Humberto Pinheiro

Design and Implementation of a Robust Current Controller for VSI Connected to the Grid Through an LCL Filter

This paper describes the design and implementation of a discrete controller for grid-connected voltage-source inverters with an LCL filter usually found in wind power generation systems. First, a theorem that relates the controllability of the discrete dynamic equation of the inverter with LCL filter and the sampling frequency is derived. Then, a condition to obtain a partial state feedback controller robust to grid impedance uncertainties and based on linear matrix inequalities is proposed. This controller guarantees the stability and damping of the LCL filter resonance for a large set of grid conditions without requiring self-tuning procedures. Finally, an internal model controller is added to ensure asymptotic reference tracking and disturbance rejection, significantly reducing the impact of grid background voltage distortion on the output currents. Experimental results are presented to support the theoretical analysis and to demonstrate the system performance.

Robust Controller for DFIGs of Grid-Connected Wind Turbines

This paper proposes a new robust controller in a stationary reference frame for doubly fed induction generators (DFIGs) of grid-connected wind turbines. Initially, a DFIG dynamic model is derived from the voltage and flux equations in αβ coordinates, where uncertainties and disturbances intrinsic to the system are accounted for as perturbation terms are added to the nominal model. Then, a controller design procedure that guarantees the DFIG stability under uncertainties and disturbances at the grid side is presented in detail. It is demonstrated that a very fast dynamic behavior can be obtained with the proposed controller, which improves the transient response of the grid-connected DFIG, particularly under conditions of unbalanced voltage dips resulting from asymmetrical network faults. In order to conform with the fault ride-through capability requirements, this paper proposes a new reference strategy, which is divided into normal and fault operation modes. Experimental results are given to support the theoretical analysis and to illustrate the performance of the grid-connected DFIG with the proposed controller.

Adaptive Current Control for Grid-Connected Converters With LCL Filter

This paper presents a discrete-time adaptive current controller for grid-connected voltage source PWM converters with LCL-filter. The main attribute of the proposed current controller is that, in steady state, the damping of the LCL resonance does not depend on the grid characteristic since the adaptive feedback gains ensure a predefined behavior for the closed-loop current control. Simulation and experimental results are presented to validate the analysis and to demonstrate the good performance of the proposed controller for grid-connected converters subjected to large grid impedance variation and grid voltage disturbances.

Comparison of digital control techniques with repetitive integral action for low cost PWM inverters

This paper presents a comparison among digital control techniques with repetitive integral action applied to voltage-source PWM inverters. As a result of the repetitive integral action, these digital control schemes can reduce steady-state errors and distortions caused by unknown periodic disturbances, which usually result from the input source and output load. Moreover, these digital control schemes measure only the output voltage, decreasing the amount of sensors and the overall system cost. The control laws, stability analysis, common and distinguishing features of these control algorithms are discussed. Experimental results from a PWM inverter (110 V/sub RMS/, 1 kVA) controlled by a low cost microcontroller are presented to demonstrate the control techniques performance under different load conditions, output filters and command strategies.

Analysis and Design of a New High-Efficiency Bidirectional Integrated ZVT PWM Converter for DC-Bus and Battery-Bank Interface

This paper proposes a high-efficiency bidirectional integrated zero-voltage transition (iZVT) pulsewidth-modulation (PWM) converter for dc-bus and battery-bank interface. The proposed converter can operate as battery charger when the utility is within its acceptable voltage range and can supply energy to critical loads when the utility fails. The converter practically eliminates both low- and high-frequency current ripple on the batteries, thus maximizing battery life without penalizing the volume of the converter. Moreover, soft switching of all switches is achieved using the proposed integrated auxiliary commutation circuit (iACC). Just one iACC is used to provide soft-switching conditions for the three converters that compose the system: the preregulator (boost), the battery charger (bidirectional converter operating as a buck), and the backup converter (bidirectional converter operating as a boost). This auxiliary circuit has few components and low reactive energy, increasing the systems overall efficiency. Experimental results based on a 580-W prototype are presented to validate the analysis and the proposed design procedure and to demonstrate the performance of the proposed approach

Self-sustained oscillating resonant converters operating above the resonant frequency

This paper presents a new control technique for resonant converters. Unlike conventional variable frequency control which externally imposes the switching frequency, the proposed scheme is based on controlling the displacement angle between one of the resonant circuit variables, typically the current through the resonant inductor, and the voltage at the output of the inverter. As a result, zero-voltage switching (ZVS) can be ensured over a wide operating range. The proposed control technique cam be applied for series, parallel, and series-parallel resonant converters. As an example, the static characteristics and dynamic model of a series-parallel resonant converter with the proposed controller are derived and the system behaviour is investigated in detail. Experimental results are given to demonstrate the operation of resonant converters with the proposed controller and to validate the analysis.

Discrete-Time Sliding Mode Observer for Sensorless Vector Control of Permanent Magnet Synchronous Machine

This paper proposes a sensorless vector control that combines two discrete-time observers to estimate the rotor speed and position of permanent magnet synchronous machines (PMSM). The first one is a sliding mode (DSM) current observer and the second one is an adaptive electromotive force (EMF) observer. Initially, the sliding conditions that assure the sliding motion around the sliding surface are derived and a design procedure to the DSM current observer is developed. Moreover, using discrete-time adaptive Lyapunov based EMF observer the rotor speed and position are obtained. Experimental results validate the theoretical analysis and demonstrate the very good performance of the proposed discrete-time sensorless vector control.

Analysis and design of a single stage power factor corrected full-bridge converter

A simple PWM full-bridge converter that integrates input power factor correction (PFC) with DC-DC conversion is examined in the paper. The converters operation is analyzed, and the results of the analysis are used to determine its steady-state characteristics. A design procedure for the selection of components is derived and demonstrated with an example. The feasibility of the converter and its ability to satisfy the IEC 1000-3-2 Class D requirements for electrical equipment are shown with results obtained from an experimental prototype.

Generalized Carrier-Based Modulation Strategy for Cascaded Multilevel Converters Operating Under Fault Conditions

This paper proposes a simple modulation approach for symmetrical and asymmetrical cascaded multilevel converters, suitable for operation under normal and fault conditions. Initially, the set of all possible converter common-mode voltages for linear operation is derived, considering the status of each converter cell. It is demonstrated that the methods previously described in the literature belong to the derived set. By properly selecting the common-mode voltage, the entire converter voltage synthesis capability can be achieved under any fault condition. The proposed modulation approach has been applied for both symmetrical and asymmetrical cascaded multilevel converters. Simulation and experimental results show the effectiveness of the proposed modulation approach and validate the theoretical analysis.

A Three-Phase UPS That Complies With the Standard IEC 62040-3

This paper proposes a down-sampled discrete-time internal-model-based controller in the synchronous reference frame with a reduced number of poles. This controller is suitable for three-phase pulsewidth modulation inverters with output transformer for double-conversion uninterruptible power supply applications. It is demonstrated that the use of a down-sampled rate and fewer poles in the internal model results in a number of benefits, among which are the following: 1) improvement of the transient response; 2) increase of the stability margin of the closed-loop system; 3) a straightforward implementation in fixed-point digital signal processor (DSP) and microcontroller implementation as well as a reduction of the required memory space; and 4) a simple solution for the saturation of the output transformer. As a result, it is possible to obtain output voltages with reduced total harmonic distortion while ensuring good transient performance for both linear and nonlinear loads. To confirm the advantages claimed for the proposed synchronous reference dq frame internal-model-based controller and to demonstrate the steady-state and transient performance under the test conditions of the International electrotechnical commission standard 62040-3, the experimental results from a 10-kVA space-vector-modulated three-phase inverter, which is fully controlled by a DSP TMS320F241, are presented.