Stand-alone Doubly-fed Induction Generator Research Articles

Maximum allowable hp rating of 3-phase induction motor fed through a stand-alone constant V/f controlled DFIG via RSC

This paper presents a scheme to start three-phase induction motors (IMs) directly connected to terminals of constant V/f controlled doubly fed induction generator (DFIG) through the rotor side converter (RSC). The proposed control is achieved by controlling the output voltage and frequency of stand-alone DFIG via controlling an injected voltage into the rotor circuit through the RSC. The control scheme provides a search for maximum rating of the three-phase IM which can be supplied from a DFIG. The search technique is based on using a simplified mathematical model to find the capability limits of the RSC and DFIG. It is found that these parameters depend on the stator frequency and rotor slip. Therefore, an investigation is performed to find the lowest frequency and the corresponding allowable maximum rating for the IM to be safely started. A typical example is provided in the paper for a 15 kW DFIG. It is shown that this generator could supply a three-phase IM with a maximum rating of 1-hp if it operated at nominal outputs, voltage and frequency, during start-up period. While, using the proposed technique, the same generator could start-up a three-phase IM with maximum power rating of 7.25 hp.

Direct slip angle control for rotor side converter of standalone DFIG‐DC system

This paper presents a direct slip angle control (DSAC) for rotor side power converter of standalone doubly-fed induction generator (DFIG) DC systems. The conventional direct torque control (DTC) for the rotor side converter (RSC) of the standalone DFIG-DC system is also introduced to evaluate the proposed DSAC compared to the conventional DTC method. In this regard, the proposed DSAC method is mathematically analyzed. Then, three states including output DC voltage change, wind speed change, and load change are simulated and performed for both the proposed DSAC and conventional DTC methods. Various simulations and experiments substantiate that the electromagnetic torque ripple, current harmonics, and the output DC voltage dip, caused by the sudden load-change, are reduced in the proposed DSAC method compared to the conventional DTC method. Besides, the parameter dependency is reduced in the proposed DSAC method since the stator voltage and current sensors are removed.

Robust Observer Design for Sensorless Voltage and Frequency Control of a Doubly Fed Induction Generator in Standalone Mode

The paper proposes a rotor speed observer for sensorless voltage and frequency control of a standalone doubly fed induction generator (DFIG) system. The proposed speed estimator is designed based on the sliding mode observer (SMO) technique, which makes the estimation algorithm robust under parametric variations and external disturbances. The position of the rotor is computed directly by the integration of the estimated speed by the designed observer. The proposed observer requires the measurements of stator voltage and current to estimate the speed. The design procedure of the observer does not require the mechanical parameters of the DFIG, which makes the observer design independent of the mechanical parameters. Lyapunov stability criteria are used to formulate the sliding gains for the observer. Simulation and experimental results are demonstrated to show the efficacy of the designed observer in terms of accuracy and convergence rate under different steady-state and dynamic conditions. The estimated rotor speed and position are utilized for controlling the magnitude of the stator voltage of a standalone DFIG system, which validates the performance of the observer.

Starting of induction motor fed with stand-alone DFIG

This paper presents dynamic simulation and control of stand-alone doubly fed induction generator (DFIG) loaded with 3-phase induction motors (IMs). The study reveals that direct on-line starting of large IMs causes a large voltage sag across the generator terminals as the starting current drawn reaches up to 8-9 times the rated load current. Traditionally, this problem has tackled by oversizing of the generator or employment of special starters, under the pretext of mitigating voltage sag. This work explores ways that the starting current can be reduced economically by applying constant V/f control side by side with indirect field-oriented control (FOC) applied on the rotor side converter of the DFIG. This methodology enables starting of larger IMs and mitigation of voltage sag that occurs during the start-up period. Two different rating of IMs loaded with speed-squared mechanical torque are mainly considered. Simulation results of the system behavior under study confirm the capability of the proposed control.

Improved Voltage Harmonics Compensation Strategy for a Stand-Alone DFIG Operating under Unbalanced and Nonlinear Load Conditions

The study presents an improved control and operation for a stand-alone Doubly-Fed Induction Generator (DFIG) energy system under unbalanced and nonlinear load conditions. This type of loads results in unbalance and distorted stator voltage and current at the Point of Common Coupling (PCC), with subsequent power quality degradation and electromagnetic torque pulsations of DFIG. A new and simple control strategy based Proportional-Resonant Controllers (PRCs) in the stationary reference frame is applied in the Load-Side Converter (LSC) for fully compensate the unbalance and harmonic distortion in the PCC and avoid the torque pulsations of DFIG. As a result, symmetrical and sinusoidal AC voltage is applied to the PCC, which indicates that the Rotor-Side Converter (RSC) can be still controlled with the traditional vector control strategy, to adjust the generator torque and to balance active power. This strategy is conceptually simple, it does not require for rotating coordinate transformation and the sequence decomposition process, in addition no current sensors need to be installed at load or stator side, so that the number of current sensors employed is reduced. Simulation results using MatLab/Simulink are given and show the effectiveness and validity of the proposed control scheme to eliminate the harmonics in the PCC and torque pulsations of DFIG.

Stator Flux Based Model Reference Adaptive Observers for Sensorless Vector Control and Direct Voltage Control of Doubly-Fed Induction Generator

A position sensorless operation of a doubly-fed induction generator (DFIG) is desired from considerations of cost, maintenance, cabling, and reliability. This article proposes two stator flux based model reference adaptive observers (SF-MRAOs) for the estimation of the rotor speed and position in a stand-alone DFIG. The cross product of the reference and estimated stator flux linkage vectors is fed as controller input in the existing method, thereby, making the observer model nonlinear. The first proposed method, which is based on notch filtering, offers a performance comparable to that of the existing method while reducing the number of voltage and current sensors required. The second proposed method linearizes the error input to the controller, making the controller design straight forward, and also leading to superior performance than the existing methods. The linearized SF-MRAO is shown to work successfully with good dynamic performance in vector control of a stand-alone DFIG. This article also shows that the proposed linearized SF-MRAO can be employed for obtaining the rotor current angle in direct voltage control of a stand-alone DFIG. Simulation and experimental results are presented from a laboratory prototype of a 10-hp DFIG coupled to a squirrel-cage induction motor. The sensitivity of the performance of the proposed linearized SF-MRAO to variations in machine parameters is also studied theoretically and experimentally.

Experimental behaviour analysis for optimally controlled standalone DFIG system

Metaheuristic optimisation techniques such as the Grey Wolf optimiser (GWO) and artificial bee colony (ABC) algorithms have been developed for enhancing the dynamic behaviour of wind energy conversion system. The stand-alone doubly fed induction generator (DFIG) control system based on the direct voltage control is experimentally validated for the robust independent control of the stator voltage amplitude and the consequent rotor current regulation. The GWO and ABC are used for selecting the optimal gains of the proportional-integral (PI) regulator to improve the dynamic performance and the robust stability of the DFIG system in the presence of step voltage variation and sudden load operation. Through, MATLAB™/Simulink numerical simulations, the dynamic performances of the GWO-PI and ABC-PI applied to stand-alone DFIG systems are compared with the conventional PI controller under different disturbances. Using a 3 kW DFIG test bench DSPACE DS1104 card prototype, the experimental validation justifies the superiority of proposed novel GWO-PI and ABC-PI controllers over the conventional PI control in terms of steady-state errors, maximum overshoots, settling time and rise time.

Direct voltage regulation of a stand‐alone DFIG system with non‐linear loads based on an improved‐extended state observer and SSM control

This study presents a direct voltage control strategy for a stand-alone doubly fed induction generator (DFIG) subject to non-linear loads. The voltage regulator employs a super-twisting sliding mode (SSM) control to directly calculate the rotor voltage reference with the advantage of mitigating the voltage chattering and omitting the current control loops. On the basis of the harmonic-eliminating principle under non-linear loads, an improved extended state observer is proposed and applied to estimate the model-related part in the SSM control law. The advantages are that no extra sequence decomposition operations and machine parameters are required; therefore, enhancing the robustness and simplifying the structure of the controller. Finally, the proposed method is applied to a 6 kW DFIG-based hardware system under varying speed conditions. Both simulation and experiment results are given to validate the harmonic-eliminating performance of the proposed method for the stand-alone DFIG system.

Direct voltage control of stand‐alone DFIG under asymmetric loads based on non‐singular terminal sliding mode control and improved extended state observer

This study investigates direct voltage control of stand-alone doubly-fed induction generator (DFIG) system subject to asymmetric loads. An improved voltage regulator is designed based on non-singular terminal sliding mode control and an improved extended state observer (ESO). Due to the salient features of the improved ESO, the rotor current transducer is not required and the sinusoidal fluctuation in the sliding surface caused by asymmetric loads is eliminated. As a result, satisfactory balanced stator voltage can be generated under asymmetric loads. The proposed method is characterised by the rapid dynamic response and the elimination of sequence decomposition operations. Finally, the proposed method is applied to a 6 kW DFIG-based hardware system under various loads and varying speed conditions. Both simulation and experiment results are given to validate the effectiveness and robustness of the proposed method for stand-alone DFIG system.

Stand-Alone DFIG FOC Sensitivity and Stability Under Mismatched Inductances

Inaccurate machine parameters can cause orientation errors and instability in field-oriented control schemes relying on model-based estimations. This paper analyzes the accuracy and stability of two field-oriented control schemes for a stand-alone doubly fed induction generator (DFIG), where the field orientation is affected by the stator inductance and stator/mutual inductance ratio. After deriving a reduced-order model accounting for parameter mismatch, the paper deduces a formula to calculate the field orientation error as a function of the inductance mismatch, revealing parameter sensitivities. A stability analysis is then carried out proving that overestimating the stator/magnetizing inductance ratio may trigger instabilities in case of a high load level, whereas underestimation allows always stable operation. The theoretical insight is supported with simulation and test results on a laboratory rig.

Sensorless Frequency and Voltage Control in the Stand-Alone DFIG-DC System

A sensorless stand-alone control scheme of a doubly fed induction generator (DFIG)-DC system is investigated in this paper. In this layout, the stator voltage is rectified by a diode bridge that is directly connected to a dc bus. The rotor-side voltage source inverter is the only controlled converter required in this system and is directly powered by the same dc bus created by the stator-side rectifier. DC voltage and stator frequency are regulated by two independent proportional-integral regulators that give the references for inner current controllers implementing field-oriented control. As it is capable of creating a stable and regulated dc bus, this system can be conveniently adopted to supply dc loads or to form a dc grid. Due to the constraint imposed by the stator diode bridge, the DFIG has to operate under a constant stator voltage, and the conventional stator field-oriented control implemented in stand-alone ac DFIG must be modified. This paper presents the control structure and the theoretical framework for the controller synthesis. Simulation and experimental validations on a small-scale rig are included.

Wind-driven stand-alone DFIG with battery and pumped hydro storage system

A wind-driven doubly fed induction generator (DFIG) along with the battery and pumped hydro storage plant (PHSP) has been devised for supplying isolated loads. PHSP-based storage system is economical and viable for the MW level wind-turbine system. The proposed scheme employs a squirrel-cage induction machine (SCIM) coupled with reversible pump turbine for PHSP. The battery storage is also included in this system to cope up with the intermittent nature of wind and fast-changing load. A simple control strategy has been implemented for maintaining the set values of voltage magnitude and frequency at the stator terminals of DFIG, which serve as a virtual grid for connecting ac loads and SCIM. Based on the availability of power in the wind, PHSP and battery, various operating modes of the proposed system have been clearly identified for supplying the isolated loads. These operating modes are clearly demonstrated through the analysis developed for this purpose and validated through experimental results. The salient features of the proposed system over the existing stand-alone wind-driven generators are (i) structural simplicity, i.e., employing only one power electronic converter, (ii) wide speed operation of wind-driven DFIG, (iii) reduced battery capacity, (iv) high energy storage using PHSP and (v) availability of continuous power to the isolated loads.

Experimental Implementation of Doubly Fed Induction Generator-Based Standalone Wind Energy Conversion System

This paper presents the advantages of integrating battery energy storage system (BESS) with doubly fed induction generator (DFIG)-based standalone wind energy conversion system (SWECS). A comparison between DFIG-based SWECS with and without BESS is presented in this paper. Control strategies for both rotor-side converter (RSC) and load-side converter (LSC) are presented for regulating voltage and frequency for variable-speed DFIG-based SWECS. By integrating BESS, this proposed standalone DFIG is able to extract maximum power. A prototype of this DFIG-based SWECS is developed using a digital signal processor (DSP)-dSPACE-DS1103. This standalone DFIG is tested under nonlinear and dynamic loads in both the cases. Dynamic performance of proposed standalone DFIG is also verified for variable wind speeds.

Harmonics Rejection in Stand-Alone Doubly-Fed Induction Generators With Nonlinear Loads

This letter proposes a novel method to reject $6n \pm 1$ harmonics in the stator voltage at the point of common coupling (PCC) for a stand-alone doubly-fed induction generator (DFIG) supplying a balanced three-phase diode rectifier. These harmonics are rejected by using the rotor current controller in the fundamental synchronous reference frame. The proposed control algorithm is based on a proportional integral controller and a bank of resonant filters. In this frame, each resonant filter in the rotor current controller has the possibility of rejecting one pair of the positive and the negative stator harmonic voltages at the PCC. Experimental results with 2.2-kW DFIG are shown in this letter.

Direct Voltage Control With Slip Angle Estimation to Extend the Range of Supported Asymmetric Loads for Stand-Alone DFIG

The control system for stand-alone doubly fed induction generator (DFIG) is commonly based on the “sensorless” direct voltage control. This paper adopts the direct voltage control and employs negative-sequence compensation through rotor-side converter to support asymmetric “unbalanced” loads. This paper demonstrates the limitation of the conventional direct voltage control to obtain the slip angle, and the consequent limitation of the negative-sequence compensation control to support full range of asymmetric loads. To overcome these limitations, this paper proposes a new estimator of the angle of the rotor current in the synchronous reference frame. The proposed estimator requires one DFIG parameter only which is the stator inductance, which can be measured allowing using real machine parameter. This paper proposes a simple method to integrate the proposed estimator into the direct voltage control, which enables it to obtain the slip angle, and it enables the negative-sequence compensation to support full range of asymmetric loads. Experimental results for a 1.1-kW prototype DFIG are provided to validate the proposed method and to demonstrate its advantages over the conventional direct voltage control.

Analysis and compensation of voltage unbalance of a DFIG using predictive rotor current control

This paper analyzes the unbalance problem of a stand-alone doubly-fed induction generator (DFIG) under unbalanced loads and proposes a compensation method to balance the stator output voltage. The proposed compensation method is developed based on a predictive current control (PCC) method implemented in the rotor current controller. The development of the PCC is based on the discrete model of the DFIG to predict an appropriate average rotor voltage vector to eliminate the rotor current error in the following switching period. The identified rotor voltage vector is then applied to the rotor-side converter (RSC) by using space-vector modulation (SVM) with constant switching frequency. To improve the control performance, a compensation method for time delay based on the prediction of the future rotor current at the end of current sampling period also is investigated. The proposed control scheme was tested by experiments with 2.2 kW DFIG to demonstrate its excellent steady-state performance as well as extremely fast dynamic response.

Analysis and Control of Wind-Driven Stand-Alone Doubly-Fed Induction Generator with Reactive Power Support from Stator and Rotor Side

Hybrid-excited system has been formulated for the operation of wind-driven Doubly-Fed Induction Generators (DFIGs) for stand-alone power supplies. Here, apart from the rotor side excitation of DFIG employing SPWM inverter supplied through set of batteries, the generator is partially excited by fixed capacitor at the stator terminals. The steady-state equivalent circuit and its analysis for the proposed system have been devised and presented. The selection of stator capacitor has clearly been brought out. An attempt is also made for sizing of various components such as inverter and battery for the given machine rating and other operating conditions. Phasor diagram representation of the proposed system for typical operating conditions is also furnished. The stator voltage magnitude and frequency are maintained constant by closed loop control developed using TMS320LF2407A Digital Signal Processor (DSP) controller. Efficacy of the proposed hybrid-excited stand-alone DFIG system has been demonstrated under steady-state and dynamic operating conditions with experimental results obtained on a 3-phase, 4-pole, 415 V, 50 Hz, 5 kVA generator in the laboratory.

A novel voltage and frequency controller for standalone DFIG based Wind Energy Conversion System

This paper presents a new speed-sensorless control strategy for a stand-alone doubly-fed induction generator supplying energy to an isolated load. The method is based on the root mean square (rms) detection scheme. The generated stator voltage is controlled via rotor currents. Amplitude of stator voltage and its frequency are controlled simultaneously. The output signals from the voltage controllers are the reference signals for the rotor current amplitude and frequency of the stator voltage is regulated with the help of frequency control loop. This developed direct voltage control method is applicable for both the balanced and unbalanced load and also for standalone and grid connected mode. The control pulses for the rotor side converter is supplied by the hysteresis controller which is operated on the error signal calculated between actual and reference rotor currents. This paper also provides the brief idea about the voltage and frequency control on modern Autonomous DFIG based Wind Energy Systems via single phase mathematical model of standalone DFIG system. A short state-of-the-art review on mechanical position/speed sensorless control schemes for autonomous DFIG based WESs is presented, which helps the present researcher and students working in this area. These include stator flux oriented control techniques; direct voltage control techniques; MARS observer based techniques for autonomous DFIG-based variable-speed WESs. Simulation results obtained from a 2MVA DFIG system, prototype in MATLab/Simulink, are presented and discussed in this paper.

Harmonic Compensation With Zero-Sequence Load Voltage Control in a Speed-Sensorless DFIG-Based Stand-Alone VSCF Generating System

Doubly fed induction generator (DFIG)-based stand-alone variable-speed constant-frequency systems reported so far cannot supply nonlinear unbalanced load current containing cophasor components. This paper presents three topologies of a stand-alone DFIG which can handle this type of load. The harmonic/imbalance component of the load current in each case is supplied by the stator-side converter. In the first option, a Δ/ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Y</i> transformer with neutral connection supplies the load. However, the harmonic current circulating in the delta winding still distorts the load phase voltage beyond the acceptable limit. As a solution, the zero-sequence load voltage is actively controlled through the stator-side converter with the load neutral connected to the midpoint of the dc-link capacitor. However, this configuration also fails when the load current contains dc component as in half-wave rectifiers. Finally, a configuration with an additional inverter leg connected to the load neutral is proposed in conjunction with the zero-sequence stator voltage controller. Simulation and experimental results from this last configuration demonstrate its excellent load voltage regulation property while supplying various nonlinear and unbalanced loads. The control algorithm is implemented without any speed/position sensor.

Intelligent control of doubly‐fed induction generator systems using PIDNNs

AbstractAn intelligent control for a stand‐alone doubly‐fed induction generator (DFIG) system using a proportional‐integral‐derivative neural network (PIDNN) is proposed in this study. This system can be applied as a stand‐alone power supply system or as the emergency power system when the electricity grid fails for all sub‐synchronous, synchronous, and super‐synchronous conditions. The rotor side converter is controlled using field‐oriented control to produce 3‐phase stator voltages with constant magnitude and frequency at different rotor speeds. Moreover, the grid side converter, which is also controlled using field‐oriented control, is primarily implemented to maintain the magnitude of the DC‐link voltage. Furthermore, the intelligent PIDNN controller is proposed for both the rotor and grid side converters to improve the transient and steady‐state responses of the DFIG system for different operating conditions. Both the network structure and online learning algorithm are introduced in detail. Finally, the feasibility of the proposed control scheme is verified through experimentation.Copyright © 2011 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society