1,720,992 research outputs found
DFIG Topologies for DC Networks: A Review on Control and Design Features
No full textThe doubly fed induction machine has been traditionally adopted in adjustable-speed ac power generation drives in order to take advantage of the reduced rating for the power electronic interface. Aside this well-established application where the doubly fed induction generator (DFIG) is controlled by a back-to-back converter, recent literature records a growing interest toward un-conventional DFIG drives for dc power generation, combining DFIG high control freedom with simplified power electronic interfaces to achieve an overall cheap and fully controllable system. Despite several concepts have been demonstrated on small-scale rigs, there is a lack of systematic comparison among different topologies and control solutions. This paper bridges this gap by providing a review of recent topologies, their control, design and performance, and operation issues. As major novelties, this paper includes off-spec performance comparison of different torque-ripple mitigation strategies, discussion of sizing requirements for generator and power electronics, fundamental aspects of the behavior under voltage dips, and priorities and challenges for future research on the subject
A Self-Sensing Stator-Current-Based Control System of a DFIG Connected to a DC-Link
No full textThis paper presents a self-sensing technique for the field-oriented control and frequency regulation of a doubly fed induction generator connected to a dc-link. In this system, the stator circuits are connected to a dc-link through a diode bridge and the rotor circuits are controlled by a voltage source inverter connected to the same dc-link. As the diode bridge maintains the fundamental harmonics of the stator current and voltage approximately in phase, an almost zero average d-axis stator current results in the stator flux reference frame. This property is used to estimate the slip angle, required to implement the field orientation in the self-sensing technique in loaded conditions. At no load the system is controlled using a different methodology. The analysis and synthesis of the control chains are presented by analytic relations. The sensitivity study shows that the method exhibits reduced sensitivity to the parameter mismatch, resulting in a small orientation error. Experimental results confirm the good performance of the proposed method
Field-Weakening Control for Efficiency Optimization in a DFIG Connected to a DC-Link
No full textThe field-weakening operation of a doubly fed induction generator (DFIG) connected to a dc-link is analyzed in this paper, in order to optimize the efficiency. In the considered DFIG-dc system, the stator feeds a constant-voltage dc link by a diode bridge, and the rotor current is controlled using a voltage-source inverter connected to the same dc link. Since the stator voltage amplitude is imposed by the dc-link, a variation in stator flux magnitude results in a frequency change. However, in this system, a stator frequency variation over a wide range can be accepted, if the rated flux is not exceeded. Thus, the stator flux amplitude can be adjusted through the magnetization current component by the voltage-source inverter and according to the load level, in order to reduce losses in the machine and in the inverter. This paper presents an optimization analysis and simplified formulae determining the optimal reference magnetization current in the control of the system. Conversely to field weakening in conventional drives, in this case, the enabling of field weakening control is not dependent on rotor speed, but depends on the reference torque. The proposed optimal control is validated through simulation and experimental results
Inner control method and frequency regulation of a DFIG connected to a DC link
Full text linkIn this paper, an inner loop for the control and frequency regulation of the doubly fed induction generator connected to a dc link through a diode bridge on the stator is presented. In this system, the stator is directly connected to the dc link using a diode bridge, and the rotor is fed by only a pulsewidth-modulated (PWM) converter. If compared to the DFIG connected to an ac grid, this system uses one PWM inverter less and a much less expensive diode bridge. Thus, the cost of power electronics is reduced. The application in mind is for dc networks such as dispersed generation grids and microgrids. These networks use several elements that should work together. Usually, these elements are connected with each other by power electronic devices in a common dc link. This paper presents a control system for the inner control loop in order to regulate the torque and the stator frequency. Simulation and experimental results show that the system works properly and is able to keep the stator frequency near the rated value of the machine. © 1986-2012 IEEE
Minimization of Torque Ripple in the DFIG-DC System Via Predictive Delay Compensation
No full textTorque ripple caused by stator current and flux harmonics is one of the main issues in the doubly fed induction generator (DFIG)-dc system, which inherently has to operate with distorted waveforms produced by the diode commutation. This paper proposes a torque-ripple mitigation strategy based on a predictive estimation of the reciprocal of flux linkage. The predictive estimation compensates for the intrinsic delay in the actuation of the torque-ripple rejection signal through the rotor current control loops. Unlike other approaches relying on complex current regulators with selective harmonic tracking, this strategy is based on well-established proportional-integral (PI) controllers for the rotor currents. PI current controllers can then still have bandwidth values typical of usual DFIG systems. Simulations and experiments on a test-rig show that the compensation strategy achieves a strong torque ripple reduction and is very robust against stator frequency variations
Sensorless Frequency and Voltage Control in the Stand-Alone DFIG-DC System
No full textA 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
Voltage control in a DFIG-DC system connected to a stand-alone dc load
No full textThis paper proposes a control scheme for a stand-alone DFIG-DC system where the stator and the rotor are connected to a dc load by an uncontrolled diode bridge and a VSI respectively. The stator frequency and the dc voltage regulation are achieved by a field oriented control, however, with respect to the classical ac stand alone DFIG, the role of the rotor currents is swapped, namely: the d-axis and q-axis rotor currents are used for the frequency and for the dc voltage regulation respectively. Such a choice is due to the peculiarity of the operation of the diode bridge connected to a constant voltage dc link. The control scheme is presented and design criteria for the voltage controller are reported. The performances of the system are verified by simulations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Minimum-loss control strategy for a dual-VSI DFIG DC System
Full text linkThis article addresses the minimum-loss control of the dual voltage-source inverter (VSI) and doubly fed induction generator (DFIG) system connected to a dc link. The minimum-loss operating conditions for field-oriented control based on the airgap flux are obtained analytically using Lagrange multipliers and validated with numerical optimization. As the main contribution of this article, the analysis accounts for core and VSI losses, providing the optimal stator frequency law and rotor/stator d-axis current split ratio, and an implicit expression for the optimal flux trajectory formulated as equality between suitable d-axis and q-axis loss functions. In the proposed implementation, this implicit condition is enforced by using a proportional-integral controller and avoiding look-up tables. Furthermore, the stator and rotor VSI controls are implemented in two independent digital signal processors with no communication, which may ease the use of off-the-shelf VSI units. The optimal conditions and control strategy are fully validated by simulations and experiments on a prototype. The main scope of application is wind-energy dc-grid technology
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