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FeCo-based soft magnetic alloys are commonly used in macroscale devices to improve its electromagnetic performance, whereas they have been barely used in the microscale. Current FeCo alloy micromanufacturing processes present some difficulties like low structural strength, oxidation at high temperature processes, stoichiometry mismatches in deposition processes and tough workability. In this work, a microcutting of FeCo-2 V-based soft magnetic alloys process is presented and described as an alternative method to obtain microparts with high magnetic properties and good geometrical finish. The results of the machining process are analysed by varying the machining parameters such as depth of cut, tool diameter, rotation speed and feed speed on simple machining operations. The study has been done for 50-μm-diameter endmill and 250-μm-diameter endmill tools. It concludes that the roughness is minimized when machining parameter combination is 0.24 mm/min of feed rate and 8 μm of depth of cut size for a 250-μm-diameter tool, while for a 50-μm-diameter tool, the selected feed rate is 0.24 mm/min, for a depth of cut between 2 and 4 μm. An automated precision 3-axis CNC station is used. Shapes needed for actuators such as angular slots, disks, or slender square geometries are shown in this work with excellent magnetic and mechanical properties. Additionally, a complementary electropolishing process is described. This process helps to eliminate burr in edges and residuals of the milling operation. This study demonstrates that micromilling can be a good alternative for microfabrication of FeCo-2 V components, suitable for precision microassemblies on MEMS.

This article presents a systematic robust current control design approach for three-phase voltage-source converters. Robustness is guaranteed by combining intrinsic passive properties of the impedance uncertainty at the point of common coupling together with stability results from a passivity-based control theory. This approach ensures stability against typical uncertainty sources at mid and high frequencies, such as cable resonances or other converters interaction, with significant less conservative performances than the obtained with the traditional robust control theory. The approach uses multiobjective controller synthesis formulation that allows us to logically combine robustness requirements with performance objectives avoiding heuristic iteration over the control structure and parameters. The controller synthesis is performed by means of a nonsmooth $\mathcal {H}_{\infty }$ optimization technique that tunes all free parameters of a vector-based controller function, which constraints its structure. This results in a synthesized controller with lower order than those obtained with convex optimization definitions of the $\mathcal {H}_{\infty }$ control problem. The design methodology is validated in time and frequency domain by means of theoretical analysis and experimental results with three usual grid filters: L , LCL, and LLCL .

The progressive electrification of railways involves an increasing number of power electronic converters connected to the railway catenary, which may compromise its stability. Both the converter for traction and the converter for auxiliary power systems (APS) behave as constant power loads (CPL) and interact negatively with the catenary impedance producing voltage instability. This article applies quantitative feedback theory (QFT) to design an ac voltage controller for the APS converter that shapes the dc input admittance of the converter by performing only ac side-control without a dc-side feedback loop. The QFT enables to design a low order controller that satisfies multiple performance specifications in systems with high uncertainty as is the case of the train system. The proposed control guarantees catenary stability while ensuring ac output voltage reference tracking and providing robustness to unmodeled uncertainties. As an additional contribution, the article presents an algorithm for including input admittance specifications in the QFT design process. The proposed control has been evaluated on an experimental platform that recreates the train system. Experimental results show that the controlled system meets railway standards and correctly shapes the specified dc input impedance.

One-stage and two-stage grid connected PV systems are compared in terms of compactness for both, 1000-V and 1500-V PV string voltage levels. Design space of the inverter’s output filter is investigated for case when dc bus voltage is very well controlled to its minimum value and when it varies in the full operating range. Full multivariable optimization is conducted for the analyzed design spaces in order to investigate possible compensation of the increase of the total system volume caused by the addition of a dc/dc stage by achieving more compact inverter stage for better controlled dc bus voltage. It is concluded that for the same efficiency, filter designs with lower dc bus voltage can occupy $\approx$ 50% less volume compared to the case when dc bus varies in the full operating range. Hybrid, multilevel, partial power processing topologies that employ new classes of 650-V and 900-V WBG devices, that are already reported in the literature and that occupy $\approx$ 0.8 -0.9dm 3 for the design rated for 1500-V PV system and 20kW of the output power are very good candidates for a dc/dc stage that can control dc bus voltage at the input of the inverter stage and that can improve significantly compactness of the inverter stage and of the whole system. All the analysis and estimations are justified by comprehensive simulations of the 1500-V grid-connected PV inverter in PSIM package.

This paper proposes a controller tuning methodology for voltage-current droop-based DC-DC converters in DC microgrids to reduce the output capacitance. This minimization is cost saving and implies lower fault currents. However, it leads to higher DC voltage variability during load transients, which requires an output impedance shaping by control means to reduce over or undershoot. The proposed control structure and problem definition simultaneously takes into account that the solution must achieve the impedance shaping, performance and stand-alone stability objectives. This comprises a multi-objective problem which is effectively formulated here and, then, solved by a non-smooth $\mathcal {H}_\infty $ optimization technique that tunes all free parameters. For comparison purposes, this tuning methodology is applied to several droop proposals, and the proposed droop is able to reduce the output capacitance of bidirectional buck-type and boost-type half-bridge converters by 37.5% and 23.08%, respectively, with respect to previous proposals. The designs are validated in time and frequency domains by means of theoretical analysis and experimental results on DC microgrid prototypes with bidirectional buck-type or boost-type half-bridge converters.

This paper presents a systematic approach on voltage controller design for three-phase inverters with passivity-based robust stability capabilities. Such design involves several requirements as stand-alone stability margins and tracking behavior along with input impedance shaping for passivity-based robustness. Besides, symmetrical three-phase systems are expressed as multiple-input multiple-output systems, which makes controller synthesis intricate. Then, the proposed multi-objective approach synthesizes the controller by means of a non-smooth ${{\mathcal{H}}_{\infty} }$ optimization technique that tunes all free parameters of the corresponding vector-based controller function. We present the design methodology for the two most usual grid-forming strategies: single-loop and dual-loop (cascaded with inner current control). Both designs are validated in time and frequency domain by means of theoretical, simulation and experimental results.

This paper introduces a novel method of designing voltage controllers to improve the stability problem in the Common-Connection Point of the DC catenary railway system, using the QFT control technique. The considered stability problem is due to the interaction between the resonance of the LC filter connected to the DC catenary with the traction and auxiliary converters (3-phase inverters) it feeds, which behave like CPLs. An stability improvement is achieved by shaping the closed-loop input impedance of a three-phase inverter using impedance-based stability criteria. The obtained simulation results, under unfavourable conditions, verify the theoretical development.

Different architectures of 1000VDC and 1500VDC grid-connected PV systems for different AC grid voltage levels are compared in this paper with respect to energy harvesting during one year period. One stage architecture is compared with two two-stage concepts already presented in literature and industry - "mini-boost" and "string-optimizer". An idea of mini-boost considers a booster rated for a portion of the nominal power that operates only in the cases when MPP voltage is lower than minimum allowed DC bus. Otherwise it is by-passed and the inverter operates as in one-stage architecture. Another concept, "string-optimizer" can be considered as a buck-boost topology that controls DC bus voltage at the minimum value. In this way range of energy harvesting operation is increased and performances of the inverter are improved. However, buck-boost topology is less efficient than a boost and, additionally in this case, it needs to be rated for the full system power. All the analysis and comparison is based on comprehensive PV panel and converter loss models, real enviromental conditions measurements and real and commercially available circuit components. Experimental verification of the employed models is provided by the measurements conducted on 22kW grid-tied solar inverter.

This paper introduces a novel QFT voltage controller that allows the input admittance shaping of power converters in frequency domain, allowing to define its magnitude and its phase. The destabilizing effect associated with the constant power load behavior of power converters is mitigated with this input admittance shaping, because it has been shaped using an impedance-based stability criterion, opposing argument criteria. The obtained simulation results, under unfavourable conditions, verify the theoretical development.

This paper presents a new approach on robust current control of LCL-type grid-tied voltage source converters. This control method takes advantage of grid impedance nature to manage its unavoidable uncertainty at every point of common coupling. The Passivity-based Robust Control framework minimizes the converter impact over grid stability based on the passivity index of the impedance uncertainty, which leads to less conservative designs. System robustness is improved by means of a passivity constraint while keeping a performance goal. In order to overcome the non-convex behavior of the proposed multi-objective problem, we present a two-step approach design to find an acceptable solution. The methodology is initialized obtaining a controller via ${\mathcal{H}_\infty }$ synthesis procedure, which achieves the performance objective. Then, if the passivity constraint is not fulfilled, the non-smooth optimization technique will enforce it while taking care of the trade-off with the performance objective. Numerical and experimental results verify the achievement of the proposed objectives in time and frequency domain.

Purpose:To compare rotational stability, centration and visual outcomes provided by three trifocal lens models that have the same optical zone design but different material, composition, and/or toricity.Methods:The study included 78 patients with symmetric bilateral intraocular lens implantation. The lenses under evaluation were trifocal intraocular lenses made of hydrophilic acrylic material: a spherical lens 26% hydrophilic acrylic (POD FineVision), a similar lens but having a toric design (POD Toric FineVision), and a trifocal lens 25% hydrophilic acrylic material (FineVision/MicroF). Moreover, the lenses share the same optical zone design. The lenses’ rotational stability and centration were measured by means of the PIOLET software, which relies on recording and image processing techniques to determine lens rotation and centration based on slit-lamp images. We also assessed patients’ visual quality by means of 25, 40, and 80 cm VA tests.Results:The best centration results were achieved with the POD Toric FineVision model, although the differences were not statistically significant. As for lens rotation, it was below 5° in all cases under study. Regarding VA, all subjects attained at least 0.3 logMAR for far distance uncorrected VA, at 80 cm VA was about 0.2 logMAR, at 40 cm it was above 0.15 logMAR, and at 25 cm it was about 0.3 logMAR for both lens types.Conclusion:All three intraocular lens models yield excellent visual results at far, near as well as intermediate distances. The POD FineVision and POD Toric FineVision models, with double C-loop design, yielded the best results centration-wise and rotation-wise. Differences had no clinical relevance.

This paper focuses on the design of an active-damping controller that shapes the dynamics of an LCL grid connected voltage source converter (VSC) to mimic an L -filter connected VSC. This makes possible to use simpler classical proportional plus resonant or proportional plus integral controllers design methods to regulate the grid current with minimum penalties in both performance and stability margins. The active-damping design follows a model-reference approach and is synthesized through a (sub)optimal $\mathcal {H}_\infty$ algorithm. The controller uses the grid current and point of common coupling voltage, although it allows the selection of a different set of measurements. Effectiveness of the method is demonstrated with time- and frequency-domain experimental results.

This paper combines the well-known linear quadratic Gaussian (LQG) control and frequency-adaptive resonators and presents a frequency-adaptive multiresonant LQG state-feedback current controller for LCLfiltered voltage-source converters connected to a distorted grid. The paper also provides a design guideline and procedure based on robust control criteria which, in combination with the linear quadratic regulator (LQR) technique, offers flexibility in the control structure and automatizes the design of the controller. The frequencyadaptive resonators, based on second-order IIR resonators and on an on-line tuning algorithm, and the robustness criteria considered for the design process offer robustness in the face of grid voltage disturbances.The controller is evaluated and validated in a 9-kVA VSC setup configured as a rectifier. This work was supported in part by the Government of Spain through the Ministerio de Economía, Industria y Competitividad and Agencia Estatal de Investigación under Grants ENE2014-57760-C2-2-R, RTC-2015-3803-3, DPI2017-88505-C2-2-R and DPI2017-92258-EXP.

© 20xx IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other work This paper presents a current controller that shapes, in the frequency domain, the input admittance of voltage-source converters connected to the grid. The controller is obtained by means of a H8 synthesis procedure, which minimizes the difference between the application closed-loop input admittance and a model-reference defined by the designer. This formulation achieves good accuracy in both modulus and phase. The proposed methodology allows the fulfilment of other current control objectives, such as current tracking, by defining frequency regions where each objective is desired. Experimental results show the good response of the proposed controller, both in frequency and time domain _

© 20xx IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting /republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works This paper presents a grid current control of a grid connected Voltage Source Converter (VSC) with an LCL filter. The control method enables to shape the input admittance of the converter in addition to track a given current reference. By specifying a low resistive admittance profile, a suitable controller is obtained by using an H8 synthesis, which will actively damp the filter resonance and the harmonics/inter-harmonics from the grid in a wide frequency range. Additionally, the proposed system presents good robustness with respect to grid uncertainty.

This paper presents the design of the current control loop of a three-phase voltage source converter (VSC) connected to the grid by applying the Quantitative Feedback Theory (QFT). This technique allows a robust controller design able to maximize performance for a certain previously known uncertainty and it creates faster and lower order controller than other robust techniques. In this work, the QFT technique has been adapted to achieve sine-wave reference tracking. The obtained experimental results verify the theoretical development.

This paper presents a current control scheme for a voltage-source converter connected to the grid through an LCL filter. The proposed current control is based on the linear quadratic (LQ) Gaussian (LQG) servo controller technique, which combines the LQ regulator and the Kalman filter (KF). The use of the KF instead of a Luenberger estimator improves the quality of the estimation of state variables in noisy environments, increasing the noise immunity of the control loop. This paper focuses on giving some criteria and practical hints to choose both the KF and LQ design parameters to achieve null steady-state tracking error, channel decoupling, and noise immunity. Furthermore, this paper analyzes the steady-state KF approximation that allows a significant decrement in the computational complexity of the algorithm at nearly no cost in accuracy. Simulation and experimental results prove the correct operation of the proposed LQG current control and the possible advantages of using KF to estimate the state vector.

This paper proposes improvements to the control method based on the complete dynamical model published by Komurcugil and Kukrer, which is very simple by operation in the stationary reference frame and lacks ac-line-current sensors. Nevertheless, the basic control method is very sensitive to variations of the line inductances, which provide difficulties in many industry applications. The modifications proposed in this paper overcome this disadvantage using a fuzzy logic controller to tune parameters and do more robust the basic control in view of line inductance parameters variations. Unfortunately, the new tuning method needs information about the reactive line current; therefore, ac-line-current reconstruction algorithm based on the dc-link current measurement has been included to keep the same number of sensors as the basic solution. Simulation and experimental results have proved the correct system operation at wide variations of the line inductance.

This paper proposes a grid impedance monitoring system for distributed power generation electronic interfaces. The system estimates the grid equivalent impedance and voltage source from the voltage measurements performed at the point of common coupling. The estimation algorithm is based on a recursive least-squares algorithm implemented in the complex field. Simultaneously, the system evaluates the quality of the estimation, minimizing its influence on the grid and detecting islanding situations. The proposed system performance has been evaluated under experimental testing.

This paper describes the hardware and software modules of a remote Internet-based laboratory for experimentation with a 120 KVA multilevel power converter. The key feature of this laboratory is the utilization of a multilevel converter that can be controlled and supervised remotely in a secured way. A wide variety of laboratory experiments are supported, from grid connection to ac motor control. The users can choice the control structure, the control parameters and the kind of load and get the graphical results of the measurements, all in real time.

New energy concepts such as distributed power generation systems (DPGSs) are changing the face of electric distribution and transmission. Power electronics researchers try to apply new electronic controller solutions with the capacity of implementing new and more complex control algorithms combined with internal high-speed communication interfaces. Thus, it is possible to monitor, store, and transfer a large number of internal variables that can be sent online to local or remote hosts in order to take new set points of different generation units. With this objective, this paper presents the design, implementation, and test of an industrial multiprocessor controller based on a floating-point digital signal processor (DSP) and a field-programmable gate array, which operate cooperatively. The communication architecture, which has been added to the proposed electronic solution, consists of a universal serial bus (USB), implemented with a minimum use of the DSP core, and a controller area network (CAN) bus that permits distributed control. Although the proposed system can be readily applied to any DPGS, in this paper, it is focused on a 150-kVA back-to-back three-level neutral-point-clamped voltage source converter for wind turbine applications.

Modern energy concepts as Distributed Power Generation are changing the appearance of electric distribution and transmission and challenging power electronics researchers, which try to develop new solutions of electronic controllers. The aim is to enable the implementation of new and more complex control algorithms to verify the last standards related to the grid energy quality for new power converters, and, also, for equipments which nowadays are operating. This paper presents the design, implementation and test of a novel real-time controller for a Neutral Point Clamped (NPC) (three-level) multilevel converter based on a floating-point Digital Signal Processor (DSP) and on a Field-Programmable Gate Array (FPGA), by operating in a cooperative way. Although the proposed system can be readily applied to any power electronic application, in this work, it is focused on the next system: a 150 kVA back-to-back three-level NPC Voltage Source Converter (VSC) for wind power applications.

As the number of wind power applications with power electronic interfaces in the grid increases, it is becoming unacceptable to disconnect the generating units every time disturbances occur, especially under voltage dips, as was a common practice in the past. Keeping the converter online during unbalanced voltage, and guaranteeing the actual standards of the converter connected to the grid, is becoming a very critical issue. From these goals, the design of a robust back-to-back neutral point clamped (three levels) voltage source converter of 150 kVA is developed in this paper. The converter is divided into two main parts: the power electronic system and the control electronic system. Concerning the first part, on the one hand, the paper presents the designs of active and passive components as insulated gate bipolar transistor, free-wheeling diodes, clamping diodes, grid filter, DC-bus capacitors, etc.; and on the other hand, the converter requirements are analyzed to ride through real grid conditions, i.e., unbalanced voltage dips. Concerning the control electronic system, the chosen electronic structure and the task distribution between the two processors used are shown.

This paper proposes an H ∞ current controller that allows power converter input admittance shaping, both in modulus and in phase, in a three phase voltage source converter to grid connection through an L filter. In addition to this new feature, controller allows grid current tracking. Proposed controller tries to solve then trade-off problem between stand-alone converter performance (current tracking) and its influence on global stability (admittance control) by an intuitive controller design. Time and frequency domain simulated results demonstrate the good performance of the proposed method.

This paper introduces an H ∞ current controller that allows the shaping of power converters' input admittance in frequency domain, allowing to define not only its magnitude but also its phase. In addition to admittance shaping, some current tracking specification are also introduced to the algorithm demonstrating that both admittance shaping and tracking performance are possible, but not at the same frequencies. The correct performance (both in time and frequency domain) of the proposed method is demonstrated in an L-filter grid-connected single-phase VSC.

This paper presents an optimal LQR control for a static-series compensator (SSC) and propounds a design criterion for the choice of the LQR weights in order to mitigate grid-voltage dips. The authors present a SSC state-space model that considers the modelling of grid, transformer and R-L load. The proposed control works in the Stationary-Reference Frames (SRF) and implements AFC resonant controllers, allowing the direct control of unbalanced dips in a three wire grid-load connection.

This work proposes and compares two multivariable identification methods that obtain the discrete state-space model of a grid-connected Voltage Source Converter with LCL filter. The two methods are based on two of the so-called subspace identification methods: the Multivariable Output Error State sPace (MOESP) and the Numerical algorithm for Subspace State Space System Identification (N4SID) methods. That kind of methods is based on the QR decomposition and the singular value decomposition (SVD) and works with black-box model structures, which are particularly flexible in the choice of the model representation. The so-obtained discrete state-space model of the converter could be used to design directly a controller for the system using advanced control techniques as optimal control, robust control, etc.

The paper proposes a multivariable identification method to identify the discrete state-space model of a grid-connected voltage-source converter and LCL filter. The method is based in one of the so-called subspace identification methods: the N4SID method. That kind of methods are based on the QR and the Singular Values decompositions and works with black-box state-space model structures, which mean particularly flexibility in the choice of the model representation. The so obtained discrete state-space model of the converter could be used to design directly the control of the system using advanced control techniques as robust control, optimal control, etc.

This paper proposes the use of a robust loop-shaping H ∞ procedure for the current control loop of VSCs connected to the grid through LCL filters. The proposed method yields robust controllers for systems connected to grids whose equivalent impedance is only known in a predefined range. The design procedure is based on simplified classical loop-shaping principles. The result is a discrete-time controller, suitable for being executed on a Digital Signal Processor and executable in real-time that is robust for the considered conditions. The procedure has been verified by simulation and experimental testing.

This article presents the results of a study that looks at how participation in a robot competition conducted by university students in engineering contributed the development of basic professional skills in the creators of the robots. The study was performed by circulating one survey to the participants of robot competitions in two different contexts: The first, using open questions was done using participants of the prestigious international competition EUROBOT during the 2007 final held in France; The second, more extensive survey, was done using a wide selection of participants from the competition held at the University of Alcala.

This work proposes a novel method based on a prediction error minimization (PEM) algorithm to identify the discrete state-space model of a LCL filter which is used as interface between a Voltage Source Converter and the utility grid. This model is used to design directly a discrete state-space current control based on a servo controller. The so-obtained model makes easier the use of advanced control techniques as Linear Quadratic Regulators, state estimators, Robust Control, etc. The main advantage of using an identification process against to design the controller starting from the manufacturer data is that the control is not sensitive to modeled errors. The method can be applied to closed-loop systems, being suitable for adaptive control designs.

This paper proposes a method based on a prediction error minimization (PEM) algorithm to identify the discrete state-space model of a LCL filter used as interface between a Voltage Source Converter and the electrical grid. This model can be used to design directly a discrete current controller in the state space using techniques as Linear Quadratic Regulator, state estimators, etc. The method can be applied to a closed-loop system, being suitable for adaptive control designs.

Grid-converters in applications such as active rectifiers, active filters, uninterruptible power supplies, and distributed generation systems need an optimal control of the active and reactive powers interchanged with the grid. It reduces considerably the total harmonic distortion of grid currents. Anyway, though grid converters have optimal controllers, with a perfect tuning of all constants, the final response can be seriously compromised if the acquired analog signals are very noisy. This situation is very frequently, especially in medium and high power grid converters. This work proposes different algorithms, as digital filters and Kalman filters, which have as objective reduce the noise to the input of converter controllers. In this way, the quality of the current grids improves considerably.

The aim of this paper is to propose a multivariable current controller of voltage source converters (VSC) connected to the grid through an LCL filter. The proposal is a linear quadratic (LQ) servocontroller, which uses a fullorder estimator to obtain the full state vector. Two kinds of estimators are proposed and compared: prediction estimator and Luenberger estimator. The behavior and robustness of the controller is tested in presence of grid disturbances and mismatches in the model parameters.

New energy concepts such as Distributed Power Generation are changing the face of electric distribution and transmission. Power electronics researchers try to apply new electronic controller solutions enabling the implementation of new and more complex control algorithms. This paper presents the design, implementation and test of the Field-Programmable Gate Array (FPGA) tasks of a novel real-time controller for a NPC (Neutral Point Clamped) (three-level) multilevel converter. The controller is based on a floating-point Digital Signal Processor (DSP) and a FPGA, by operating in a cooperative way. The proposed system is instantiated to the next system: a 150 kVA back-to-back three-level NPC VSC (Voltage Source Converter) for wind turbine applications.

This document presents interesting results in a grid converter synchronization based on the p.u. magnitudes in the positive reference frames. This methods introduces small modifications respect to the sPLL (software phase locked loop) presented by the authors in previous works. The main idea is to compare the two sPLL systems: in the first one, the error signal is Vdp (positive refence frame); whereas in the second one, the error signal is Vdp divided by its magnitude. For this reason, the method receives the name: "p.u. magnitude". The desired objective is to reflect correctly the abrupt phase changes in the control synchronization system of the grid converters, which will be more important under hard unbalance dips and phase-angle jumps in the grid system. On other hand, in the introduction of this paper, the new "generalized functions block" for the synchronization systems are suggested that are used to represent the models in great part of the different works about this topic. This generalization includes two blocks which are denominated: SIB (signal interface block) and CB (calculator block).

This work presents a recursive algorithm to estimate the grid equivalent impedance and generator from the current and voltage measurements performed in the common coupling point, PCC, of a power converter. The method is based in a recursive least squares algorithm performed over the complex space. The described method works on-line, has low computational overhead and does not require the injection of disturbances in the grid.

This paper proposes a new recursive algorithm to estimate the grid impedance from the current and voltage measurements performed in the common coupling point. The method is based in a recursive least squares algorithm performed over the complex space. The described method has low computational overhead, does not require the injection of disturbances in the grid and simultaneously estimates the grid equivalent impedance and the grid equivalent generator.

The objective of this paper is to analyse the influence of a non-ideal grid over hysteresis controllers of converters connected to the grid through both L and LCL filters. The paper will describe the different possible configurations in both cases, analysing the effect of a general R-L grid. Limit values for both grid inductance and resistance are given for the L structure. In the case of LCL connection, the analysis will overview important issues such as active damping, and switching behaviour giving limits to stability as a function of grid impedance value.

The rapid increase in current harmonics in the AC systems due to large introduction of solid state power switching devices is deteriorating the power quality. Consequently the interest in active filters is growing. However for their correct operation it is imperative to know the harmonic parameters such as magnitude and phase. Hence many algorithms have been developed to detect the harmonics. The frequency-domain-based algorithms (generally adopting the Fourier Transform) request high computation resources. The time-domain techniques work on the instantaneous values of signals, therefore they request fewer calculations. By these reasons, the time-domain algorithms are frequently used in active filter control. In this work, a review of principal time-domain methods is carried out. Also, their principal problems are indicated and the possible solutions and improvements are proposed. The analysis is validated by experimental results obtained with digital signal processor TMS320C6713.

Nowadays the most used multilevel topology for variable-speed wind turbines (WTs) is the a back-to back three-level NPC (Neutral Point Clamped) VSCs (voltage source converter). One of the most critical elements in the design of this type of converters is the DC-bus capacitors. The calculus of these depends on the same factors that the same two-levels topology, and, moreover it is function of a low frequency ripple due to the current by NP point. In this work, the value of these capacitors is obtained from the analytical equations of the DC-bus voltage ripple due to the i DC (DC-bus current) and the i NP (NP current). These expressions are verified through simulation and practical results.

The intensive use of power converters and other non-linear loads in industry and by consumers in general, has produced an increasing deterioration of the power systems voltage waveforms. The presence of harmonic distortion in power lines results in greater power losses in the distribution systems, interference problem in communication systems and, sometimes, in operation failures of electronics equipments. Owing to these problems and to preserve the power quality, the active filters are used for harmonic elimination. In this filters, a perfect harmonic identification is fundamental, therefore, nowadays many identification algorithms are available, such as, DFT, Fourier Series, ect. However, in this work a more robust algorithm based on Kalman filtering is presented. Also, several grid voltage models in state variable are shown, since the Kalman Filter needs a model of the evaluated system (in this case, of the grid). Moreover, the advantages of this method with respect to the most used technique, FFT, are commented.

This paper proposes a new control scheme for the neutral-point balance in single-phase three level neutral-point clamped converters. The control is addressed, considering the plant under study a Non-Linear Time-Variant system and applying a quasi-exact linearization to it. This way it is possible to apply classic control techniques. The method depicted is simple, general and suitable to back and boost topologies as well as to inverter or rectifier operating modes. Correct operation is verified under both simulation environment and experimental operation. Experimental operation has been verified with linear and non-linear load.

Due to the intensive use of power converters and other non-linear loads in industry and by consumers in general, it can be observed an increasing deterioration of the power quality. The presence of harmonics in power lines results in greater power losses in the distribution system, interference problems in communication systems and, sometimes, operation failures of elecronic equipments. Owing to these problems, active filters are used for harmonic elimination and they need a reliable identification method to operate efficiently. Many algorithms have been developed to estimate the harmonic parameters such as magnitude and phase. In this work, it is reported a comparison of identification techniques for single-phase systems, moreover, improvements on these methods and a new algorithm based on correlators are proposed. This comparison can guide in choosing a suitable technique in each application. Simulation and experimental results (obtained with digital signal processor TMS320C6713) validate the proposed comparison.

A very interesting feature of VSC based back-to-back converters connected to the grid is their capacity to work in the four quadrants, and consequently their capability to change the converter power flow direction eventually. Variable speed wind turbines represent one of the main applications of these devices. The objective of this article is the study of the influence of R/sub L/ and C/sub DC/ value over a back-to-back converter from a classic linear control point of view. The analysis that is presented in this article begins with the extraction of two different models for the DC bus which permits the deduction of ranges of nominal values for the DC bus capacitor (C/sub DC/) and for the load resistance (R/sub L/) which ensures system stability. A stability analysis of these models as a function of the operating mode of the VSC (rectifier, inverter), of the DC-bus capacity and of the value and sign of the current across the DC-bus is the objective of the second part of this article. Finally, a control scheme for both models is proposed. Special interest is dedicated to the effects of DC-bus parameters (R/sub L/ and C/sub DC/) over the controller operation and to a correct chose for that values: an analytic method for constant design is presented. Control system correct operation is verified under a non-linear back-to-back simulation environment.