E. Dominguez

Analysis of the Power Balance in the Cells of a Multilevel Cascaded H-Bridge Converter

Multilevel cascaded H-bridge (CHB) converters have been presented as a good solution for high-power applications. In this way, several control and modulation techniques have been proposed for this power converter topology. In this paper, the steady-state power balance in the cells of a single-phase two-cell CHB is studied. The capability to be supplied with active power from the grid or to deliver active power to the grid in each cell is analyzed according to the dc-link voltages and the desired ac output voltage value. The limits of the maximum and minimum input active powers for a stable operation of the CHB are addressed. Simulation results are shown to validate the presented analysis.

Conventional Space-Vector Modulation Techniques Versus the Single-Phase Modulator for Multilevel Converters

Space-vector modulation (SVM) is a well-suited technique to be applied to multilevel converters and is an important research focus in the last 25 years. Recently, a single-phase multilevel modulator has been introduced, showing its conceptual simplicity and its very low computational cost. In this paper, some of the most conventional multilevel SVM techniques have been chosen to compare their results with those obtained with single-phase multilevel modulators. The obtained results demonstrate that the single-phase multilevel modulators applied to each phase are equivalent with the chosen well-known multilevel SVM techniques. In this way, single-phase multilevel modulators can be applied to a converter with any number of levels and phases, avoiding the use of conceptually and mathematically complex SVM strategies. Analytical calculations and experimental results are shown, validating the proposed concepts.

New Space Vector Modulation Technique for Single-Phase Multilevel Converters

Single-phase multilevel converters are suitable for medium power applications as photovoltaic systems and switched reluctance machines. An overview of possible modulation methods including carrier-based pulse width modulation and space vector modulation techniques for multilevel single-phase converters is presented. A new space vector modulation for this type of converters is proposed. This space vector modulation method is very simple presenting low computational cost. Different solutions for the space vector modulation are presented achieving similar output results but imposing restrictions on the power converter topology. Optimization algorithms balancing the DC-link voltage or minimizing the commutation losses are presented. Experimental results using a 150 kVA five-level diode- clamped converter are shown to validate the proposed modulation and optimization methods.

Controller design for a single-phase two-cell multilevel cascade H-bridge converter

In this paper is studied the single-phase two-cell multilevel cascade H-bridge power converter connected to the grid, acting as a synchronous rectifier. The power exchange process between the cells of the converter and the grid is analyzed. Based on this analysis and on the power converter model the stages of the controller design process are shown. A new controller for the cascade power converter is proposed, achieving the regulation of each DC-Link capacitor voltage towards its reference. The proposed controller includes a repetitive scheme in the current tracking loop, providing low current harmonic content and almost unity power factor. Simulation results have been carried out in a 10 kVA single-phase two-cell multilevel cascade H-bridge power converter model to illustrate the good performance of the proposed controller.

New trends and topologies for high power industrial applications: The multilevel converters solution

This paper reviews briefly the current scenarios where power electronics converters are being applied. In the paper, the main focus moves towards the high power applications, reviewing the different alternatives and topologies. The multilevel approach is studied in more depth, showing that is a good solution to the challenges that medium voltage-high power applications pose. Several industry examples are introduced and the most suitable modulation techniques for multilevel high power converters are explained. Among them, the recent selective harmonic mitigation method appears as a good solution to achieve a high performance. Finally the conclusions are addressed.

Recent advances in high-power industrial applications

The industrial electronics market is continuously changing following the users demand. This paper introduces the current industrial electronics applications and is focused in the medium-voltage high-power ones. The multilevel approach is the most attractive solution to achieve the challenges that medium voltage-high power applications arise. Several commercial examples are introduced and some of the last research advances related to multilevel power electronic converters are presented in this paper.

Two-dimensional modulation technique with dc voltage control for single-phase two-cell cascaded converters

In this paper, a simple feed-forward modulation technique for single-phase two-cell multilevel cascaded converters is presented. All the possible switching states of the power converter are taken into account applying a two dimensional control region. The proposed technique uses the actual values of the DC-Link capacitor voltages to obtain output phase voltages and currents with low harmonic distortion with any dc voltage in the H-bridges of the cascaded converter. The possible switching sequences of the converter are studied and, depending on the actual dc voltage values, their desired values are achieved. Simulation results are shown in order validate the proposed technique working as a synchronous rectifier.

Digital Implementation Issues for a Three-Phase Power Converter Development Using a Repetitive Control Scheme

In this paper the digital implementation of the repetitive control scheme for the three-phase two-level power converter is investigated. The proposed controller has an outer control loop to regulate the output capacitor voltage and an inner control loop for the current tracking process, which is composed of a damping term plus a repetitive control scheme, with the aim to achieve unity power factor and low THD content at the input currents. The repetitive control scheme consists on a negative feedback array of a single delay line plus a feedforward path. The implementation in a digital signal processor of the repetitive term has some issues that are pointed out in this paper. An adaptation rule to the delay parameter of the repetitive controller is proposed in order to improve the system behavior. The experimental results have been carried out in a 30 kVA three- phase power converter prototype to illustrate the good performance of the digital implementation of the controller, including a comparative study of the system behavior under the repetitive controller with the theoretical and the modified delay parameter.

Optimized Direct Power Control Strategy using Output Regulation Subspaces and Pulse Width Modulation

In this paper, a direct power control (DPC) using output regulation subspaces (ORS) for a three-phase two-level converter is presented. An optimal controller design and the use of optimized modulation techniques permit to improve the system performance minimizing the necessary grid-connection inductance. ORS plus proportional controllers are proposed for the generation of the reference vector to decrease active and reactive power errors and to achieve unity power factor. The generation of the reference vector is carried out thanks to simple pulse width modulation techniques. The computational cost of the proposed control is really low and all the calculations can be done online allowing its implementation in low cost microprocessors. Simulation and experimental results are shown in order to illustrate the good performance of the proposed control strategy

Direct Power Control for three-phase power converters under distorted input voltages

Conventional Direct Power Control (DPC) technique is a simple and efficient control strategy for three-phase power synchronous rectifiers. However its performance is deteriorated when the power rectifier is working under distorted or unbalanced input voltages. In this paper the distorted input voltages operation of the system is under concern, and a model-based DPC for three-phase power converters is designed. The obtained expressions for the input control signal permit to design an adaptive control law which ensures the power reference tracking and the high performance operation of the power system. Besides, this adaptive control law minimizes the errors introduced by the parameters uncertainties as the smoothing inductor value, the grid frequency and the input voltage harmonic content. Controller design process and stability of the system are shown. Besides, the instantaneous power reference definition to achieve sinusoidal input currents are presented. Simulation results for a synchronous three-phase power rectifier prototype are shown to validate the proposed controller.