1,721,022 research outputs found
Overcurrent estimation in a doubly-fed induction generator-DC system during a voltage dip in the dc grid
No full textThis study analyses the voltage dip behaviour of the doubly-fed induction generator (DFIG) connected to a common dc link on both the stator and rotor sides, via a diode bridge and a voltage source inverter (VSI), respectively. After a voltage dip in the dc grid, the rotor VSI can transiently lose control so that rotor currents circulate through the VSI free-wheeling diodes. However, during the uncontrolled period, both stator and rotor voltages are clamped to the same dc-link voltage and no rotor overvoltage occurs. In order to analyse the behaviour of the system during the first periods of the uncontrolled current transients, this study presents a simplified average model which is validated through simulations. Voltage dips are classified into three types depending on their severity. A simple analytical expression is derived for the maximum amplitude of the current during the voltage dip, and an overcurrent chart in the plane 'rotor speed - dip amplitude' is deduced. These results provide deep insight into the DFIG-DC system behaviour and are useful for design purpose
Minimum Loss Conditions in a Salient-Pole Wound-Field Synchronous Machine Drive
No full textThe conditions for minimum losses in a salient-pole wound-field synchronous machine (WFSM) drive are studied in this paper. The drive comprises a WFSM energized by a stator inverter and excited by a dc-dc converter both tied to a DC link. The minimum-loss operation is formulated as a nonlinear constrained optimization problem with equality constraints (e.g, torque command), and inequality constraints (flux, voltage and current limits). Lagrange multipliers are applied to solve this problem analytically. At low load, the torque demand can be met using different values for two independent electric variables (e.g. stator flux and field current magnitude). These can be optimized, thereby leading to two optimal implicit conditions. At higher load, when the stator flux reaches the maximum value, the free variables reduce to one and yield a single implicit optimal condition. For these two scenarios, the paper presents analytical derivations of the optimal conditions and numerical validation using MatLab. These conditions can be used to devise a control system optimizing the drive operation
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
Emulation of prime movers in wind turbine and diesel generator systems for laboratory use
No full textThis paper presents an experimental system with hardware-in-the-loop emulation of both a diesel engine and wind turbine prime movers. System equations are analysed to model the required dynamics of the prime movers and design the control of the emulators. Experimental and simulation results show the dynamic response of the emulated systems
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
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
Volt-ampere ratings in electronically tuned linear alternators for thermoacoustic engines
Full text linkLinear alternators (LAs) coupled to thermoacoustic engines (TAEs) provide a viable solution to extract energy from a heat source in a variety of applications such as waste heat, energy harvesting, solar thermal and biomass power generation. For the electrical power to be maximised, the acoustic impedances of LA and TAE have to match. This requirement cannot, in general, be met by relying only on the design of the LA, but can be achieved at the control level, by using a fraction of the LA inverter current to create 'electronic stiffness' which contributes to the overall stiffness tuning the resonance frequency. The same concept can, in principle, be used to replace part of the mechanical spring stiffness in order to overcome the limitations in the mechanical design, at the expense of an increase in LA and inverter ratings. The impact of electronic stiffness on LA power capability and ratings is analysed here. Two meaningful scenarios are considered in the analysis: The LA derating for resonance frequency tuning and the oversizing when springs are partially replaced by electronic stiffness. The study is supplemented with experiments on a small-scale LA test rig
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
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
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