Multicell Converter Research Articles

A complete control structure based backstepping controller design for stacked multi-cell multi-level SPWM VSC STATCOM

Currently, the installation of FACTS (Flexible Alternative Current Transmission System) devices such as synchronous static compensators (STATCOM) represent a very useful solution to improve both network stability and power quality. The effectiveness of these devices is principally determined by the used static converter (topology and the switching strategy) and the adopted dynamic law characterizing the controller structure. This paper illustrates the stacked five-level, multi-cell STATCOM compensator performances governed by a Backstepping controller technique compared to the use of the conventional PI controller (SPWM) for controlling voltage variation (sags or overshoots) affecting a given network point. The interest in using stacked multi-cell converters topology is to increase the number of voltage levels in order to ensure a good quality voltage waveform at the specified connection point. Therefore, the proposed backstepping controller based on the use of Lyapunov control functions presents a simplified structure and a reduced computational load compared to many others advanced control approaches. The proposed system is verified by running a simulation analysis with Matlab Simulink environment. The simulation results obtained show that the proposed STATCOM configuration ensures good voltage regulation at the common connection point (PCC), correction of the unity power factor on the grid side and good response time via reactive power compensation under variable load conditions.

Power Losses Reduction of Stacked Multicell Converter Fed Wind Energy Conversion System (WECS) Using Hybrid Methodology

This paper proposes a hybrid method for reduction of power losses of multicell converter in wind energy conversion system (WECS). The proposed methodology is the Artificial Intelligence (AI) based phase disposition pulse width modulation (PD-PWM). The Artificial Intelligence (AI) is the Gradient Boosting Decision Tree algorithm (GBDT). Hence it is named as GBDT-PD-PWM. Gradient boosting is a machine learning system that creates a prediction model in the group of weak prediction models, often called decision trees. The multicell converter is employed as the grid side converter (GSC) of WECS to attenuate harmonics created by nonlinear load. For stacked multicell converters (SMCs), a method of capacitor voltage balance based on PD-PWM approach is proposed.

Nonlinearities of Multisampled Phase-Shifted PWM in Unbalanced Multicell Converters

Nonlinearities of Multisampled Phase-Shifted PWM in Unbalanced Multicell Converters

Optimized Switching Frequency Voltage Balancing Schemes for Flying Capacitor Based Multilevel Converters

The redundant switching states of flying capacitor-based (FC-based) multilevel converters are used to balance the voltages of the flying capacitors. Attempts to balance capacitor voltages have ignored the switching transitions between converter switching states. In this paper, we propose a generalized voltage balancing scheme with an optimized switching frequency scheme for FC-based multilevel converters, including the classic flying capacitor multicell (FCM) converter, which needs less computation. All the switching transitions between the switching states of adjacent voltage levels are considered by the overall priority index of switching frequency. In addition, an overall priority index is utilized to balance the voltages of flying capacitors. Using these two indexes, three criteria are developed to select the proper switching state of the converter. Using these criteria, the proposed decoupled SVM scheme is implemented intuitively and simply for FC-based multilevel converters. The proposed scheme is evaluated by simulation results of a five-level FCM converter. Moreover, experimental results demonstrate the robustness of the proposed method in the static load and transient conditions.

Reduction of DC Capacitor Size in Three-Phase Input/Single-Phase Output Power Cells of Multi-Cell Converters through Resonant and Predictive Control: A Characterization of Its Impact on the Operating Region

Cascaded H-bridge drives require using a significant-size capacitor on each cell to deal with the oscillatory power generated by the H-bridge inverter in the DC-link. This results in a bulky cell with reduced reliability due to the circulating second harmonic current through the DC-link capacitors. In this article, a control strategy based on a finite control set model predictive control and a proportional-resonant controller is proposed to compensate for the oscillatory power required by the H-bridge inverter through the cell’s input rectifier. With the proposed strategy, a DC-link second harmonic free operation is achieved, allowing for the possibility of reducing the capacitor size and, in consequence, the cell dimensions. The feasibility of the proposed control scheme is verified by experimental results in one cell of a cascade H-bridge inverter achieving an operation with a capacitance 141 times smaller than required by conventional control approaches for the same voltage ripple.

A Novel Active Neutral Point-Clamped Five-Level Inverter With Single-Stage-Integrated Dynamic Voltage Boosting Feature

Application of mid-point-clamped multilevel inverters, e.g., active neutral point-clamped (ANPC) or stacked multicell (SMC) converters, have been significantly broadened in recent years due to better performance, reduced common-mode voltage and leakage current, bidirectional power flow capability and simplicity in circuit design. Nonetheless, the dc-link voltage utilization factor is only 50% (i.e., the peak of the ac output voltage is half of the dc-link voltage), which requires additional front-end boost integrated circuit to accommodate low and wide varying input voltage. In this article, a single-stage ANPC-based five-level (5 L) boost integrated inverter is proposed, which is able to meet the peak amplitude of the grid voltage even when the dc-link voltage is low and wide varying. The proposed topology is comprised of two boost inductors, four capacitors, and 10 power switches, which can be assembled with two standard T-Type modules and two additional discrete power switches. Compared with the conventional two-stage ANPC and/or SMC-5 L inverters with a front-end bidirectional boost converter, the proposed topology requires the same number of power switches, whilst providing more flexible/dynamic voltage conversion gain with a reduced total standing voltage across the switches. Operating principles, circuit analysis and modulation strategy of the proposed topology are discussed. Finally, a comparative study supported by closed-loop grid-tied experimental results are presented to confirm the potentiality and feasibility of this proposal.

New Approach of Multi-Cell Stacked Cell Inverter for Solar Photovoltaic System

In this paper, a new inverter topology dedicated to isolated or grid-connected PV systems is proposed. This inverter is based on the structures of a stacked multi-cell converter (SMC) and an H-bridge. This new topology has allowed the voltage stresses of the converter to be distributed among several switching cells. Secondly, divide the input voltage into several fractions to reduce the number of power semiconductors to be switched. In this contribution, the general topology of this micro-inverter has been described and the simulation tests developed to validate its operation have been presented. Finally, we discussed the simulation results, the efficiency of this topology and the feasibility of its use in a grid-connected photovoltaic production system.

Carrier Redistribution Pulsewidth Modulation for Five-Level Stacked Multicell Converter

This article presents a modified carrier redistribution pulsewidth modulation (CRPWM) method for the five-level stacked multicell converter (SMC). The proposed CRPWM method is modified by exchanging the arrangement of the carriers of phase disposition pulsewidth modulation (PDPWM). With the ingenious carrier arrangement, the proposed PWM technique equalizes the utilization of phase leg voltage redundancies corresponding to the charging and the discharging state of each floating capacitor during one switching period of all the switches. The proposed modulation strategy has the same harmonic performance of line-to-line voltage with PDPWM, while the switch utilization is the same as phase-shifted pulsewidth modulation (PSPWM). Therefore, the voltage balancing of floating capacitors is achieved during a minimum switching period even with the influence of load current change and inappropriate carrier ratio is considered. The modified CRPWM is analyzed in theory and the voltage balancing ability and output harmonics performance are compared with PSPWM and traditional CRPWM. Simulation and experimental results on the laboratory prototype five-level SMC confirm the validity of the proposed CRPWM technique.

On modeling and real-time simulation of a robust adaptive controller applied to a multicellular power converter

Introduction. This paper describes the simulation and the robustness assessment of a DC-DC power converter designed to interface a dual-battery conversion system. The adopted converter is a Buck unidirectional and non-isolated converter, composed of three cells interconnected in parallel and operating in continuous conduction mode. Purpose. In order to address the growing challenges of high switching frequencies, a more stable, efficient, and fixed-frequency-operating power system is desired. Originality. Conventional sliding mode controller suffers from high-frequency oscillation caused by practical limitations of system components and switching frequency variation. So, we have explored a soft-switching technology to deal with interface problems and switching losses, and we developed a procedure to choose the high-pass filter parameters in a sliding mode-controlled multicell converter. Methods. We suggest that the sliding mode is controlled by hysteresis bands as the excesses of the band. This delay in state exchanges gives a signal to control the switching frequency of the converter, which, in turn, produces a controlled trajectory. We are seeking an adaptive current control solution to address this issue and adapt a variable-bandwidth of the hysteresis modulation to mitigate nonlinearity in conventional sliding mode control, which struggles to set the switching frequency. Chatter problems are therefore avoided. A boundary layer-based control scheme allows multicell converters to operate with a fixed-switching-frequency. Practical value. Simulation studies in the MATLAB / Simulink environment are performed to analyze system performance and assess its robustness and stability. Thus, our converter is more efficient and able to cope with parametric variation.

Design Aspects for Achieving High Bandwidth Series and Parallel Multicell Converters

Series and parallel multicell converters (SMCs and PMCs) have been conventionally used in high-voltage and high-power applications. Recently, due to the improved efficiency with the increase of the number of cells, they are also applied in small power converters with high power density. The higher the number of cells, the higher the apparent switching frequency, so the filter requirements are reduced, which means less energy is stored in the passive components. This potentially allows for faster dynamic responses in closed-loop operation, i.e., higher bandwidth of the converters. This article highlights the filter design and the importance of using an appropriate modulator for SMCs in order to reach high bandwidths, mainly when the number of cells increases. It is underlined, through a comparison with PMC, that SMC topologies are more sensitive to the modulation strategy. SMC and PMC performances of 2-kW converters are evaluated and compared through simulations implemented in software PLECS and validated using an experimental test bench.

FCS-MPC with Nonlinear Control Applied to a Multicell AFE Rectifier.

The use of controlled power converters has been extended for high power applications, stacking off-the-shelve semiconductors, and allowing the implementation of, among others, AC drives for medium voltages of 2.3 kV to 13.8 kV. For AC drives based on power cells assembled with three-phase diode rectifiers and cascaded H-bridge inverters, a sophisticated input multipulse transformer is required to reduce the grid voltage, provide isolation among the power cells, and compensate for low-frequency current harmonics generated by the diode-based rectifiers. However, this input multipulse transformer is bulky, heavy, and expensive and must be designed according to the number of power cells, not allowing total modularity of the AC drives based on cascade H-bridges. This study proposes and evaluates a control strategy based on a finite control set-model predictive control that emulates the harmonic cancellation performed by an input multipulse transformer in a cascade H-bridge topology. Hence, the proposed method requires conventional input transformers and replaces the three-phase diode rectifiers. As a result, greater modularity than the conventional multicell converter and improved AC overall input current with a THD as low as 2% with a unitary displacement power factor are achieved. In this case, each power cell manages its own DC voltage using a nonlinear control strategy, ensuring stable system operation for passive and regenerative loads. The experimental tests demonstrated the correct performance of the proposed scheme.

New Control Approach of Multicell Stacked Cell Inverter for Solar Photovoltaic System

This paper presents the study, modelling, and simulation of the DCSVM (Duty Cycle Space Vector Modulation) control technique applied to a new inverter topology dedicated to isolated or grid-connected photovoltaic systems using the MATLAB/Simulink software. This inverter is based on the structures of a stacked multicell converter (SMC) and an H-bridge. This new topology allows the voltage stresses of the converter to be distributed among several switching cells. It also allows the input voltage to be divided into several fractions so that the number of switching power semiconductors is reduced. The DCSVM is a control technique that generates control signals to the two-level voltage converter as well as the intermediate times. The main advantage of this control technique is the reduction in the number of calculations, especially for the trigonometric functions and the generation of the reference voltage. In this contribution, the general topology of this microinverter is described and the DCSVM control technique is presented. Finally, simulation results, the efficiency of this topology, and the validity of the DCSVM control in a grid-connected PV generation system are discussed. The results obtained are very satisfactory.

Adaptive Control and Formal Stability Analysis of Single-Phase Flying Capacitor Multicell DC-AC Inverters

Multicell converters and inverters using flying capacitors are usually used for high voltage power conversion applications. This study is carried out in a general context concerning an N-cell flying capacitors converter described by an (N + 1)th order average mathematical model. A nonlinear adaptive backstepping controller suitable for regulating the voltages across the N-flying capacitors is proposed with the aim to reach equitably distributed stresses on the converter switches. The controller also aims at generating a sinusoidal voltage with a desired amplitude and frequency at the output of the converter as well as to estimate, in real time, the load resistance. Special attention is devoted to the stability analysis of the whole closed-loop system with load estimation by using Lasalle’s invariance principle and Lyapunov method. The system performance is tested by means of numerical simulations showing that the proposed controller meets its objectives in terms of tracking, regulation, robustness and adaptation and demonstrating that it outperforms the classical backstepping controller.

Voltage-Balancing Control in Stacked Multicell Converter Based on Single Space-Vector Modulation

This article introduces voltage-balancing control methods in stacked multicell converters based on single space-vector modulation. Two different control strategies for capacitors voltage are presented. The basic vectors with the smallest common-mode voltage are chosen to accomplish the voltage-balancing control. The nearest vector is selected for the first proposed method, and the appropriate switch combinations of each phase are determined by the cost function of floating-capacitor voltages and neutral point (NP) voltage. For the second proposed method, we consider sacrificing a little current performance to provide more possibility of switching state selection. The switching state of each phase is determined according to the voltage-balancing control of floating capacitors, and the optimal switching state with the smallest NP voltage is selected from the two nearest adjacent basic vectors. Experimental results compared with the existing space vector pulsewidth modulation method are presented to verify the effectiveness of the proposed methods.

Z(TN) Observability Concept Analysis for Power Flying Capacitors of Multicells Converters

—In this paper, a new Z(TN) observability concept is proposed for power flying capacitors of multicell converter based on ordering of sequences. The aim is to solve the problem of capacitor’s voltages estimation by tacking account the hybrid behavior appearing in the power multicell converters and the ordering of the sequences. Furthermore, an analysis of obervability is introduced. Some illustrative results of a3-cell-converter with two legs are given in order to show the efficiency of the proposed approach by using high order sliding mode observer. The applicability of the designed observers are emphasized by the robustness test with resistance load variation. Finally, experimental results are presented in order to illustrate the performance of the proposed approach.

A Fast-Decoupled Space Vector Modulation Scheme for Flying Capacitor-Based Multilevel Converters

The space vector modulation (SVM) scheme is popularly implemented in multilevel converters. However, in flying capacitor-based (FC-based) multilevel converters, it becomes extremely challenging to implement due to the redundancy in phase voltage levels. The intrinsic property of line voltage redundancy of SVM in addition to the phase voltage redundancy of FC-based converters results in considerably increased computational complexity and storage requirements. In fact, one of the most important concerns about FC-based converters is the voltage control of the FCs. This article proposes a generalized decoupled SVM scheme for the FC-based multilevel converters, including the classic flying capacitor multicell (FCM) converters, which is less computationally complex. For balancing the FC voltages, an overall priority index is employed to determine the switching state with the best balancing characteristics. The proposed decoupled SVM scheme along with the capacitor voltage balancing scheme can be implemented intuitively and simply for all FC-based multilevel converters. The simulation results of a five-level FCM converter show the effectiveness of the proposed method in the regulation of FC voltages. Moreover, the proposed voltage balancing method is robust to static load and transient conditions. Also, experimental results demonstrate the effectiveness of the proposed method.

Finite Control Set—Model Predictive Control with Non-Spread Spectrum and Reduced Switching Frequency Applied to Multi-Cell Rectifiers

Multi-cell converters are widely used in medium-voltage AC drives. This equipment is based on power cells that operate with low-voltage-rating semiconductors and require an input multipulse transformer. This transformer cancels the low-frequency current harmonics generated by the three-phase diode-based rectifier. Unfortunately, this transformer is bulky, heavy, expensive, and does not extend the existing power cell (three-phase rectifier—Direct Current (DC) voltage-link—single-phase inverter) to the transformer. In this study, a harmonic cancelation method based on finite control set-model predictive control (FCS–MPC), extending the power cell’s modularity to the input transformer. On the other hand, it considers treating the two disadvantages of the FCS–MPC: High switching frequency and spread spectrum. The details were developed in theory and practice to obtain satisfactory experimental results.

Model predictive control of a hybrid stacked multicell converter with voltage balancing and fault tolerance capability

Due to the merits of low conduction losses and superior output power quality, a hybrid stacked multicell converter (HSMC) was put forward as a competitive alternative in the medium/low-voltage high-efficiency applications. A nine-level HSMC (9L-HSMC) can be seen as a combination of a five-level SMC (5L-SMC) and two-level half-bridge, and proper modulation and control strategy are essential for the HSMC to obtain the optimal output performance. This work develops a finite control-set model predictive control (MPC) strategy for 9L-HSMC to balance the capacitor voltages and maintain the desired outputs, rendering the absence of the linear regulator and pulse width modulation (PWM) modulator possible. The fault-tolerant operation can also be achieved flexibly when open-circuit faults occur at active bidirectional switches in the 5L-SMC bridge, and the performance is verified by simulations. Finally, experiments under several typical operating conditions are implemented to verify the effectiveness of 9L-HSMC with the developed MPC strategy.

Controllability Analysis and Verification for High-Order DC–DC Converters Using Switched Linear Systems Theory

Output voltages of renewable energy sources, such as photovoltaic units and fuel cells, are normally low, and multistage boost converters or single-stage high step-up converters are usually employed to increase the output voltage to be connected to the electric grid. Among all converter technologies, single-stage high-order dc–dc converters with multiple cells have gained significant popularity in recent years due to their high power density, low cost, and high efficiency. With an increased number of cells, the number of switching variables and dynamic components in multicell converters also increases, leading to control difficulties and consequently system malfunction. In order to understand and ensure the controllability of these converter systems, in this article, we establish high-order mathematical models for typical single-stage multicell high-order dc–dc converters and study their controllability using switched linear systems theories. The structure controllability, state controllability, and output controllability of the converters are analyzed in detail based on the proposed mathematical model. The theoretical analysis is substantiated by simulation and experimental verification on the improved four-cell switched-inductor dc–dc converter. The outcome of this article will provide a theoretical framework and useful references for parameter and control designs for high-order converters in practical applications.

High-Performance 3-Phase 5-Level E-Type Multilevel–Multicell Converters for Microgrids

This paper focuses on the analysis and design of two multilevel–multicell converters (MMCs), named 3-phase 5-Level E-Type Multilevel–Multicell Rectifier (3Φ5L E-Type MMR) and 3-phase 5-Level E-Type Multilevel–Multicell Inverter (3Φ5L E-Type MMI) to be used in microgrid applications. The proposed 3-phase E-Type multilevel rectifier and inverter have each phase being accomplished by the combination of two I-Type topologies connected to the T-Type topology. The two cells of each phase of the rectifier and inverter are connected in interleaving using an intercell transformer (ICT) in order to reduce the volume of the output filter. Such an E-Type topology arrangement is expected to allow both the high efficiency and power density required for microgrid applications, as well as being capable of providing good performance in terms of quality of the voltage and current waveforms. The proposed hardware design and control interface are supported by the simulation results performed in Matlab/Simulink. The analysis has been then validated in terms of an experimental campaign performed on the converter prototype, which presented a power density of 8.4 kW/dm3 and a specific power of 3.24 kW/kg. The experimental results showed that the proposed converter can achieve a peak efficiency of 99% using only silicon power semiconductors.