Stator Power Research Articles

Torque controller of a doubly‐fed induction generator impelled by a DC motor for wind system applications

The authors propose robust non-linear controllers for a wind system. The proposed controllers are based on a combination of the block control linearisation and super-twisting algorithm, as a second-order sliding mode technique. The main control loop is designed to regulate the electromagnetic torque and stator power factor of a doubly-fed induction generator which is connected to the grid. The generator is mechanically coupled with a DC motor, which can emulate the wind turbine operation. Therefore, an auxiliary control loop is designed to control the DC motor velocity. Additionally, a grid-side converter controller is proposed to regulate the DC-link voltage into AC/DC/AC converter, and displacement factor of the energy flow between the rotor and the grid. A robust stability analysis of the complete closed-loop system under external disturbances is presented. The robustness of the proposed control schemes is validated in real time using a workbench, which consists of a motor-generator group, AC/DC/AC electronic drive and dSPACE DS 1104 controller boards.

Hybrid Sliding Mode Control of DFIG with MPPT Using Three Multicellular Converters

This paper deals with hybrid sliding mode control of Doubly Fed Induction Generator DFIG with Maximum Power Point Tracking MPPT connected by rotor side to three bridges of Multicellular Converters MCCs. The hybrid aspect of the converters is taken into consideration which includes the continuous and discrete states of the converters. The vector control is used to command the DFIG speed and reactive stator power. The currents in Park d-q reference are controlled using hybrid sliding mode. The sliding surfaces are developed using Lyapunov stability method. The developed controller allows decoupled control of the stator active and reactive power. The final results are illustrated at the end of this paper to present the advantages of the control method developed in this paper.

Decoupled Direct Control of Natural and Power Variables of Doubly Fed Induction Generator for Extended Wind Speed Range Using Feedback Linearization

The electromagnetic torque, stator power, and the grid side converter power outputs of a doubly fed induction generator (DFIG) are differentiated to obtain a feedback linear relationship with the rotor voltage vectors serving as the control inputs. The derived model is such that can allow for a transition of DFIG to operate at low wind speed by shorting its stator terminal to one another. The torque and power quantities are the state variables of the system, which are insensitive to coordinate system orientation. Steady-state models are used to obtain optimal operating regimes with respect to minimal electrical losses. Decoupled torque control is developed by field orientation, while decoupled power control is derived from voltage orientation of the DFIG. Transients originating from transitioning into low wind speed operation are investigated. Robustness of each of the decoupled controllers against parameter variations is investigated by locating the poles of its closed-loop transfer function. The results for a 5-Hp machine are presented.

Hybrid Fuzzy Sliding Mode Control of a DFIG Integrated into the Network

This paper presents the study of a variable speed wind energy conversion system using a Doubly Fed Induction Generator (DFIG) based on a Fuzzy sliding mode control (FSMC) applied to achieve control of active and reactive powers exchanged between the stator of the DFIG and the grid to ensure a Maximum Power Point Tracking (MPPT) of a wind energy conversion system. However the principal drawback of the sliding mode, is the chattering effect which characterized by torque ripple, this phenomena is undesirable and harmful for the machines, it generates noises and additional forces of torsion on the machine shaft. In order to reduce the chattering effect, the Sign function of sliding mode controller’s discontinuous part is replaced by a fuzzy logic; we will have the fuzzy sliding mode controller (FSMC). The FSMC makes it possible to combine the performances of the two types of controllers (SMC and FLC) and eliminates the chattering effect. The proposed control algorithm is applied to a DFIG where the stator is directly connected to the grid and the rotor is connected to a three-level converter structure NPC to suppress low level harmonics, higher frequencies will be filtered out by the machine. Second goal of this paper is to extract a maximum of power; the rotor side converter is controlled by using a stator flux-oriented strategy. The decoupling created by the control between active and reactive stator power allows keeping the power factor close to unity. Simulation results show that the wind turbine can operate at its optimum energy for a wide range of wind speed. Both simulation and validation results show effectiveness of the proposed control strategy is in terms of power regulation. Moreover, the fuzzy sliding mode approach is arranged so as to reduce the chattering produced in the generated power that could lead to increased mechanical stress because of strong torque variations. DOI: http://dx.doi.org/10.11591/ijpeds.v3i4.4072

Analysis on Power Output Capability and its Power Control Strategy of DFIG Wind Turbine under Unbalanced Grid Voltage

Wind turbines are required by gird codes that active and reactive power should be fed into power grid during gird faults such as voltage sags. Power output capability is important for DFIG wind turbine to provide active and reactive power support. Constraints of stator, rotor maximum current and maximum output voltage of rotor-side converter (RSC) of DFIG are considered to analyze stator power output limit of the DFIG under unbalanced voltage sags, and newton-raphson method is used to solve the power limits, then the power control strategy based on power limit analysis is proposed. Simulation, calculation and comparison are carried out by a 1.5MW DFIG wind turbine model, and the effectiveness of the analysis and the proposed control strategy is verified.

Performance Enhancement of PMSG Systems with Control of Generator-side Converter Using d-axis Stator Current Controller

This paper presents a performance enhancement of a permanent magnet synchronous generator (PMSG) system with control of generator-side converter for a wind turbine application. This method uses zero d- axis stator current control to minimize winding losses of the generator. The electromagnetic torque of the generator is correlated with the magnitude of the qaxis stator current, while the daxis stator current is regulated at zero, the control scheme decouples the dq-axis stator current control through a vector con- trol for the generator-side converter. This paper also presents mathematical analysis of the active power and stator power factor, and the maximum power point tracking (MPPT) operation. Simulation re- sults are provided to guarantee the proposed control scheme, in which the performance enhancement and eciency are evaluated.

Robust control based on the Lyapunov theory of a grid-connected doubly fed induction generator

This paper discusses the robust control of a grid-connected doubly-fed induction generator (DFIG) controlled by vector control using a nonlinear feedback linearization strategy in order to ameliorate the performances of the control and to govern the developed stator active and reactive power in a linear and decoupled manner, in which an optimal operation of the DFIG in subsynchronous operation is given, as well as the control stator power flow with the possibility of keeping stator power factor at a unity. The use of the state-all-flux induction machine model gives place to a simpler control model. So, to achieve this objective, the Lyapunov approach is used associated with a sliding mode control to guarantee the global asymptotical stability and the robustness of the parametric variations.

Determination of Steady-State and Dynamic Control Laws of Doubly Fed Induction Generator Using Natural and Power Variables

A doubly fed induction generator model is presented whereby the natural and power variables are the state variables. The natural variables are the electromagnetic torque (Te), the reactive torque (Tr), the magnitude of the rotor flux linkage (λr), the magnitude of the stator flux linkage (λs), and the rotor speed (ωr). The power variables are the real power (Pf) and reactive power (Qf) generated/absorbed by the grid-side converter into/from the grid. Simulation of the dynamic natural variable model of the induction machine is compared with a vector variable simulation. Steady-state operating regions are established for various power factor operations. The optimal stator power factor operation is estimated. A direct control of torque and power variables is developed. The robustness of the developed control against rotor parameter variation is investigated using small signal analysis and is compared with vector control. Results are shown for a 5-hp machine.

A unified power electronic controller for wind driven grid connected wound rotor induction generator using line commutated inverter

The implementation of a simple power converter for a wound rotor induction generator employing a three phase diode bridge rectifier and a line commutated inverter in the rotor circuit for super synchronous speeds has been proposed. The detailed working of the system in power smoothing mode and maximum power point tracking mode is presented. The current flow in the rotor circuit is controlled (by controlling the firing angle of the line commutated inverter) for controlling the stator power in both the modes. An 8 bit PIC microcontroller has been programmed to vary the firing angle of the line commutated inverter. Experiments have been carried out on a 3-phase, 3.73 kW, 400 V, 50 Hz, 4-pole, 1500 r/min wound rotor induction generator and the results obtained with the generator supplying power in both the modes are furnished. The complete scheme has been modeled using MATLAB/SIMULINK blocks and a simulation study has been conducted. The experimental waveforms are compared with the simulation results and a very close agreement between them is observed.

Application of fuzzy logic control algorithm as stator power controller of a grid-connected doubly-fed induction generator

This paper discusses the power outputs control of a grid-connected doubly-fed induction generator (DFIG) for a wind power generation systems. The DFIG structure control has a six diode rectifier and a PWM IGBT converter in order to control the power outputs of the DFIG driven by wind turbine. So, to supply commercially the electrical power to the grid without any problems related to power quality, the active and reactive powers (Ps, Qs) at the stator side of the DFIG are strictly controlled at a required level, which, in this paper, is realized with an optimized fuzzy logic controller based on the grid flux oriented control, which gives an optimal operation of the DFIG in sub-synchronous region, and the control of the stator power flow with the possibility of keeping stator power factor at a unity.

Performance Study of Brushless Doubly-Fed Machines in Variable Speed Constant Frequency Wind Power Generation

The brushless doubly-fed machine (BDFM) is a new type of induction machine with good application in wind power generation. In this paper, the design method for the stator winding and rotor winding of BDFM in detail on the basis of operation principle of the machine is introduced. As an example, The performance simulation for a 480kW cage rotor BDFM with 8-pole stator power winding and 4-pole control winding can be considered under the condition of no-load, rated load and variable speed constant frequency operation based on field-circuit coupled finite element analysis. Simulation results show that the design method has good performance and is feasible in variable speed constant frequency wind generation systems.

Improvement of dynamic performance of doubly fed induction generator-based wind turbine power system under an unbalanced grid voltage condition

With the current increase in wind power penetration into the energy market, control and operation of wind turbine generators becomes a major research topic. Wind turbine based on doubly fed induction generator (DFIG), which is sensitive to grid disturbances, is widely used. Under an unbalanced grid voltage condition, oscillations of the DFIG's electromagnetic torque and instantaneous stator powers strongly affect the dynamic performance of the DFIG. In this study, a new configuration based on vector proportional'integral (VPI) controller is proposed to eliminate such oscillations. This new configuration is employed in the rotor side converter (RSC) of the DFIG. With the proposed VPI control strategy, decomposition of sequential components and mathematical complexity are reduced. Compared with the conventional field-oriented control based on the standard single PI controller, the VPI controller can successfully eliminate torque and stator power oscillations. The effectiveness of the proposed control strategy is validated through simulation results obtained on a 1.5'MW DFIG-based wind turbine system model built in MATLAB/Simulink.

Maximal Wind Energy Tracing of Brushless Doubly-Fed Generator under Flux Oriented Vector Control

The aim of this paper is to present the complete modeling and simulation of a grid-connected brushless doubly fed induction generator (BDFG) for variable wind energy conversion is used to assess a maximum power point tracking MPPT strategy for different velocities. The decoupling control of active and reactive powers for BDFG has been developed using PI controllers. The performance of proposed stator power winding flux oriented vector control is examined. The complete system is simulated in the Matlab/Simulink environment and the computer simulation results obtained confirm the effectiveness and validity of MPPT strategy.

DFIG Power Generation Capability and Feasibility Regions Under Unbalanced Grid Voltage Conditions

Several strategies have been proposed for operating doubly-fed induction generator (DFIG)-based wind turbine under unbalanced grid voltage conditions. This study focuses on those strategies in which the rotor-side power converter aims at eliminating the oscillations affecting the electromagnetic torque and the stator reactive power. Given the limited size of the DFIG's power converters, and hence their tolerable current and voltage boundaries, this study analyzes which DFIG power generation capability is under unbalanced grid voltage, and therefrom derives rotor current and stator power controllable ranges. Besides, the feasibility regions of the DFIG are also deduced for different types of imbalance. Furthermore, and based on the outcome of previous analysis, a modified rotor current limiter, as also its equivalent stator power limiter, is proposed. In contrast to the conventional ones, the limiters proposed here take into account that imbalances may arise in the grid voltage. As a consequence, system's overall performance is considerably enhanced under unbalanced grid voltage conditions. Finally, simulation results establish the validity of the treated issues.

A Fault Ride-Through Technique of DFIG Wind Turbine Systems Using Dynamic Voltage Restorers

This paper proposes a low-voltage ride-through technique of a doubly fed induction generator (DFIG) wind turbine system using a dynamic voltage restorer (DVR). For effective control of the DVR, digital all-pass filters are used for extracting the positive-sequence component from the unbalanced grid voltage since they have the advantages of giving a desired phase shift and no magnitude reduction over conventional low- or high-pass filters. Using the positive-sequence component, the phase angles for the positive- and negative-sequence components of the grid voltage are derived. A control algorithm for the DVR that is dual voltage controllers only is implemented for the two sequence components in the dq synchronous reference frame. In order to achieve the power rating reduction of the DVR, the stator power reference for the DFIG is reduced during faults. In addition, a control scheme of pitch angle system is applied to stabilize the operation of the wind turbine system in the event of grid faults. PSCAD/EMTDC simulations show the effectiveness of the proposed technique and a feasibility of reducing the power rating of DVR for the fault ride-through capability of DFIG. The validity of the proposed control scheme for the DVR has also been verified by experimental results.

Effect of Rotor Excitation Voltage on Steady-State Stability and Maximum Output Power of a Doubly Fed Induction Generator

The effect of rotor excitation voltage on steady-state stability and output power of a doubly fed induction generator (DFIG) is investigated. Wind turbine mechanical torque-speed characteristics, as well as generator electromagnetic torque-speed characteristics, are considered in the derivation of the steady-state stability region for the magnitude and phase angle of the rotor excitation voltage. To evaluate the maximum output power of a DFIG, detailed expressions for stator power, rotor power, stator loss, rotor loss, and electrical power are derived as functions of the generator speed and the magnitude and phase angle of the rotor excitation voltage. The effect of stator resistance on the magnitude and phase angle of the resultant optimal rotor excitation voltage, which gives maximum output power and minimum loss, is examined. Experimental results are given to demonstrate the effectiveness of the theoretical analysis.

Model-Based Predictive Direct Power Control of Doubly Fed Induction Generators

This paper presents a predictive direct power control strategy for doubly fed induction generators (DFIGs). The method predicts the DFIG's stator active and reactive power variations within a fixed sampling period, which is used to directly calculate the required rotor voltage to eliminate the power errors at the end of the following sampling period. Space vector modulation is then used to generate the required switching pulses within the fixed sampling period that results in a constant switching frequency. The impact of sampling delay on the accuracy of the sampled active and reactive powers is analyzed, and detailed compensation methods are proposed to improve the power control accuracy and system stability. Experimental results for a 1.5-kW DFIG system demonstrate the effectiveness and robustness of the proposed control strategy during power steps, and variations of rotating speed and machine parameters. System performance for tracking varying stator power references further illustrates the dynamic performance of the proposed method.

Direct Torque Control of Doubly Fed Induction Generators Connected to Grids with Unbalanced Voltage

This article describes a direct control method for doubly fed induction generators that are connected to networks with unbalanced grid voltage. As the proposed method employs direct feedback of the torque and reactive power of the doubly fed induction generators in the control of power generation, it requires less computational power than methods using cascaded control loops. In the proposed algorithm, the feedback and control variables are separated into symmetrical components; they are independently controlled to i) realize the minimization of the undesirable stator power ripples or torque ripples arising from the unbalanced voltages in the three-phase supply and ii) regulate the torque and reactive power independently at variable speeds. The proposed method is validated by computer simulation and experimental results.

Analysis of Three-phase Induction Motor Performance under Different Voltage Unbalance Conditions Using Simulation and Experimental Results

The performance of a three-phase squirrel cage induction motor fed by an unbalanced voltage supply has been investigated in this article. Matlab/Simulink software is used for simulation of the motor using simple (conventional) and exact models under different unbalanced voltage conditions. Simulation results are verified by performing experimental tests on a 1.5-kW standard three-phase induction motor. With the aid of simulation and experimental results, variations of stator current, motor efficiency, power factor, and rotor ripple have been investigated under various steady-state unbalanced voltage conditions. It is shown that, in addition to the voltage unbalance factor, the magnitude of positive and negative sequence components of the unbalanced supply voltage have an important role on motor performance. For instance, motor power factor decreases with the increase of positive sequence voltage component, even if the voltage unbalance factor is constant. Also, the motor efficiency decreases with the increase of the voltage unbalance factor, even if the positive sequence component of the supplied voltage is constant. It is shown that the stator current unbalance factor and motor losses will increase (decreasing efficiency) with the increase of the voltage unbalance factor under constant load conditions. Moreover, the voltage unbalance factor has a negative impact on the electromagnetic torque peak due to the decrease in the positive sequence voltage component. Using different voltage unbalance factors, the motor derating factor is also calculated, where four different limits are considered. The results imply conservative limits set by standards for motor operation under unbalanced voltage conditions. A comparison between the experimental and exact model simulation results confirmed a maximum discrepancy of 4% between the results, indicating the validity of the employed exact model for motor analysis in unbalanced voltage conditions.

CONTROLLER VALIDATION AND VERIFICATION FOR A DOUBLY-FED VARIABLE SPEED WIND GENERATOR SYSTEM

The paper deals with a variable speed wind turbine fitted with a DFIG (Doubly-Fed Induction Generator) vector control system. A new rotor active power feedback, under the cascade regulation superordinated both to the DFIG rotor current internal feedback and the DFIG stator power external feedback has been introduced. The DFIG vector control system fitted with an external rotor active power feedback enables the wind turbine rotation speed to be controlled by means of the rotor active power reference. By introducing a rotor active power feedback into the DFIG vector control system, a power flow control in the generator rotor circuit is rendered possible in both stationary and dynamic wind turbine operation modes. The DFIG vector control structure has proved to be particularly suited to wind turbines with pronounced wind speed time changes. The paper also discusses the wind turbine dynamic characteristics with and without a feedback rotor active power within the DFIG vector control system at varying and sudden variations in rotation speed.