saving cost. The simulation results based on MATLAB/Simulink using a 2 MW DFIG and the experimental results based on the 7.5 kW DFIG validate the effectiveness and feasibility of the DVR-based low-voltage ride through strategy.

Reactive power control of DFIG wind turbines for power oscillation damping under a wide range of operating conditions

This study analyses the effect of replacing existing synchronous generators (SGs) equipped with power system stabilisers (PSS) by doubly fed induction generator (DFIG) based wind farms on the damping of power system oscillations in a multi-machine power system. A power system stabiliser was designed to enhance the capability of DFIG to damp power systems oscillations. The validity and effectiveness of the proposed controller are demonstrated on the widely used New England 10-machine 39-bus test system that combines conventional SGs and DFIG based wind farms using eigenvalue analysis and non-linear simulation. The non-linear simulation is used to demonstrate how the damping contribution of DFIG based wind farms is affected by different operating conditions within the same wind farm and stochastic wind speed behaviour. The results show that installing conventional fixed parameters PSS within reactive power control loop of DFIG rotor side converter has a positive damping contribution for a wide range of operating conditions. Furthermore, the results clearly show that DFIG based wind farms equipped with the proposed farm level PSS can damp power system oscillations more effectively than SGs PSS.

Integrated Protection of DFIG-Based Wind Turbine With a Resistive-Type SFCL Under Symmetrical and Asymmetrical Faults

This paper presents a novel protection scheme for in-grid doubly fed induction generator (DFIG) by using a resistive-type superconducting fault current limiter (SFCL) connected in a series with the DFIG rotor winding. The basic principle, prototype design, and system modeling are presented to integrate the DFIG and SFCL modules into one electrical-superconducting-mixed device. The comprehensive evaluation of the proposed scheme and the comparative study with the conventional crowbar-based scheme are carried out. The results obtained show that the proposed scheme is favored to limit the peak values of the fault rotor current, dc-link voltage, and electromagnetic torque oscillations, and thus to protect the rotor-side converter, dc-link capacitor, and mechanical parts under symmetrical and asymmetrical faults.

Sensorless Nonlinear Control of Wind Energy Systems with Doubly Fed Induction Generator

The problem of controlling doubly fed induction generators (DFIG) associated with wind turbines is addressed. The control objective is twofold: maximum power point tracking and reactive power regulation in the DFIG. Unlike previous works, we seek the achievement of this control objective without resorting to physical sensors of mechanical variables (e.g., wind turbine velocity and DFIG rotor speed). Interestingly, wind velocity is also not assumed to be accessible to measurements. The control problem is dealt with using an output feedback controller designed on the basis of the nonlinear state-space representation of the controlled system. The controller is constituted of a high-gain nonlinear state observer and a nonlinear sliding state feedback mode. Using tools from Lyapunov’s stability, it is formally shown that the closed-loop control system, expressed in terms of the state estimation errors and the output-reference tracking errors, enjoys a semi-global practical stability. Accordingly, it is possible to tune the controller design parameters so that it meets its objectives with an arbitrarily high accuracy, whatever the initial conditions are. These theoretical results are confirmed by simulations involving wide range variation of the wind speed.

Synchronous mode operation of DFIG based wind turbines for improvement of power system inertia

Inertia emulation methods exist to compensate for the reduced inertial support provided by doubly fed induction generator (DFIG) based wind turbines. Instead of emulating inertia, this paper proposes to temporarily convert DFIGs to synchronous generators, enabling supply of real inertia to the system. In order to achieve this, the voltage supplied to the DFIG rotor needs to be made independent of the grid frequency. Feeding the rotor with a fixed dc voltage while it is rotating at synchronous speed enables the DFIG to operate in synchronism with the grid and couple the inertia of its rotating mass to the power system. The rotor side converter of a DFIG can be controlled to function as the dc voltage source, allowing convenient switching between the two operation modes according to system requirements.

Conceptual Design and Evaluation of a Resistive-Type SFCL for Efficient Fault Ride Through in a DFIG

To satisfy the fault ride through (FRT) requirements from both the grid side and the in-grid doubly fed induction generator (DFIG) itself simultaneously, we propose a novel FRT scheme by adopting a resistive-type superconducting fault current limiter (SFCL) connected in series with the DFIG rotor. In this paper, the conceptual design and performance evaluation of the SFCL for potential FRT applications are presented. To wind the three-phase noninductive bifilar pancake coils, 324-m SuperPower SF12100-type ReBCO tapes are adopted, whereas one Cryomech AL600-type Gifford-McMahon refrigerator is used to form a liquid nitrogen $(\text{LN}_{2})$ circulated cooling system. Two main ac loss contributions of the hysteresis loss and the flux flow loss are calculated and discussed to achieve accurate SFCL resistance modeling. Finally, case studies with regard to a 1.5-MW DFIG-based wind turbine system are carried out to verify the feasibility of the proposed SFCL-based FRT scheme. With the designed SFCL, the fault rotor current is significantly reduced from about 4396 (4.94 p.u.) to 1406 A (1.58 p.u.) during the most severe condition. The fault grid voltage is also improved from about 89.7 (0.13 p.u.) to 103.5 V (0.15 p.u.) to comply with the international grid codes. In addition, the equivalent SFCL resistance and its corresponding ac loss contributions are discussed to evaluate the integrated performance of the designed SFCL during a grid fault.

Direct Torque Control of Saturated Doubly-Fed Induction Generator using High Order Sliding Mode Controllers

The present work examines a direct torque control strategy using a high order sliding mode controllers of a doubly-fed induction generator (DFIG) incorporated in a wind energy conversion system and working in saturated state. This research is carried out to reach two main objectives. Firstly, in order to introduce some accuracy for the calculation of DFIG performances, an accurate model considering magnetic saturation effect is developed. The second objective is to achieve a robust control of DFIG based wind turbine. For this purpose, a Direct Torque Control (DTC) combined with a High Order Sliding Mode Control (HOSMC) is applied to the DFIG rotor side converter. Conventionally, the direct torque control having hysteresis comparators possesses major flux and torque ripples at steady-state and moreover the switching frequency varies on a large range. The new DTC method gives a perfect decoupling between the flux and the torque. It also reduces ripples in these grandeurs. Finally, simulated results show, accurate dynamic performances, faster transient responses and more robust control are achieved.

Improved Direct Torque and Reactive Power Control of a Matrix-Converter-Fed Grid-Connected Doubly Fed Induction Generator

Doubly fed induction generator (DFIG) is the most popular variable-speed generator for wind energy application due to its superior performance, lower cost, and control flexibility. Instead of back-to-back converters, matrix converter (MC) can be a good alternative for connecting the DFIG rotor to the grid. This paper presents a novel lookup-table-based hysteresis controller for a variable-speed constant-frequency DFIG supplied from a direct MC. For the machine control, a modified direct torque controller (DTC) with reactive power and speed control in the outer loops has been used. The reactive component of the MC input current has been also controlled using a hysteresis controller considering all active switch states based on estimated input current and load power factor. This upgradation of the input current control switching table reduces the distortion in the grid-supplied current waveform. Overall, the performance of the DFIG with MC is experimentally shown to be superior while using the proposed controller compared with a standard DTC-based control or space-vector-pulsewidth-modulation-based vector control of MC-fed DFIG reported in the literature.

Mitigation of operational losses in matrix converter fed DFIG for WECS

With the change in the wind profile, the speed of doubly fed induction generator (DFIG) of wind energy conversion system (WECS) changes from super synchronous to sub synchronous range and vice versa. DFIG is operated to generate quality power; hence, power at rotor side is controlled using matrix converter taking power from grid and feeding back power to the grid depending on the wind profile. Input voltage required to be fed to the rotor side of DFIG is always changing and depends on the speed of the available wind. During the operation when power is fed from grid to rotor of DFIG, a high-voltage stress continues across the switches of power electronic converter (PEC). In existing topologies of the matrix converter used with the DFIG in WECS, the constant voltage stress at the power electronic switch (PES) is available which causes the higher losses across the switch. This also causes common mode voltage (CMV) which leads to the over voltage stress. This may cause winding insulation damage and bearing failure of the DFIG. Furthermore, higher dv/dt of CMV raises the leakage current which causes the thermal stress and electromagnetic noise to the equipments installed near the matrix converter. In this paper, the work done is focused on the mitigation of operational losses in matrix converter fed doubly fed induction generator for wind energy conversion system. The overall loss in the matrix converter and mitigation of common mode voltage is achieved, which improves the overall efficiency and the performance of the wind energy conversion system.

RBFNN-based adaptive crowbar protection scheme designed for the doubly fed induction generator in large-scale wind farms

The doubly fed induction generator (DFIG) and its relative technique, especially in the case where crowbar protection is provided for enhanced low-voltage-ride-through (LVRT) performance, have been hot issues for a long time. In this paper, the wind farm LVRT standard and the dilemma that the conventional crowbar protection encounters are introduced in the first place. Then, the variation of the DFIG rotor faulted current is analyzed, with part of its duration being considered as a black box. Therefore, the radial basis function neural network (RBFNN) is utilized for the black box modeling. Based on above studies, an adaptive crowbar protection scheme is finally put forward and assessed. Emphasis is mainly placed on the black box modeling, error analysis, and the investigation of its impact on the wind farm LVRT capability. DIgSILENT-based simulation results indicate that, with the novel scheme, it becomes possible for the crowbar to be adaptively switched out at a certain reasonable moment, so as to generate reactive power within the faulted duration. As a result, the steady-state voltage of the point of common coupling (PCC) is enhanced and the safety of the rotor-side convertor (RSC) can be ensured at the same time. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

DFIG Machine Design for Maximizing Power Output Based on Surrogate Optimization Algorithm

This paper presents a surrogate-model-based optimization of a doubly-fed induction generator (DFIG) machine winding design for maximizing power yield. Based on site-specific wind profile data and the machine's previous operational performance, the DFIG's stator and rotor windings are optimized to match the maximum efficiency with operating conditions for rewinding purposes. The particle swarm optimization-based surrogate optimization techniques are used in conjunction with the finite element method to optimize the machine design utilizing the limited available information for the site-specific wind profile and generator operating conditions. A response surface method in the surrogate model is developed to formulate the design objectives and constraints. Besides, the machine tests and efficiency calculations follow IEEE standard 112-B. Numerical and experimental results validate the effectiveness of the proposed technologies.

Fault Ride-Through Capability Enhancement of DFIG-Based Wind Turbine With a Flux-Coupling-Type SFCL Employed at Different Locations

Doubly fed induction generators (DFIGs) have attracted a wide interest for wind power generation, but they suffer from high sensitivity to grid disturbances, particularly grid faults. In this paper, a modified flux-coupling-type superconducting fault current limiter (SFCL) is suggested to improve the fault ride-through (FRT) capability of DFIGs. The SFCL's structure and principle is first presented. Then, considering that the SFCL can be installed at a DFIG's different locations, its influence mechanism to the DFIG's FRT capability is analyzed, and some technical discussions on the design of the SFCL are carried out. Furthermore, the simulation model of a 1.5-MW/690-V DFIG integrated with the SFCL is built, and the performance analysis is conducted. From the results, introducing the SFCL can effectively limit the fault currents across the DFIG's stator and rotor sides, and when the stator side is selected as the installation site, the terminal-voltage sag can be also improved, which helps prevent the disconnection of the DFIG from the power grid.

A novel method for obtaining virtual inertial response of DFIG‐based wind turbines

AbstractThis paper investigates virtual inertia control of doubly fed induction generator (DFIG)‐based wind turbines to provide dynamic frequency support in the event of sudden power change. The relationships among DFIGs' virtual inertia, rotor speed and network frequency variation are analysed, and a novel virtual inertia control strategy is proposed. The proposed control strategy shifts the maximum power point tracking (MPPT) curve to the virtual inertia control curves according to the frequency deviation so as to release the ‘hidden’ kinetic energy and provide dynamic frequency support to the grid. The calculation of the virtual inertia and its control curves are also presented. Compared with a PD regulator‐based inertial controller, the proposed virtual inertia control scheme not only provides fast inertial response in the event of sudden power change but also achieves a smoother recovery to the MPPT operation. A four‐machine system with 30% of wind penetration is simulated to validate the proposed control strategy. Simulation results show that DFIG‐based wind farms can provide rapid response to the frequency deviation using the proposed control strategy. Therefore, the dynamic frequency response of the power grid with high wind power penetration can be significantly improved. Copyright © 2015 John Wiley & Sons, Ltd.

Detection of rotor electrical asymmetry in wind turbine doubly‐fed induction generators

This study presents a new technique for detecting rotor electrical faults in wind turbine doubly-fed induction generators (DFIGs), controlled by a stator field-oriented vector control scheme. This is a novel method aimed at detecting and identifying rotor electrical asymmetry faults from within the rotor-side inverter control loop, using the error signal, to provide a future method of generator condition monitoring with enhanced detection sensitivity. Simulation and experimental measurements of the proposed signals were carried out under steady-state operation for both healthy and faulty generator conditions. Stator current and power were also investigated for rotor electrical asymmetry detection and comparison made with rotor-side inverter control signals. An investigation was then performed to define the sensitivity of the proposed monitoring signals to fault severity changes and a comparison made with previous current, power and vibration signal methods. The results confirm that a simple spectrum analysis of the proposed control loop signals gives effective and sensitive DFIG rotor electrical asymmetry detection.

Cooperative Control of SFCL and SMES for Enhancing Fault Ride Through Capability and Smoothing Power Fluctuation of DFIG Wind Farm

This paper deals with a cooperative control of a resistive type superconducting fault current limiter (SFCL) and a superconducting magnetic energy storage (SMES) for enhancing fault ride through (FRT) capability and smoothing power fluctuation of the doubly fed induction generator (DFIG)-based wind farm. When the system faults occur, the SFCL is used to limit the fault current, alleviate the terminal voltage drop, and transient power fluctuation so that the DFIG can ride through the fault. Subsequently, the remaining power fluctuation is suppressed by the SMES. The resistive value of the SFCL as well as the superconducting coil inductance of the SMES are simultaneously optimized so that a sudden increase in the kinetic energy in the DFIG rotor during faults, an initial stored energy in the SMES coil, an energy loss of the SFCL, and an output power fluctuation of the DFIG are minimum. The superior control effect of the cooperative SFCL and SMES over the individual device is confirmed by simulation study.