Brushless Doubly Fed Induction Generator Research Articles

Control & Design for FOPID Based Line-Side Converter of the Brushless Doubly-Fed Induction Generator

This paper deals with the operation of doubly fed induction generator (DFIG) with an integrated active filter capabilities using grid-side converter (GSC). The main contribution of this work lies in the control of GSC for supplying harmonics in addition to its slip power transfer. The rotor-side converter (RSC) is used for attaining maximum power extraction and to supply required reactive power to DFIG. This wind energy conversion system (WECS) works as a static compensator (STATCOM) for supplying harmonics even when the wind turbine is in shutdown condition. Control algorithms of both GSC and RSC are presented in detail. The proposed DFIG-based WECS is simulated using MATLAB/Simulink.

Control & Design for FOPID Based Line-Side Converter of the Brushless Doubly-Fed Induction Generator

Control & Design for FOPID Based Line-Side Converter of the Brushless Doubly-Fed Induction Generator

Dual Synchronous Rotating Frame Current Control of Brushless Doubly Fed Induction Generator Under Unbalanced Network

This article investigates the modeling and control of brushless doubly fed induction generator (BDFIG) under an unbalanced network. The system behavior of BDFIG under the unbalanced grid voltage is analyzed. The control strategies and different control targets of BDFIG in balanced and unbalanced networks are discussed. In order to achieve control targets, a current control strategy consisting of a primary controller and a secondary controller for the control winding is proposed. The primary controller is implemented in the positive-sequence synchronous rotating frame (SRF) to ensure the dynamic response of the system without sequence components separation, whereas the secondary controller is implemented in the negative-sequence SRF to control the negative-sequence current component only. A feedforward compensation for the negative-sequence current control is used to mitigate the sequence components separation influence on the dynamic response of the secondary controller. Experimental results demonstrate the effectiveness of the control strategy in suppressing the power and current oscillation of a BDFIG system under the unbalanced network.

Influence of Pole-Pair Combinations on the Characteristics of the Brushless Doubly Fed Induction Generator

The brushless doubly fed induction generator (BDFIG) is an alternative to the doubly fed induction generator (DFIG), widely used in wind turbines which avoids the need for brush gear and slip rings. The choice of pole numbers for the two stator windings present in the BDFIG sets the operating speed, typically in the medium speed range to eliminate a gearbox stage. This paper focuses on how both the total number of poles and the assignment of poles between the windings affect machine performance. Analytical expressions have been developed for parameters including pull-out torque, magnetizing current and back-iron depth. The results show that the pole count can be increased without unduly compromising pull-out torque and that in cases where more than one combination of pole number is acceptable only the back iron depth is significantly affected. In addition an output factor has been introduced to enable a direct comparison to be made with conventional DFIGs. The torque density of a brushless DFIG is compromised to a degree relative to a comparable DFIG as a consequence of the presence of two magnetic fields and finite element analysis is needed to achieve an optimized design. Finally, predictions of the performance of multi-MW machines are made based on data from an existing 250 kW machine which show that suitable efficiencies can be obtained and excessive control winding excitation can be avoided.

Sensorless Voltage Control Schemes for Brushless Doubly-Fed Induction Generators in Stand-Alone and Grid-Connected Applications

The aim of this paper is to investigate the sensorless voltage control schemes for the promising wound-rotor brushless doubly-fed induction generator (BDFIG) in both the stand-alone and grid-connected systems. For sensorless target, a new rotor position observer using the space vector flux relations of BDFIG is suggested. The proposed sensorless algorithm does not require any additional observers for obtaining the generator speed by directly detecting the rotor position, which may reduce the cost of the overall control system. For the voltage control purpose, a direct voltage control (DVC) scheme is applied for the stand-alone systems. However, this control strategy did not handle the control of the generated-voltage phase angle and also did not consider the phase sequence issue which are very essential provisions for the smooth connection target in grid-connected applications. Hence, DVC strategy cannot guarantee all the intended provisions for a soft and smooth connection process with the grid side and therefore, an effective grid-synchronization algorithm is proposed. A comprehensive analysis to confirm the functionality of the proposed control strategies is discussed through both simulation and experimental results. The presented study in this paper verifies the efficacy of the proposed schemes for sensorless voltage control target of BDFIG in both the stand-alone and grid-connected applications with a good transient behavior.

Indirect Vector-Controlled Brushless Doubly-Fed Twin-Stator Induction Generator for Wind Energy Conversion Application

Wind energy conversion systems (WECSs) seem certain to play a major part in the world’s energy future due to their known high power capacity. The maximum power tracking is unavoidable due to the wind velocity variation and the non-linear relation with the turbine mechanical power. Commercial wind turbines are commonly coupled to either doubly-fed induction generators (DFIGs), wound rotor synchronous generators (WRSG) or permanent magnet synchronous generators (PMSGs). The DFIG-based WECS has several advantages over others. One of which is the power converter in such systems only deals with rotor power, hence the converter rating can run at reduced power rating. However, DFIG has the famous disadvantage of the presence of slip rings which leads to increased maintenance costs and outage times. Hence, brushless doublyfed induction machines (BDFIMs) can be considered as a viable alternative at the penalty of complicated controller requirement and limited decoupling control capability due to the machine’s non-linearity. In this paper, an enhanced performance indirect vector controller is proposed for WECS based on brushless doubly-fed twin-stator induction generator (BDFTSIG). The presented controller offers (i) simplified implementation, (ii) decoupled active-reactive power control, and (iii) a wide range of operation. The proposed controller performance is investigated under various loading conditions showing enhanced transient and minimal steady-state oscillations in addition to complete active/reactive power decoupling. The rigorous simulation and experimental results verify the claimed controller effectiveness under all possible operating conditions for sub- and super-synchronous speed range.

Sensorless Control of Brushless Doubly Fed Induction Generator with Nonlinear Loads for Stand-Alone Power Generation Systems

This paper presents a sensorless control scheme for the stand-alone brushless doubly fed induction generator (BDFIG) feeding nonlinear loads. The fundamental and harmonic components of the distorted power winding (PW) voltage caused by the nonlinear loads are extracted and controlled separately. A rotor speed observer is employed to estimate the speed based on the PW voltage and control winding (CW) current without the need of any other machine parameters except for the number of pole pairs. Since the d- and q-axis references of the CW current from the PW voltage control loop contain both dc and ac components, which cannot be tracked easily by conventional PI controllers, a CW predictive current controller is designed to regulate the CW current. Finally, the performance of the proposed control scheme is verified by comprehensive experiments on a 35-kVA prototype BDFIG.

Full-Parameter Identification Model Based on Back Propagation Algorithm for Brushless Doubly Fed Induction Generator

Brushless doubly fed induction generator (BDFIG) has great potential due to its high reliability and low maintenance cost. To achieve high-performance modeling and control, BDFIG resistances and inductances are necessary. However, the existing identification methods either required professional structure knowledge or special excitations and setups, or only estimated part of parameters. Thus, this letter proposes a multilayer full-parameter identification model based on the back-propagation (BP) algorithm for BDFIG, which is constructed with electric quantities as nodes and parameters as adjustable weights, and utilizes the electric quantities measured from regular operations as data. According to the fitting error obtained by comparing the model outputs with the easily measured references, the BP algorithm is applied to update the weights until the error is sufficiently small. Then, all resistances and inductances can be extracted directly from the weights. Such an identification methodology can be easily embedded into existing BDFIG systems. The simulations and experiments verify its feasibility and accuracy.

Indirect Torque Control of a Cascaded Brushless Doubly-Fed Induction Generator Operating With Unbalanced Power Grid

This article describes an indirect torque control algorithm for a cascaded brushless doubly-fed generator (CBDFIG). The control algorithm presented in the article provides a possibility for CBDFIG to work with an unbalanced power grid applying different strategies: electromagnetic torque oscillations reduction, generated active power oscillation reduction, generated phase currents balancing, sinusoidal control currents under grid imbalance . The proposed control method is based on a cascaded brushless doubly-fed induction generator equivalent model and requires fewer tuning-demanding elements in comparison to classic control methods. The control algorithm presented in the paper was implemented and tested on a CBDFIG machine model in a simulation environment PSIM and on a real physical stand with a cascaded brushless doubly-fed generator.

Neuro-fuzzy Sliding Mode Controller Based on a Brushless Doubly Fed Induction Generator

The combination of neural networks and fuzzy controllers is considered as the most efficient approach for different functions approximation, and indicates their ability to control nonlinear dynamical systems. This paper presents a hybrid control strategy called Neuro-Fuzzy Sliding Mode Control (NFSMC) based on the Brushless Doubly fed Induction Generator (BDFIG). This replaces the sliding surface of the control to exclude chattering phenomenon caused by the discontinuous control action. This technique offers attractive features, such as robustness to parameter variations. Simulations results of 2.5 KW BDFIG have been presented to validate the effectiveness and robustness of the proposed approach in the presence of uncertainties with respect to vector control (VC) and sliding mode control (SMC). We compare the static and dynamic characteristics of the three control techniques under the same operating conditions and in the same simulation configuration. The proposed controller schemes (NFSMC) are effective in reducing the ripple of active and reactive powers, effectively suppress sliding-mode chattering and the effects of parametric uncertainties not affecting system performance.

Control of a BDFIG Based on Current and Sliding Mode Predictive Approaches

The Brushless Doubly Fed Induction Generator (BDFIG) is a great attraction nowadays thanks to its high reliability and high low voltage ride-through capability. In variable speed generation systems, the most used control techniques are the vector control, direct torque/power control. Vector control is highly sensitive to parameters variations, while the two others offer high amount of ripples reducing power quality. This paper therefore proposes a Sliding Mode Power Predictive Control (SMPPC) and an Indirect Model Power Predictive Control (I-MPPC) for a simple and efficient power control of the BDFIG. The SMPPC takes current components in the αβ frame as sliding surfaces derived from the I-MPPC based on current predictions to track the desired active and reactive powers in the BDFIG. A cost function defined as currents’ quadratic errors is used to choose the appropriate voltage vector for control winding supply, and the Particle Swarm Optimization algorithm is used to determine the optimal gain for the SMPPC. Simulations on MATLAB/Simulink are carried out to show the effectiveness of the control schemes as well as their robustness to some parameters variations. More so, power compensation is used to improve the capability of the control strategy to provide sinusoidal and balanced currents under unbalanced grid conditions. The proposed methods compared to direct power control show good performances in terms of total harmonic distortion reduction, low ripples and robustness, strengthening the idea of using the BDFIG as an alternative to new generation of wind energy conversion systems.

Sensorless Control of Ship Shaft Stand-Alone BDFIGs Based on Reactive-Power MRAS Observer

This article proposes a rotor position observer based on a reactive power model reference adaptive system (MRAS) for sensorless direct voltage control (DVC) strategy of the stand-alone brushless doubly fed induction generators (BDFIGs). The proposed MRAS observer is based on the control winding (CW) reactive power and the power winding (PW) field orientation. The suggested new sensorless control method does not require flux computation, which can ensure the simplicity and efficacy of the proposed method. The stability analysis of the new observer is investigated and confirmed with the concept of hyperstability and using Popov's criterion methodology. The robustness of the new control method is analyzed under parameter uncertainties. Furthermore, comprehensive simulation and experimental results are obtained with a prototype wound-rotor BDFIG. The corresponding results confirm the capability and effectiveness of the proposed sensorless DVC strategy with a good transient behavior under different operating conditions for the stand-alone BDFIG system. Moreover, these results assure that the presented rotor position observer is robust via any machine parameter change.

An Improved Rotor Speed Observer for Standalone Brushless Doubly-Fed Induction Generator Under Unbalanced and Nonlinear Loads

The conventional control methods for brushless doubly-fed induction generator (BDFIG) normally employ mechanical sensors to acquire the information of rotor speed, which brings many disadvantages in the cost, complexity, reliability, etc. This paper presents an improved rotor speed observer (RSO) for the sensorless operation of standalone BDFIGs, which is based on the power winding (PW) voltage and control winding (CW) current. In order to eliminate the impact of unbalanced and nonlinear loads on the RSO, second-order generalized integrators (SOGIs) and low-pass filters (LPFs) are introduced to pre-filter the PW voltage and CW current, respectively. Through comprehensive parameter design, the response speed of the improved RSO will be not lower than that of the basic RSO with ensuring the filtering effect of these additional filters. In addition, the proposed RSO is independent to machine parameters except the pole pairs. Comprehensive experiments are conducted and results verify the proposed improved RSO applied to the standalone BDFIG. Also, the applicability of the proposed RSO on another dual-electrical-port machine, doubly-fed induction generator, is confirmed by simulation results.

Robust position observer for sensorless direct voltage control of stand-alone ship shaft brushless doubly-fed induction generators

Robust position observer for sensorless direct voltage control of stand-alone ship shaft brushless doubly-fed induction generators

Experimental and finite element studies of a 250 kW brushless doubly fed induction generator

This study studies, using experimentally verified finite element (FE) analysis, various performance measures of the brushless doubly fed induction generator (BDFIG) when magnetic wedges are used for closing stator open slots. The BDFIG is an attractive generator solution for offshore wind power and can replace doubly fed slip-ring induction generators. It is shown in this study that the use of magnetic wedges, commonly used in large induction machines, reduces the stator windings magnetising currents in a BDFIG, reflected in the values of magnetising inductances. They also increase the leakage flux in the stator slots, leading to a larger series inductance in the equivalent circuit. The series inductance significantly affects the BDFIG performance as well as the rating of its converter. The effects of magnetic wedges on BDFIG air-gap flux, synchronous torque, and stator tooth top saturation is also investigated. Two-dimensional FE analysis of an experimental 250 kW BDFIG is used in this study, verified by experimental measurements.

Model predictive virtual power control of brushless doubly‐fed induction generator for fast and smooth grid synchronisation

This study presents a model predictive virtual power control (MPVPC) method for brushless doubly-fed induction generator (BDFIG) during the grid synchronisation process. Predictive virtual power model is developed for the first time based on the defined virtual power and state-space equation of BDFIG. The control characteristics of virtual power are analysed to explain clearly how to realise grid synchronisation condition through the control of virtual power. The MPVPC controller is then designed to unify the control structure during the transition from the grid synchronisation mode to the grid connected mode, and only flux feedback needs to be changed between these two modes. The consistency between theoretical analysis and actual performance is confirmed by the results of both numerical simulation and experimental tests. As demonstrated, the proposed MPVPC controller can achieve fast and smooth grid synchronisation and excellent decoupled control of active power and reactive power.

Negative Sequence Voltage Compensating for Unbalanced Standalone Brushless Doubly-Fed Induction Generator

During the unbalance operation of a standalone brushless doubly-fed induction generator (BDFIG), the negative sequence component causes significant unbalanced effect on the voltage and current of the power winding (PW). In this paper, a new control strategy is proposed to minimize the voltage unbalance effect of a standalone BDFIG under unbalanced loads. The proposed control strategy consists of two parts. The first part represents the conventional direct voltage control strategy to control the frequency and magnitude of the positive sequence PW voltage. The second part is the proposed strategy to minimize the negative sequence component in the output voltage of PW with a negative sequence compensator in the machine side converter. The dual second-order generalized integrator is used to extract negative-sequence component of the PW voltage. Proportional integral controllers are used to obtain reference of the control winding (CW) current which compensate the negative sequence component of the PW voltage. The proportional-integral-resonant controller is applied to regulate the CW current in positive reference frame. A prototype 30 kVA BDFIG test platform is developed in the laboratory to verify the proposed strategy.

Improved Coordinated Control of Standalone Brushless Doubly Fed Induction Generator Supplying Nonlinear Loads

This paper proposes an improved coordinated control of standalone brushless doubly fed induction generator (BDFIG) supplying nonlinear loads. Due to the existence of nonlinear loads, the voltage at the point of common coupling becomes distorted with a large number of low-order harmonics, which severely degrades the performance of power generation systems. In order to eliminate the included low-order harmonics, a coordinated control method of harmonic mitigation has been presented in this paper for the control of both machine side and load side converters. Then, a weight factor is introduced to the proposed control method for distributing the contribution of harmonic mitigation between the two converters. Comprehensive experimental results on a 30 kVA BDFIG are presented to verify the proposed control method.

Stand-Alone Brushless Doubly Fed Generation Control System With Feedforward Parameters Identification

Due to the low maintenance cost and high reliability, brushless doubly fed induction generator (BDFIG) has great potential in remote stand-alone applications. For good dynamic performance, the feedforward compensation is used in the BDFIG system. However, its performance highly depends on machine parameters and rotor angle, and thus complicated parameter measurements or estimations are required. To overcome this problem, this paper proposes an adaptive control system, in which the required feedforward parameters such as compensation coefficients and the transformation angle can be identified online by fully utilizing the control action of the inner loops. With the proposed system, precise machine parameters are needless and the encoder can also be canceled, and therefore the overall system becomes more cost-efficient and robust. Detailed analysis and designs of the adaptive control and identification are given in this paper. Experimental results from a BDFIG prototype prove the feasibility and effectiveness of the proposed system.

Crowbarless Symmetrical Low-Voltage Ride Through Based on Flux Linkage Tracking For Brushless Doubly Fed Induction Generators

The significant benefits of brushless doubly fed induction generators (BDFIGs) are higher reliability and lower maintenance cost due to the lack of a brush gear. Recent studies on BDFIGs have shown promising applications in wind power generation. The low-voltage ride-through (LVRT) capability of wind turbines is one of the most important properties specified in the grid codes of many countries. A crowbarless LVRT control strategy based on flux linkage tracking for a BDFIG under symmetrical voltage dips is proposed in this article. A novel and simple controller in static reference frame is presented to implement flux linkage tracking. To both suppress the control winding (CW) current and increase power winding (PW) reactive current, a time-sharing control and CW current-first control solution is proposed. In the most severe case, where the BDFIG rotor speed is 100% of the maximum rotor speed and under 100% symmetrical voltage dips, the experimental results show that the CW current peak can be limited to 2.01 p.u. of the CW current rating and the torque ripple is small during the fault. The experimental results also show the possibility of the BDFIG to ride through the severest voltage dips without extra hardware, such as a crowbar.