Stator Of Doubly Fed Induction Generator Research Articles

Control of DFIG-based microgrid and seamless transition from stator connected and disconnected modes

This study aims to ameliorate the contribution capability of doubly-fed induction generator (DFIG) to participate in standalone microgrid operation. The islanded microgrid consists of a solar photovoltaic array for solar energy conversion and battery energy storage in addition to DFIG-based wind energy conversion system. Using a simplified control approach, the study describes multi-mode operation of a DFIG-based AC/DC microgrid using a stator-side solid-state transition switch (SSTS). Using SSTS operation, the DFIG stator can be seamlessly disconnected and reconnected from the point of common coupling without interrupting power to the loads in the microgrid. Additionally, non-ideal AC loads can be handled efficiently without the need for computationally exhaustive approaches to enhance stator voltages and currents.

A sensorless control strategy for automatic start‐up and grid‐synchronization of doubly fed induction generator in wind energy conversion system

SummaryIn a wind energy conversion system (WECS) based on a doubly fed induction generator (DFIG), a fast and smooth automatic synchronization of the DFIG with the utility grid is always desirable. Additionally, elimination of the need for a speed encoder makes the synchronization process much more reliable. This paper proposes a modified voltage controller‐based automatic grid‐synchronization strategy for the DFIG‐WECS using a rotor‐current‐dependent model‐reference‐adaptive‐system (MRAS) for rotor‐speed/position estimation. A V/f control‐based starting strategy is used to start the DFIG. An improved decoupled DFIG stator voltage controller is proposed to enhance the transient response during the DFIG grid synchronization. The modified controller is implemented in a grid‐voltage‐oriented reference frame to synchronize the DFIG‐stator with the grid. A robust modified MRAS with higher noise immunity and reduced machine parameter dependency is used to estimate the rotor‐speed/position during synchronization control. To validate the performance of the proposed strategy, a DFIG‐based WECS is modeled and simulated using an OPAL‐RT real‐time simulator, where a motor‐generator setup is modeled to have an equivalent WECS configuration. As compared with the latest reported work, this approach reduces the synchronization time by 16.99% while ensuring a smooth synchronization with no stator inrush‐current, even without any speed/position encoder.

Coordinated Control for Seamless Integration of Wind Energy Conversion System With Small Hydrogenerator Through Modified Notch Filters

This paper deals with the seamless integration of doubly fed induction generator (DFIG) based wind energy conversion system (WECS) with local grid constituted by a permanent magnet based small hydrogenerator (PMHG). The seamless integration is carried out using a solid state static transfer switch (SSSTS). The WECS-PMHG system is operated using a coordinated control strategy based on modified notch filters. The modified notch filters are adopted for the computation of (a) sensorless speed and position of the DFIG rotor, (b) fundamental constituents of unbalanced/nonlinear load current and (c) filtered DFIG stator voltages and phase angle. Moreover, the notch filters are also used for mitigating the effects of unbalance/nonlinearity in load currents on the stator and rotor currents of DFIG as well as PMHG currents. Additionally, the control methodology ensures no power interaction between the WECS and the PMHG during the synchronization process. This enables transient-free WECS and PMHG currents. Further, the coordinated control regulates the amplitude and frequency of system voltages even amidst wide variations in wind speed and load. The DFIG stator and wind currents are injected at unity power factor with the system voltages, while the excitation requirement of the DFIG is met by its machine side converter. The validness of the control is demonstrated experimentally through a developed laboratory-scale test bench.

Dual Converter Operating With Floating Capacitors Connecting Open-End Winding Doubly-Fed Induction Generator to the DC Microgrid

This paper presents a novel configuration to connect a three-phase doubly-fed induction generator (DFIG) to the dc microgrid. The proposed topology connects an uncontrolled con- verter and a full-controlled converter in series with the generator stator windings in an open-end winding (OEW) configuration. The full-controlled converter is connected to an independent dc link based on floating capacitors. The paper discusses the converter operating principles, pulse-width modulation (PWM), DFIG power control, and the floating capacitors' voltage reg- ulation. The work compares the proposed and conventional systems regarding current harmonic distortions, torque ripple, and semiconductor losses. Simulations and experiments show steady-state and transient results for a 0.56-kW DFIG operating at the rated power. Finally, it was verified that the proposed topology presents lower power losses and low-distorted voltages and currents over the DFIG stator windings.

Analysis of low voltage ride-through capability and optimal control strategy of doubly-fed wind farms under symmetrical fault

Given the “carbon neutralization and carbon peak” policy, enhancing the low voltage ride-through (LVRT) capability of wind farms has become a current demand to ensure the safe and stable operation of power systems in the context of a possible severe threat of large-scale disconnection caused by wind farms. Currently, research on the LVRT of wind farms mainly focuses on suppressing rotor current and providing reactive current support, while the impact of active current output on LVRT performance has not been thoroughly discussed. This paper studies and reveals the relationship between the limit of reactive current output and the depth of voltage drop during LVRT for doubly-fed induction generator (DFIG) based wind farms. Specifically, the reactive current output limit of the grid-side converter is independent of the depth of voltage drop, and its limit is the maximum current allowed by the converter, while the reactive current output limit of the DFIG stator is a linear function of the depth of voltage drop. An optimized scheme for allocating reactive current among the STATCOM, DFIG stator, and grid-side converter is proposed. The scheme maximizes the output of active current while satisfying the standard requirements for reactive current output. Compared to traditional schemes, the proposed LVRT optimization strategy can output more active power during the LVRT period, effectively suppressing the rate of rotor speed increase, and improving the LVRT performance and fault recovery capability of wind farms. Simulation results verify the effectiveness of the proposed scheme.

Wind Turbine Gearbox Diagnosis Based on Stator Current

Early detection of faults in wind energy systems can reduce downtime, operating, and maintenance costs while increasing productivity. This paper proposes a method based on the analysis of generator stator current signals to detect faults in a wind turbine gearbox equipped with a doubly fed induction generator (DFIG). A localized parameter model was established to simulate the vibratory response of a two-stage gear system under healthy and faulty conditions. The simulation was performed in the MATLAB/Simulink environment. The results include a detailed analysis of the mechanical part of the gearbox, highlighting mesh stiffness, output speed, and accelerations. Additionally, the electrical part was evaluated based on the current supplied by the doubly fed induction generator. The results were presented in the case of healthy gears and in the presence of faults such as a broken or cracked tooth. Fast Fourier transform (FFT) analysis was employed to detect gear defects in the stator current signal. The presence of a crack or broken tooth in the gearbox induces modulation of the DFIG stator current signals according to the shaft frequencies corresponding to the faulty gear. These findings provide a preliminary basis for the detection and diagnosis of this type of failure.

Islanding Operation and Seamless Resynchronization in DFIG–SPV System Equipped With Functional Enhancements Through ATLLAD Control Framework

Doubly-fed induction generator (DFIG) and solar photovoltaic (SPV) array equipped hybrid wind-solar generation system is increasingly acknowledged as a viable techno-economic solution for remote-area electrification. Despite the progress made in this frontier, two major hurdles still persist. The first hurdle relates to the power inadequacy for local loads and the mode transitional features in DFIG-SPV system, during grid outages, which are rather frequent in remote-areas. Another issue in DFIG-SPV system at remote-areas is its functional vulnerability at the irregular grid conditions like unbalance and distorted voltages. Taking these aspects into consideration, this paper implements a new arc-tangent least logarithmic absolute difference (ATLLAD) based control framework for DFIG-SPV system. The operational scheme ensures disruption-free load power at islanding scenarios, while, the control strategy enables seamless transitions without inducing overcurrents or voltage spikes during mode switching. To deal with DFIG-side functional vulnerability at irregular grid conditions, ATLLAD-based control uses an innovative approach to impart an additional degree of freedom for the simultaneous suppression of DFIG torque oscillations and reduction in DFIG stator current disturbances. Meanwhile, the enhancement at the grid-side involves injection of improved quality grid currents despite the irregularities in the grid voltages and the load currents. Test results from an experimental-rig of the DFIG-SPV system illustrate satisfactory performance of ATLLAD control approach.

Blackstart and Fault Ride-Through Capability of DFIG-Based Wind Turbines

Grid-forming wind turbines must be able to re-energize transmission branches, motor loads, unbalanced loads, and power electronics loads following a blackout. Further, they must ride through faults that can arise during restoration. To tackle these challenges, this paper sets forth a grid-forming control strategy for wind turbines using doubly-fed induction generators (DFIGs). In particular, a special set of feedforward control loops are engineered to mitigate the impacts of restoration transients as well as symmetrical and asymmetrical faults on the regulation of DFIG rotor currents and stator voltages. This paper also leverages two-axis anti-windup regulators to bound two-axis voltage and current commands within ratings. The controller performance is demonstrated via a high-fidelity simulation model of a notional wind-dominant grid.

IMPROVED ADAPTIVE NONLINEAR CONTROL FOR VARIABLE SPEED WIND-TURBINE FED BY DIRECT MATRIX CONVERTER

This paper proposes a robust decoupling power algorithm based on a doubly fed induction generator (DFIG) for variable speed wind-turbine (WT). The DFIG rotor circuit is fed by the direct matrix converter (DMC), which presents several features such as no need to the dc-bus voltage, sinusoidal supply, rotor side waveforms, bidirectional power flow, and adjustable input power factor. The 18 bidirectional switches are controlled using the Venturini modulation technique. On the other hand, the DFIG stator circuit is connected directly to the grid. The nonlinear control strategy based on feedback linearization is applied to control the stator power (Ps and Qs) independently using the rotor quadrature and direct currents (irq and ird), which present the images of the previous stator powers. Some limitations appear in the power algorithm using the conventional pi controller, especially in power tracking, error, and quality. In this context, the model reference adaptive controller (MRAC) presents an alternative solution, a robust and efficient controller proposed instead of the pi controllers to control stator powers. Finally, the simulation results confirm that the proposed algorithm could work under hard conditions and demonstrate that the wind energy conversion system (WECS) provides enhanced dynamic responses in transient and steady states and good power quality delivered to the grid.

Robust sensor-less sliding mode of second-order control of doubly fed induction generators in variable speed wind turbine systems based on a novel MRAS-ANFIS observer

The current study proposed a robust sensor-less sliding mode second-order based on a super twisting algorithm (STA-SMSO) approach using a new observer Model Reference Adaptive System-Adaptative Neuro-Fuzzy Inference System (MRAS-ANFIS). This model was applied to a doubly fed induction generator (DFIG) wind turbine running under variable wind speed and DFIG fed with a power voltage source without a speed sensor, while the control objective was used to regulate independently, the active and reactive power DFIG stator were decoupled by using the field-oriented control technique. Additionally, this process reduced the cost of the control scheme and the size of DFIG by eliminating the speed sensor (encoder). In order to improve the traditional MRAS, the MRAS-ANFIS observer was proposed to replace the usual PI controller in the adaptation mechanism of MRAS with an Adaptative Neuro-Fuzzy Inference System (ANFIS) controller. The estimation of rotor position was tested and discussed under varying load conditions in low, zero, and high-speed region. The results mentioned that the proposed observer (MRAS-ANFIS) presented an attractive feature, such as guarantees finite time convergence, good response on speed wind variations, high robustness against machine parameter variations, and load variations compared to the conventional MRAS observer and MRAS-Fuzzy. Hence, the estimated rotor speed converged to their actual value has the capacity for estimating position in deferent region (low/zero/high) of speed.

Self-Synchronizing Control Enabling Disruption-Free Operation and Seamless Mode Transitions in Wind–Solar Based Hybrid AC/DC Microgrid

This paper introduces a novel self-synchronizing control architecture for the power converters in a hybrid AC/DC microgrid (HMG), wherein, the doubly fed induction generator (DFIG) and the solar photovoltaic (SPV) array are utilized for wind and solar generation, respectively. The self-synchronizing control enables the HMG to remain functional and provides a disruption-free power at all the four identified operation modes. Different operation modes are enforced due to grid islanding or DFIG wind generator disconnection scenarios. Meanwhile, the shifting among these four operation modes culminate into eight possible transition phases. The HMG's self-synchronizing control is devised to achieve seamless operation at all possible transition phases, and avoids any voltage spikes or transient overcurrents. Such a seamless transition functionality becomes increasingly vital in case of the DFIG-SPV based HMG. This is due to the direct interconnection of DFIG stator with the utility grid that renders the system largely vulnerable to any un-synchronized transition. Therefore, major research emphasis of this work is on the development of a novel methodology for ensuring a disruption-free operation and seamless mode transitions in a DFIG-SPV based HMG. The HMG control is based on a new affine projection sign algorithm (APSA), which supplements the HMG with unity power factor operation and load unbalance/harmonics mitigation. The viability of the HMG and its self-synchronizing control at various mode transition phases and system dynamics are extensively tested on an experimental rig developed in the laboratory. Test results endorse the feasibility of the self synchronizing control, while, a comparative analysis demonstrates the superiority of the APSA scheme over other state-of-art approaches.

Incipient detection of stator inter‐turn short‐circuit faults in a Doubly‐Fed Induction Generator using deep learning

Wind turbines are increasingly expanding worldwide and Doubly-Fed Induction Generator (DFIG) is a key component of most of them. Stator winding fault is a major fault in this equipment and its incipient detection is of vital importance. However, there is a paucity of research in this field. In this study, a novel machine learning-based method is proposed for incipient detection of inter-turn short-circuit fault (ITF) in the DFIG stator based on the current signals of the stator. The proposed method makes use of state-of-the-art deep learning methods along with conventional signal processing tools and general machine learning techniques. More specifically, the incipient fault detection problem is regarded as a multi-class classification problem and a Long Short-Term Memory network, which is more appropriate for time-series data is utilised for inference. Furthermore, a variant of the celebrated Empirical mode Decomposition analysis tool is used to extract some well-known statistical features among which the most informative ones are selected using a new feature selection method. Our tests using experimental data in steady-state conditions show that the proposed method can accurately detect ITF fault at its initial stage when only one turn is shorted. Moreover, its performance is considerably higher than that of a variety of machine learning-based methods.

Sensorless passivity based control of doubly-fed induction generators in variable-speed wind turbine systems based on high gain observer

The current study presents a robust sensorless control using passivity based control (PBC) combined with high gain observer (HGO). The proposed controller is applied to control the generated doubly-fed induction generator (DFIG) active and reactive power installed on a variable speed wind energy conversion system. The control objective is used to regulate independently the DFIG stator active and reactive power, which are decoupled by using the field oriented control technique. Additionally, this process leads to reduce the cost of control scheme by eliminating the speed sensor. Firstly, the DFIG is modeled under the port controlled Hamiltonian (PCH) model, as well as the method of simultaneous injection damping. Then, the DFIG is further modeled by assignment passivity based on the simultaneous injection damping and assignment (SIDA-PBC) control of the obtained model under such conditions and a comparison with the fuzzy sliding mode controller is carried out. Furthermore, the HGO is selected in order to estimate the rotor position and the speed from the measurement of the DFIG currents and voltages, and compared with fuzzy sliding mode observer. For testing the proposed control scheme performance, a 1.5 MW DFIG system is developed and simulated using MATLAB/Simulink. The obtained results demonstrate the effectiveness of the proposed control scheme in the presence of various DFIG parameters variation. Additionally, the control objective is achieved without speed sensor.

Coordinated Control for Performance Enhancement of an Islanded AC/DC Microgrid With Uninterrupted Power and Seamless Transition Capabilities

This article presents a simplistic control approach and multimode operation of a wind-solar-based ac/dc microgrid. The islanded microgrid consists of a doubly-fed induction generator (DFIG) for wind energy conversion, and a solar photovoltaic array for solar energy conversion. In this configuration, the stator terminals of DFIG are connected at the point of common intersection (PCI) of ac loads via a stator-side solid-state transition switch ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SSTS</sub> ). The developed control strategy is designed to seamlessly disconnect and reconnect the DFIG stator from the PCI based on the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SSTS</sub> operation, without interrupting power to ac and dc loads in the microgrid. The disconnection of stator terminals is required in the event of a prolonged period of no wind generation from the DFIG. The controller is synthesized to ensure no or low transient effect during the transition between the two operating modes, i.e., DFIG stator connected mode (SCM) and stator disconnected mode (SDM). Further, the power quality enhancement of stator voltages and currents amidst nonideal ac loads is ensured without need for computationally exhaustive approaches involving sequence decompositions, notch filters, resonant regulators, or load current sensors. Moreover, the power quality enhancement at the DFIG stator is achieved without injecting any undue harmonics in the DFIG rotor currents. The effectiveness of this islanded microgrid is verified through obtained results from a developed laboratory prototype.

Half-Controlled Converters Connecting Open-End Doubly Fed Induction Generator to a DC-Microgrid

This article presents a novel system to connect a three-phase doubly fed induction generator (DFIG) to a dc-microgrid. The proposed topology is based on two three-phase half-controlled converters connected in series with the generator stator windings, in an open-end winding configuration. The converters operating principles, pulsewidth modulation, power and air-gap flux controls, and zero-sequence current minimization strategies are discussed. The proposed system is compared with conventional systems in terms of current harmonic distortions, torque ripple, and semiconductor losses. Simulations and experiments were also performed, showing steady-state and transient results for the DFIG generating 500 W. Finally, it was verified that the proposed system can operate with lower power losses and maintain low-distorted voltages and currents over the DFIG stator windings.

Flexible Ripple Minimization Technique for Wind-Solar Renewable Energy System Under Unbalanced and Distorted Grid Conditions

This article focusses on power quality issues related with the operation of a wind energy conversion system employing a doubly fed induction generator (DFIG), integrated with a solar photovoltaic array, under unbalanced, and distorted grid conditions. The multiple-order DFIG electromagnetic torque ripples and grid active power ripples, resulting from the unbalanced and distorted grid, can be mitigated, however, at the cost of highly unbalanced and distorted currents. This article provides a possible solution towards a tradeoff by developing a flexible ripple minimization technique based on a cross-feedback extended second order generalized integrator for harmonics sequence separation (CextSOGI-HSS). The CextSOGI-HSS based current compensation enables a controlled minimization of ripple as per the customizable flexibility quotient. Furthermore, the current compensation technique employs an improved recursive least squares (IRLS) adaptive filtering algorithm for processing DFIG stator currents and local load currents. The IRLS algorithm improves grid currents harmonics spectra by filtering-out higher order harmonics from DFIG stator currents and nonlinear load currents, meanwhile, the effects of any load or stator currents unbalance on grid currents are also mitigated. Test results from a developed laboratory prototype confirm the efficacy of the presented flexible ripple minimization control technique.

Constant Power Factor Model of DFIG-Based Wind Turbine for Steady State Load Flow Studies

DFIG (Doubly Fed Induction Generator)-based WPP (Wind Power Plant) is the most popular type of wind-driven electric power generation configuration. The main reason for its popularity is that the DFIG system can capture wind energy more optimally than other WPP configurations. Due to the increasing penetration of WPP in power systems, the need to study its impact on power system performance is becoming increasingly important. To enable such a study to be conducted properly, the first and probably the most crucial step is to model all system components (including the WPP). This paper proposes a new steady state model of DFIG-based WPP for load flow analysis. The proposed model is derived based on DFIG power formulas (i.e., DFIG stator and rotor power formulas). The model in the present work is simple and can easily be incorporated into load flow analysis. Representation of the DFIG in both sub-synchronous and super-synchronous conditions can be carried out by using a single mathematical model. Furthermore, since DFIG can be operated at a constant power factor (i.e., unity, leading, or lagging power factor), this important feature is also considered in the proposed model development.

Study on Suppression Strategy for Broadband Sub-Synchronous Oscillation in Doubly-Fed Wind Power Generation System

In the power transmission of doubly-fed induction generators (DFIGs), sub-synchronous oscillation (SSO) can occur due to the influence of series compensation capacitance and long-distance transmission. SSO not only affects the output of the DFIG but also leads to oscillation diffusion. In order to solve the problem of disturbance in the control of the DFIG rotor side converter (RSC) under SSO, an adaptive quasi-resonant controller is proposed for the suppression of SSO. This strategy focuses on the propagation path of and frequency change in the SSO in the RSC control system and suppresses the SSO current in the wideband through the cooperative control of the back-stepping controller and the adaptive quasi-resonant controller. In this way, the stator-side output of the DFIG will not be affected by SSO, thus avoiding the amplification of the sub-synchronous power of the line by the DFIG. A simulation model and experimental platform were built to verify the suppression effect of this control strategy on the DFIG stator sub-synchronous current at different SSO frequencies. The results show that the proposed strategy has a good suppression effect on broadband SSO.

Robust Neural Control of Wind Turbine Based Doubly Fed Induction Generator and NPC Three Level Inverter

This paper presents dynamic modeling and control of Doubly Fed Induction Generator (DFIG) based on wind turbine systems, where the stator of DFIG is directly connected to the grid and the rotor was fed by a three level PWM NPC inverter. The active and reactive power control of the DFIG is based on the feedback technique by vector control method by using a classical regulator of Proportional-Integral (PI) type which allows us, in association with the looping of powers, to obtain an efficient and robust system. This approach is a very attractive solution for devices using DFIG as wind energy conversion systems; because, it is a simple, practical implementation, commonly applied in the wind turbine industry and it presents very acceptable performance, However, this control approach has certain limitations and has several causes, vector command with NPC three-level inverter pulse width modulation (PWM) is used to control the reactive power and active power of the generator. Then, use the neural network design to replace the traditional proportional-integral (PI) controller. Finally, the Matlab/Simulink software is used for simulation to prove the effectiveness of the command strategy.

An Enhanced Rotor Position/Speed Estimation Technique for Doubly Fed Induction Generator Using Stator-Side Reactive Current Variable in Model Reference Adaptive System

In this article, an enhanced rotor position/speed estimation technique of doubly fed induction generator (DFIG) is focused. The estimator adopts model reference adaptive system (MRAS). The stator-side reactive current is taken as the main working variable in the model. The distinct advantage of the proposed scheme is stated that it is completely free from estimation error due to the mismatch of both stator and rotor resistances of DFIG and also the method is insensitive to magnetizing inductance variation. On the other hand, the existing schemes depend on any or combination of the parameters. The dependencies on the number of parameters are reduced in the proposed case to make the estimation method more robust. It has various other desirable qualities, such as no use of integration or differentiation during computation, flux calculationless estimation process, catch on fly operation, capability to estimate at very low slip speed condition, and accurate estimation at both sub- and supersynchronous speed range. The other novelty of the estimator is cited that it performs accurate rotor position/speed estimation irrespective of loading condition on the machine. The real-time experimental works are performed to validate various performances of the suggested estimation technique. The obtained results confirm expected performances as per theoretical analysis.