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Modular converters such as the multilevel cascaded H-bridge (CHB) are an attractive option for multiple applications mainly because of inherent modularity and fault-tolerant operation. This article is focused on the CHB converter operating with unbalanced conditions (different dc voltages and/or modulation indexes). Under these circumstances, applying the conventional control and modulation strategies, the output voltage harmonic spectrum is degraded. In this article, a generalized variable-angle phase-shifted pulsewidth modulation (PS-PWM) technique for CHB converters with a large number of power modules (> 3) is presented. The method considers all possible cells’ combinations to form groups and assigns the role of each cell in the group. This cell role defines the identifier of the cell in the variable-angle PS-PWM technique. In the steady state, in each group of cells, the harmonic distortion of the CHB output voltage located at twice the carrier frequency ƒc is eliminated, while the distortion at 4ƒc is also diminished. Experimental results show how the proposed technique achieves superior harmonic performance without introducing any significant disadvantage. [ABSTRACT FROM AUTHOR]

Finite control set model predictive control (FCS-MPC) chooses at each sampling period the input control vector state without using a modulator. Therefore, it belongs to the direct control family. As a consequence, the conventional implementation of FCS-MPC presents variable switching frequency. This makes difficult the design of the output filter. This article investigates the period control approach (PCA-MPC) to achieve a fixed switching pattern, similar to that of modulated strategies placed at a desired frequency. PCA-MPC cannot reach a reference switching frequency without a steady-state error. Nonetheless, it is more than capable of reaching a fixed switching pattern, with clean spectrum at a wide range of switching frequencies. This feature is used here to design an embedded control objective to reach the desired switching frequency. The implementation of the cost function is done through modification of PCA-MPC measurements. These modifications allow one to obtain fast and precise estimates of the switching frequency, which, in turn, permits a fast control. The performance of this control strategy is evaluated through simulation and corroborated by experimental validation on a two-level three-phase, power converter. [ABSTRACT FROM AUTHOR]

One of the main drawbacks with the use of finite control set model predictive control (FCS-MPC) in power converters is the variable switching frequency. When not taken into consideration, it can vary randomly depending mainly on the operating point of the system and the sampling time. This will produce a wide distributed voltage and current spectrum, causing audible noise, resonances, and poor steady-state behavior. To address this issue, a new algorithm, based on the control of the switching period within the cost function, is proposed in this paper. The algorithm shows to be very effective in achieving a regular commutation pattern, similar to pulse width modulation (PWM) techniques, which is evidenced even by similarities in the sideband harmonics generated in the voltage spectrum. The algorithm is easy to design and implement, demanding very low computation power, and enables a combination of the advantages of FCS-MPC with the benefits of a PWM-like power quality. Simulations and experimental results are presented to validate the proposed method, which are compared to previously reported FCS-MPC techniques dealing with the same challenge. [ABSTRACT FROM PUBLISHER]

Multilevel cascaded H-bridge converters have become a mature technology for applications where high-power medium ac voltages are required. Normal operation of multilevel cascaded H-bridge converters assumes that all power cells have the same dc voltage, and each power cell generates the same voltage averaged over a sampling period using a conventional phase-shifted pulse width modulation (PWM) technique. However, this modulation method does not achieve good results under unbalanced operation per H-bridge in the power converter, which may happen in grid-connected applications such as photovoltaic or battery energy storage systems. In the paper, a simplified mathematical analysis of the phase-shifted PWM technique is presented. In addition, a modification of this conventional modulation method using variable shift angles between the power cells is introduced. This modification leads to the elimination of harmonic distortion of low-order harmonics due to the switching (triangular carrier frequency and its multiples) even under unbalanced operational conditions. The analysis is particularized for a three-cell cascaded H-bridge converter, and experimental results are presented to demonstrate the good performance of the proposed modulation method. [ABSTRACT FROM PUBLISHER]

The cost reduction of power-electronic devices, the increase in their reliability, efficiency, and power capability, and lower development times, together with more demanding application requirements, has driven the development of several new inverter topologies recently introduced in the industry, particularly medium-voltage converters. New more complex inverter topologies and new application fields come along with additional control challenges, such as voltage imbalances, power-quality issues, higher efficiency needs, and fault-tolerant operation, which necessarily requires the parallel development of modulation schemes. Therefore, recently, there have been significant advances in the field of modulation of dc/ac converters, which conceptually has been dominated during the last several decades almost exclusively by classic pulse-width modulation (PWM) methods. This paper aims to concentrate and discuss the latest developments on this exciting technology, to provide insight on where the state-of-the-art stands today, and analyze the trends and challenges driving its future. [ABSTRACT FROM PUBLISHER]

Multilevel cascaded H-bridge converters have found industrial application in the medium-voltage high-power range. In this paper, a generalized modulation technique for this type of converter based on a multidimensional control region is presented. Using the multidimensional control region, it is shown that all previous modulation techniques are particularized versions of the proposed method. Several possible solutions to develop a specific implementation of the modulation method are addressed in order to show the potential possibilities and the flexibility of the proposed technique. In addition, a feedforward version of this technique is also introduced to determine the switching sequence and the switching times, avoiding low harmonic distortion with unbalanced dc voltages. Experimental results are shown in order to validate the proposed concepts. [ABSTRACT FROM AUTHOR]

Multilevel inverters have emerged as attractive high-power medium-voltage power-conversion systems. They are mainly controlled with high-frequency pulsewidth-modulation methods. This is not suitable for very high-power application due to significant switching losses. This paper presents an adaptive duty-cycle modulation algorithm that reduces the switching frequency to a minimum necessary to fulfill the dynamic requirements of the system. Switching losses are, therefore, strongly reduced. This is achieved by using the slope of the voltage reference to adapt the modulation period to ensure that only one-step change between two voltage levels. Simulation and experimental results are presented for a nine-level cascaded inverter. Voltage waveforms obtained for variable frequencies and amplitudes show similar switching patterns, with a reduced and near-constant number of commutations per cycle, regardless of the reference frequency and amplitude. [ABSTRACT FROM AUTHOR]