Gil D. Marques

An instantaneous active and reactive current component method for active filters

A shunt active filter based on the instantaneous active and reactive current component i/sub d/-i/sub q/ method is proposed. This new control method aims to compensate harmonics and first harmonic unbalance. To evaluate its relative performance, it is compared with the instantaneous active and reactive power p-q method under various mains voltage conditions and for different harmonic injection high-pass filters. Both methods are completely frequency-independent, however under distorted mains voltages the proposed method presents a better harmonic compensation performance. The system synthesis and implementation are performed. Simulation and experimental results are presented.

An active power filter and unbalanced current compensator

This paper presents the synthesis and performance of a shunt active power filter based on the three-phase pulsewidth modulation (PWM) voltage converter connected to the AC mains. Current harmonics and asymmetries caused by nonlinear loads can be compensated. A decoupled system in Parks variables is achieved and so simple controllers with excellent performance can be used. The controllers are implemented directly in the Parks referential. Expressions for the controllers synthesis are derived. Experimental results from a 2 kVA IGBT prototype showing excellent dynamic and steady-state systems performances are presented. The control circuit is implemented with analog and digital electronic circuits. A considerable amount of electronic circuits are needed. The method presented in this paper can also be implemented with a digital signal processor.

Four-wire current-regulated PWM voltage converter

Shunt active power filters are connected in parallel with the electricity supply network. If the AC mains has a neutral conductor, it is desirable to compensate the mains harmonic currents zero-sequence components. This can be achieved with a four-wire pulsewidth modulation voltage converter connected to the AC mains. In this case, the three-phase and the neutral AC currents must be controlled. A generalization of the space-vector-based current controller in the /spl alpha//spl beta/o coordinate system is presented in this paper. With this current controller, all the current harmonic systems of positive, negative, and zero sequence can be injected by the converter and, thus, compensated on the AC mains. The system is also useful to compensate unbalanced currents of fundamental frequency. A useful benefit of this system is that it is possible to control the converter four-wire currents with equal hysteresis errors. Simulation and experimental results are presented.

DC voltage control and stability analysis of PWM-voltage-type reversible rectifiers

A PWM voltage rectifier has useful characteristics on its DC and AC sides. On its DC side, a DC-link unidirectional voltage is obtained and bidirectional power transfer capability is possible by reversing the flow direction of the DC-link current. On its AC side, near sinusoidal current waveforms and AC four-quadrant operation can be obtained, leading to high-quality power being exchanged between the power converter and the mains. The use of AC filters becomes unnecessary. The rectifier DC voltage must be regulated to a constant value. In this paper, three solutions for the DC voltage control are presented. In the first solution, the DC voltage is controlled by acting upon the quadrature component of the power converter fundamental Parks voltages with relation to the mains voltages. Slow responses are necessary because of stability reasons. Also, load power variations produce both active and reactive power variations in the power converter AC side. To improve the DC voltage response, a second control solution is presented. The power converter currents in Parks coordinates must be controlled. The DC voltage is controlled by controlling the direct Parks current component and, thus, acting only on the active power of the converter AC side. Faster responses are achieved. In this case, load power variations do not produce reactive power variations in the converter AC side. The third control solution is a simplified version of this last one. Experimental results from a 2 kVA IGBT-based prototype showing good system dynamic performance are presented.

A DFIG Sensorless Rotor-Position Detector Based on a Hysteresis Controller

A simple sensorless method for the detection of the mechanical rotor position of the wound-rotor induction machine in order to implement stator-flux orientation is described and evaluated in this paper. The method is based on the phase comparison of the actual and the estimated rotor currents using the classical model of the machine. It can be conceptually implemented in the rotor or in the stator reference frames. It has some similarity to the model reference adaptive system methodology, but uses a hysteresis comparator instead of a proportional integral (PI) controller. In this way, the method does not need parameter determination for the controllers and shows a considerable independence of parameter uncertainties. Simulation and experimental results show that the method is appropriate for the vector control of the doubly fed induction generator (DFIG) because it leads to the decoupling of active and reactive power chains. It is also appropriate for the control of the DFIG in transients like those when voltage dips occur.

Air-Gap-Power-Vector-Based Sensorless Method for DFIG Control Without Flux Estimator

A sensorless method for the direct estimation of the slip position of the wound-rotor induction machine is introduced, discussed, and experimentally validated in this paper. The slip position is necessary for the implementation of control by flux orientation. The method proposed is based on the phase comparison of an estimated air-gap power vector and the measured rotor currents in a common reference frame. The proposed method can be implemented in the rotor or in the field reference frames with a hysteresis or proportional-integral controller. The method is sensitive to the stator no-load active and reactive powers, but this dependence is not very important. Simulation and experimental results show that the method is appropriate for the vector control of the doubly fed induction machine. Unlike other methods, this method gives acceptable results when the load is small. In addition, it does not need a flux estimator for the implementation of flux orientation.

Desktop fast-field cycling nuclear magnetic resonance relaxometer

In this paper a new type of Fast Field Cycling (FFC) Nuclear Magnetic Resonance (NMR) relaxometer with low power consumption (200W) and cycle to cycle field stability better than 10(-4) is described. The new high-permeability magnet was designed to allow for good magnetic field homogeneity and allows for the sample rotation around an axis perpendicular to magnetic field, operating with magnetic fields between 0 and 0.21T. The power supply of the new relaxometer was specially developed in order to have steady state accurate currents and allow for magnetic field switching times less than 3ms. Additional control circuits were developed and included to compensate the Earth magnetic field component parallel to the field axis and to compensate for parasitic currents. The main aspects of the developed circuits together with some calibrating experimental results using the liquid crystal compounds 5CB and 8CB are presented and discussed.

Stator Frequency Regulation in a Field-Oriented Controlled DFIG Connected to a DC Link

Dispersed generation systems need several elements that should work together. One possible way to connect these elements is through a common dc link. This paper discusses a new control method for the doubly fed induction generator when connected to a dc link. Considerable benefits are obtained in this configuration, because a simple diode bridge can be used on the stator and only an ac/dc power electronic inverter is needed to control the rotor currents. As the ac mains do not impose the stator frequency, this should be regulated to get an appropriate operation of the induction machine. This paper proposes a method to regulate the stator frequency and control the torque based on adjustments of the d - q rotor reference currents by using the field-oriented control. The analysis and synthesis of the control system based on a simplified model is discussed. Simulations and experimental results show that the proposed control exhibits good dynamics in the torque control and in frequency regulation.

New Sensorless Rotor Position Estimator of a DFIG Based on Torque Calculations—Stability Study

This paper presents a new sensorless method and its stability study for the estimation of the mechanical rotor position of the wound-rotor induction machine. The main purpose of this study is to implement the control of the doubly fed induction generator (DFIG). The method is based on the model reference adaptive system and uses the electromagnetic torque as the working error variable. The method does not need any information about the stator or rotor fluxes, and so, it is only indirectly dependent of the flux dynamics. As other methods proposed recently, this can also be implemented in the rotor or in the stator reference frames and with hysteresis or with proportional-integral controllers. The stability analysis gives an instability region on the rotor current dq plane described by a circle whose diameter is the no-load stator current. The method is robust to parameter variations depending only weakly on a single parameter. Simulation and experimental results show that the method is appropriate for the vector control of the DFIG although needing an additional approach to stabilize the system in the instability region.

Torque Ripple Reduction in a DFIG-DC System by Resonant Current Controllers

A DFIG connected to a dc bus by a diode rectifier and a unique reduced-power PWM converter is considered in this paper. With respect to the traditional ac-grid connected DFIG, such a layout avoids the grid-side PWM converter and is an interesting solution to integrate the DFIG in a dc microgrid together with other generating units, loads as well as storages. The peculiarity of the DFIG, which allows the control of the rotor current space vector independently of the mechanical position, offers two important benefits when the stator is connected to a constant voltage dc grid by a diode bridge: it avoids the need to boost the flux amplitude at low speed, and it allows to considerably reduce the torque ripple due to the diode commutation. This last issue is developed in this paper by using a field-oriented control scheme based on proportional-integral and resonant Controllers. The proposed control is validated through simulation and experiments.