Rotor Current Vector Research Articles

Doubly Fed Induction Machine-Based DC Voltage Generator with Reduced Oscillations of Torque and Output Voltage

The doubly fed induction machine (DFIM)-based DC voltage generator is equipped with a stator-connected diode rectifier. The six-pulse diode rectifier as a nonlinear circuit introduces harmonics in the stator and rotor current and distorts the machine stator voltage, as well as the stator flux. This causes electromagnetic torque oscillations and instantaneous power components oscillations. The torque oscillations adversely impact the mechanical parts of the drive-train and oscillations of the p component of instantaneous power influence DC-bus voltage oscillations. The oscillations can be somewhat cancelled by control methods. However, cancellation of electromagnetic torque is not strictly coupled with cancellation of oscillations of the p component of instantaneous power. The paper presents an analysis of influence of the control methods aimed at a reduction of torque oscillations on the output voltage oscillations level in the stand-alone DFIM-based DC voltage generator. Field-oriented control FOC with current controllers and space vector modulation-based direct torque control DTC-SVM with flux module regulation have been compared with control in which electromagnetic torque is one of the commanded variables, whereas the second variable is the dot product of stator flux and rotor current space vectors. The contributions of this paper are the introduction of a new variable in the second control path in the DTC-SVM method instead of flux vector length and the proof that it can reduce torque and DC-bus voltage oscillations in the DFIG-DC system. Additionally, this paper reveals that for proper stator-to-rotor-turns ratio of a doubly fed machine necessary for reduction of the rotor converter power, lower DC-bus voltage can be obtained than is required for full realization rotor side voltage requested by rotor current controllers. This is the reason why, regardless of the control method, torque oscillations cannot be always fully cancelled, and a comparative study of the methods at these conditions has been conducted in simulation and in laboratory tests.

FREQUENCY-PARAMETRIC CONTROL SYSTEM OF AN ASYNCHRONOUS ELECTRIC DRIVE WITH AN OBSERVER OF THE ANGLE BETWEEN THE VECTORS OF THE STATOR CURRENT AND THE MAGNETIZATION CURRENT

In industry, especially metallurgy, a large number of previously installed electric drives based on an asynchronous motor with a phase rotor with uneconomical parametric control systems are used, the advantage of which is the possibility of removing part of the sliding energy from the rotor circuit, and the ability to obtain an increased inrush current multiplicity. The use of frequency converters in such an electric drive allows you to adjust the speed of rotation of the stator field and maintain sliding and losses in the rotor circuit at a minimum level. However, when the rotor windings are shorted, the multiplicity of the starting torque decreases. Additional possibilities for maintaining an increased starting torque of an asynchronous motor with a phase rotor with frequency control are provided by the inclusion of a frequency-dependent active-inductive resistance (IR) in the rotor circuit. In this case, the electric drive will be frequency-parametric. Using an observer of the mutual position of the stator and rotor current vectors, working on the basis of processing the measured signals of instantaneous values of the stator and rotor currents, allows you to build a scalar control system with stator current correction, bringing the operation of the electric drive to the mode with minimal losses in the motor windings.

CONSTRUCTION OF ROTOR ANGLE IDENTIFIER IN VECTOR CONTROL SYSTEMS OF DOUBLE FEED MACHINES

In asynchronous electric drives with vector control on the rotor, it is necessary to calculate the value of the sine and cosine of the angle of rotation of the rotor relative to the stator to form control actions. When using angle sensors, complex structural tasks can arise — placement and reliable mounting of the sensor on the shaft and, accordingly, the task of the overall layout of the unit. For high-power machines, the tasks of developing and creating the design of the sensor itself arise. If serial rotor angular position sensors can be used, the task of placing and mounting the sensor is no less difficult. In these cases it is necessary to deduce the second end of a shaft from the case of the engine with contact rings that complicates its design. Therefore, the urgent need to create more reliable electric drives with vector control systems on the rotor is the synthesis of identifiers of the angle of rotation of the rotor.
 Identifiers are known whose calculation algorithms are based on determining the projections of the flow coupling vectors. In the work with the use of coordinate transducers of projections of stator or rotor current vectors and equations of electromagnetic circuits of an asynchronous machine, the synthesis and subsequent analysis of the properties of the rotor position angle identifier in vector control systems of dual power machines is performed. New equations of the identifier of flux couplings are received, its stability is investigated and on conditions of stability types of electric drives in which it is possible to apply the offered identifier are defined. The stability of the vector control system and sufficient identification accuracy when using the proposed equations and functions are confirmed by the method of mathematical modeling of the recommended electric drive systems in different operating modes.

Study on Direct Torque Control Methods of a Doubly Fed Induction Machine Working as a Stand-Alone DC Voltage Generator

The article presents a comparison of different Direct Torque Control methods of a Stand-Alone Doubly Fed Induction DC-Voltage Generator, with classic field oriented control used as a benchmark. The system consists of a doubly fed induction generator the stator circuit of which is connected to the DC-bus with a diode rectifier, and the rotor circuit converter is connected to the same DC bus. The main problem of this power generation system are large torque oscillations caused by a nonlinear diode rectifier connected to the stator. Three direct torque control algorithms are described as a means of reduction of this drawback with simultaneous control of DC bus voltage and stator voltage frequency. The methods differ by the second variable used for control in parallel to the torque control path. The selected second variable is the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> component of the rotor current vector, the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> component of the stator flux or the stator flux module.

A Unified Power Control Method for Standalone and Grid-Connected DFIG-DC System

This letter proposes a unified power control method for the doubly fed induction generator dc (DFIG-DC) system, which can seamlessly transfer from the grid-connected mode to the standalone mode without changing a control strategy and vice versa. There is no need to detect the interconnection switch to identify whether it is grid connected or standalone. The dc voltage and stator active power are both positive correlations with the magnitude of a rotor current vector, which indicates that these two objectives can be combined as the unified power to generate the rotor current reference. The stator frequency can be controlled by the rotating speed of the rotor current vector without calculating the stator frequency, which can reduce the parameter dependence. Finally, the experiments based on a 1-kW DFIG-DC setup are carried out to verify the proposed unified power control method.

Rotor Current Oriented Control Method of DFIG-DC System Without Stator Side Sensors

This letter proposes a novel stator power and frequency control method of doubly-fed induction generator (DFIG)-dc system, which is based on rotor current orientation and avoids using stator side sensors. By means of establishing the relationship of the rotor current vector, stator active power, and stator frequency, the stator active power can be controlled by the angle between the rotor current vector and stator voltage vector. The stator frequency can be controlled by the magnitude of the rotor current vector. The stator active power is calculated by dc voltage and dc current at stator side. In this way, the stator voltage and current sensors can both be eliminated. Furthermore, the stator frequency and orientation angle are acquired by the power control loop, which avoids using voltage model or current model. In this way, both parameter dependency and dc sampling uncertainties can be eliminated. Finally, experiments based on a 1-kW DFIG-dc setup were carried out to verify the proposed method.

A Simplified Stator Frequency and Power Control Method of DFIG-DC System Without Stator Voltage and Current Sensors

The primary objective of the grid connected DFIG-DC system is to achieve the accurate control of stator frequency and active power. In this letter, a simplified power control method is proposed by just controlling the magnitude of the rotor current vector, which can avoid the stator voltage and current sensors. The stator active power is calculated by the product of dc voltage and dc current. The stator frequency is simply controlled by the rotating speed of the rotor current vector, which is achieved through a given rotating frame. Furthermore, the parameter dependence and dc sampling offset problems can be eliminated because the voltage model or current model, which are usually used for acquiring stator frequency and stator flux angle, can be avoided. Therefore, the robustness and stability of the stator frequency and power control can be improved. Finally, experiments based on a 1 kW DFIG-DC setup are carried out to verify the proposed method.

Direct Power Magnitude Control of DFIG-DC System Without Orientation Control

This article presents a novel direct power magnitude control of doubly fed induction generator-DC system without orientation control. The stator frequency is directly given and the angle of synchronized d-q frame is generated by the integral of stator frequency. The relationship between the stator power and rotor current vector is revealed in this article. Based on this positive correlation, the output power is easily controlled by the d-axis rotor current, and the q-axis rotor current is controlled flexibly to achieve the smooth switching from no output power to rated power. In accordance with the small-signal model, the relationship between decoupling control and coupling control is presented, and the principle to select the reference of q-axis rotor current is verified. Furthermore, the improved direct resonant control for torque ripple and harmonic current mitigation can still be applied in this novel control method. Finally, the experiment results are presented to validate the proposed method.

Switching Fault-Tolerant Control for DFIG-Based Wind Turbines With Rotor and Stator Current Sensor Faults

This paper presents a switching fault-tolerant control (SFTC) strategy for a doubly-fed induction generator-based wind turbine (DFIG-WT) subject to rotor and stator current sensor faults. A novel stator-current-loop vector control scheme is proposed for the regulation of DFIG-WT without involving rotor currents, and it is compared with the conventional rotor-current-loop vector control scheme on closed-loop stability, tracking performance, and robustness against model uncertainties and external disturbances through theoretical analysis. The SFTC strategy switches between the rotor and stator current vector controllers via a switching logic based on Kalman filter-based fault detection and isolation (FDI) scheme. The proposed SFTC strategy is able to provide superb transient and steady-state performance, strong robustness to parameter uncertainties, and high fault-tolerance capability under rotor and stator current sensor faults. Simulation studies verify the control performance of the SFTC strategy.

A Diagnostic Space Vector-Based Index for Rotor Electrical Fault Detection in Wound-Rotor Induction Machines Under Speed Transient

This paper investigates a new approach for the detection of rotor electrical faults in three-phase wound-rotor induction machines. The method is assessed in a wind energy conversion system, where the rotor side of the wound-rotor induction machine is supplied by a static converter for the control of active and reactive power flows from the generator to the electrical grid. Here, a diagnostic space vector is presented that sensibly improves the rotor fault detection in wound-rotor induction machine by combining the spectral signatures of both rotor current and voltage space vectors. The dc component of this diagnostic space vector, that is equal to zero in healthy conditions and different from zero under rotor asymmetry, is adopted as rotor fault index. The main advantages of the proposed technique are the implementation simplicity and the capability to operate even during speed transients. The technique is also validated by means of simulation and laboratory scale experimental tests.

Indirect Torque and Stator Reactive Power Control of Doubly Fed Induction Machine Connected to Unbalanced Power Network

The paper deals with the methods of either stator current or rotor current vector control of doubly fed induction machine connected to the unbalanced power network. Several alternative control targets, such as fixed electromagnetic torque, symmetrical stator current, or symmetrical rotor current, have been achieved. In spite of the selected target, the stator current is sinusoidal, however in some cases, it is unbalanced. The proposed vector control methods do not use separate negative sequence calculation and control. In some targets, filtration of signals is required in order to achieve symmetrical reference variables (positive sequence), which can be either stator or rotor current depending on the assumed target. Laboratory tests results have been presented in the steady state, as well as in transients. Short comparison of stator and rotor current controllers has been made on the basis of the laboratory test results.

A State Feedback DFIG Power Control for Wind Generation

This paper proposes a state feedback controller for doubly-fed induction generators at variable speed wind. The controller uses pole placement technique, stator flux orientation, and state feedback rotor current vector loops. None conventional PI controller is used on the control. Its algorithm calculates the voltage vector to be supplied to the rotor to certificate that the active and reactive power follow their desired reference values. Experimental results obtained in a hardware setup are carried out to validate the controller operation.

The Rotor Current Orientation Decoupling Control of Doubly-Fed Motor

This paper presents a vector control scheme of doubly-fed motor based on stator flux orientation. The scheme achieves decoupled flux and torque component of stator current through directional control of the rotor current vector. It is suitable for both motor sub synchronous operating condition and super synchronous operating condition. Simulation experiment based on the scheme is carried out by Matlab/Simulink and simulating waveforms are given to prove the correctness of the scheme.

Neural Network Model Reference Adaptive System Speed Estimation for Sensorless Control of a Doubly Fed Induction Generator

This article proposes a neural network model reference adaptive system for the rotor angle and speed estimation of the doubly fed induction generator used in wind turbines. The model reference adaptive system reference signal is the measured rotor current. The adaptive neural network adjusts the weights minimizing the rotor current vector squared error using the steepest descent algorithm. The neural network maximum stable learning rate will be determined for this application. The validity of the proposed neural network model reference adaptive system is verified and analyzed in a real prototype of 7.5-kW doubly fed induction generator. To validate the proposed estimator, the estimated rotor angle and speed in the process of connecting the doubly fed induction generator to the grid and the sensorless regulation according to a random wind speed profile are presented.

A vector-controlled doubly-fed induction generator for a variable-speed wind turbine application

This paper examines the performance of a vector-controlled doubly-fed induction generator for wind turbine use. The scheme is considered for variable-speed-range applications to produce a greater wind energy capture from varying wind velocity. A major advantage of the doubly-fed scheme over the squirrel-cage generator is the reduced volt-ampere rating of the power converter used to control the rotor current. The singly-fed scheme requires that the power converter is able to carry the full generator output. In order to produce decoupled regulation of active and reactive output power, the rotor current is controlled using field orientation principles. The measurement of rotor position and the grid voltage vector position allows the rotor current to be controlled in a reference frame closely associated with the stator magnetising current. The stator active and reactive power are also measured and fed back into outer Ps and Qs control loops. The Ps and Qs controllers produce d and q axis rotor current demands for the inner rotor current vector controller. By control of the reactive power loop the generator can be made to produce power at unity power factor. The vector control of rotor current is carried out using a commercial PWM inverter equipped with vector control hardware as standard. The inverter also produces a unity power factor sinusoidal current interface to the grid network. The performance of the scheme is examined using both simulation and experimental results. The decoupling control of active and reactive power is demonstrated and shows the ability to generate power at unity power factor.

DYNAMIC ANALYSIS OF A DOUBLY FED GENERATOR IN POWER SYSTEM APPLICATIONS

Abstract The dynamic performance and control of a doubly fed generator is investigated. It is shown experimentally that a reactive current regulator can be used to control the machine excitation level. The study initially assumes an externally controlled field frequency, so that the machine is in the controlled speed mode. Linearized analysis is used to show that larger generators may be subject to dynamic instabilities at high slip operation. It is shown that feedback of shaft speed along with the stator or rotor current vector can be used to provide stable operation.

Control Systems of Doubly Fed Induction Machine Based on Multiscalar Model

A new multiscalar model of the induction motor developed in recent years is used for control system synthesis of a doubly-fed induction machine (DFM). Scalar and vector products of the stator flux and rotor current vectors and square of the stator flux vector are chosen as new variables. Differential equations of the DFM for the chosen variables are received and steady state analysis is made. A control system with a nonlinear feedback based on the proposed model is synthesised. Stability problem appearing in the proposed system is analised and the same problem in a system based on vector methods is pointed out. For stabilization of the motor torque better results are received in the proposed system.

Static and Dynamic Properties of a Current Controlled Doubly-Fed Induction Machine

The paper presents properties, control system and experimental results of a doubly-fed current controlled induction machine. The influence of rotor current co-ordinates adjusting on an electromagnetic torque and stator reactive power is shown. The variation of torque, stator, rotor and cycloconverter reactive power and of magnetizing flux by setting the leading rotor current vector is analysed. The stator voltage oriented control with a cycloconverter applied in the rotor circuit is shown. The experimental results of the system are presented. An independent fast speed and stator reactive power control has been achieved. The system is stable in the whole range of speed, load and reactive power conditions.