Stacked 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.

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.

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.

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.

A Capacitor-Voltage-Balancing Method Based on Optimal Zero-Sequence Voltage Injection in Stacked Multicell Converter

This article introduces a capacitor-voltage-balancing method based on optimal zero-sequence voltage injection in a stacked multicell converter (SMC). The proposed method is implemented using hybrid carrier pulsewidth modulation and is capable of balancing and regulating the voltages of dc-link capacitors and floating capacitors in the topology. The dynamic model of voltage fluctuation and the coupling relationship of voltage-balancing control between dc-link capacitors and floating capacitors are analyzed. The optimal zero-sequence voltage selection method is presented to address the voltage deviations of dc-link capacitors with the maximal regulation ability for floating capacitor-voltage-balancing control. Compared with the active capacitor-voltage-balancing (ACVB) method, the proposed method calculates the compensation current of the neutral point (NP) and chooses the optimal zero-sequence voltage in the whole available range. As a result, the low-frequency voltage ripples in NP can be suppressed with excellent dynamic performance under wider load conditions. Simulation and experimental results are presented to verify the effectiveness of the proposed method.

Design of flying capacitors for n-level FC and n-level SMC

This paper presents a methodology for the design of flying capacitors for multilevel inverters. It is based on the harmonic representation of the switching functions, so that it is possible to design the flying capacitors for different operation points. Experimental results are provided to verify the presented analysis. The proposed methodology is developed for the five-level Flying Capacitor Converter and for the five-level Stacked Multicell Converter, but it can be expanded for both types of converter with a higher number of output voltage levels or for other topologies.

Logic-Form-Equation-Based Active Capacitor Voltage Balancing Control Technique for Stacked Multicell Converters

This paper proposes a novel active capacitor voltage balancing technique for stacked multicell (SM) converters. The proposed active capacitor voltage balancing control method detects the direction of the load current and measures the voltage of flying capacitors (FCs) to generate a proper switching state in order to form the required output voltage level and regulate the FC voltages at their desired reference levels. The proposed active capacitor voltage balancing control technique can modulate the output voltage through any of the pulse-width-modulation (PWM) techniques such as the phase-shifted carrier PWM or the level-shifted-carrier PWM. The foremost advantage of the proposed active control technique is that it does not demand any complex computations because it is not based on the optimization of the FC voltage-deviation cost functions. A thorough set of simulation results and experimental measurements for the five-level and seven-level SM converters are provided to validate the proposed active capacitor voltage balancing control method.

Elimination of Low-Frequency Ripples and Regulation of Neutral-Point Voltage in Stacked Multicell Converters

This paper introduces a modulation method for the stacked multicell converters (SMCs). The proposed method is implemented using phase-disposition pulse width modulation and is capable of balancing and regulating the voltages of all the capacitors in the topology, i.e., the flying capacitors and the dc-link capacitors. The proposed method is also able to eliminate the low-frequency voltage ripples that may appear in the neutral point (NP) of SMCs. In SMCs with two stacks, the NP voltage level can be generated by direct connection of the output to the dc-link NP, but also using two extra states available. This redundancy is used to regulate the NP voltage. Furthermore, since the proposed method can eliminate the low-frequency voltage ripples in the capacitors, the capacitances of all the capacitors of the SMCs can be reduced, while maintaining high quality of the output voltages and currents. This enables the use of film capacitors. Simulation and experimental results are presented from a three-phase five-level $2\times 2$ SMC to verify the effectiveness of the proposed method.

Investigation of Conduction and Switching Power Losses in Modified Stacked Multicell Converters

Considering the advantage of the modified stacked multicell converter over flying capacitor and stacked multicell converters, and noting that investigation of the conduction and switching power losses can be advantageous in the design of multilevel converters, an analytical approach to calculate and analyze conduction and switching power losses in modified stacked multicell converter is presented in this paper. First, rms and average currents flowing through the insulated-gate bipolar transistors (IGBTs) and antiparallel diodes are analytically calculated by considering the duty cycle of each IGBT and diode in terms of converter modulation index, load current, and power factor. For validation, numerical results of the derived closed-form equations for computation of the rms and average currents are compared with simulation results and experimental measurements. Numeric computations of the closed-form equations match the simulation results and experimental measurements very well, which verifies the analytic equations. Next, derived equations for rms and average currents computation are utilized to calculate the conduction power losses. Finally, switching power loss analysis is performed by numeric approach and curve fitting method.

New Flying-Capacitor-Based Multilevel Converter With Optimized Number of Switches and Capacitors for Renewable Energy Integration

The flying-capacitor-based multilevel converter is one of the well-known breeds of the multilevel power converters. This paper proposes a new flying-capacitor-based multilevel converter to minimize the number of flying capacitors (FCs) and power switches. The advantage of the proposed FC-based multilevel converter in comparison with the conventional flying-capacitor multicell converter is that it needs fewer FCs. Also, in comparison with the stacked multicell converter, the proposed multilevel converter requires fewer semiconductor switches. In order to balance the voltage of the FCs in proposed multilevel converter, a new active voltage balancing method which is fully implemented using logic-form equations is presented. The proposed voltage balancing method measures output current and FC voltages to generate switching states to produce the required output voltage level, as well as balance the FCs voltages at their reference values. The output voltage of the proposed multilevel converter controlled with suggested active voltage balancing method can be modulated with any pulse-width-modulation (PWM) method, such as phase-shifted-carrier PWM or level-shifted-carrier PWM. Simulation results and experimental measurements of proposed FC-based multilevel converter are presented to verify the performance of the proposed converter, and its novel switching and modulation strategy, which is based on the active voltage balancing method.

An Active Voltage-Balancing Method Based on Phase-Shifted PWM for Stacked Multicell Converters

This paper proposes and evaluates an active voltage-balancing method for stacked multicell converters (SMCs) using phase-shifted pulse width modulation. The method is easy to implement and can be applied to SMCs with any number of levels. The proposed method balances the voltages of the capacitors by modifying the duty cycle of each switch of the SMC using a proportional controller. The direction of the output current and the crossed effect among the duty cycles of the switches is considered and used to improve capacitor voltage balance. The performance of the proposed voltage-balancing method is verified experimentally for different loads, such as linear and nonlinear loads, and under transient conditions.

On Reducing Power Losses in Stack Multicell Converters with Optimal Voltage Balancing Method

This paper proposes a voltage balancing method for stacked multicell converters (SMCs) based on phase-disposition pulse-width modulation (PD-PWM). In order to reduce the switching transitions of the power devices, only optimal transitions between consecutive voltage levels are used. Selection of the optimal transition sequence is performed by evaluating a cost function. Significant reductions in the switching transitions and power losses of the power devices are achieved as compared to the optimal-state voltage balancing method where nonoptimal transitions are allowed. Simulation and experimental results from a seven-level 3×2 SMC verifies that the proposed PD-PWM voltage balancing method is robust under linear/nonlinear loads and transients.

Voltage-Balancing Method for Stacked Multicell Converters Using Phase-Disposition PWM

This paper presents a voltage-balancing method for stacked multicell converters (SMCs) based on phase disposition pulsewidth modulation. A cost function is evaluated to select the optimum state from a pool of redundant possible ones at each voltage level. It is demonstrated that, if sawtooth carriers are used for modulation instead of standard triangular ones, significant reduction in the switching frequency and the associated power losses of the power devices can be achieved without affecting the voltage-balancing capability. Experimental results from a seven-level SMC laboratory prototype are provided to verify the proposed voltage-balancing method against linear and nonlinear loads, as well as under transient conditions.

Flying‐capacitor stacked multicell multilevel voltage source inverters: analysis and modelling

The aim of this study is to investigate the natural voltage balancing dynamics of the single/three-phase stacked multicell converters (SMCs). First, explicit analytic solutions for switching functions’ implicit equations, which are, utilised in modulating of the SMCs operated under phase disposition phase shifted carrier pulse width modulation (PD-PSC-PWM) technique whereas being initiated from the Kepler's transcendental equation, are derived. Next, utilising above-mentioned calculated Kapteyn (Fourier–Bessel)-based infinite series, a time-variant differential equation-based analytical model for single/three-phase SMCs is suggested. This strategy reduces to a model that can facilitate the analytic investigation of the start-up behaviour, dynamic response and natural voltage balancing phenomenon for SMCs owning any number of voltage-levels and phase numbers. Furthermore, it is also inferred that under the PD-PSC-PWM modulation technique, the flying-capacitor (FC) voltages balance naturally for higher number of stacks and cells, hence substantiating that the natural balancing exists for high-level SMCs. Moreover, precise analysis of the experimental observations reveals that settling time of the FC voltages in the positive and negative stacks positioned in the immediate vicinity of the SMC's midpoint is higher than the settling time of the FC voltages in the correspondent positive and negative stacks outlying the SMC's midpoint.

Detailed and comprehensive mathematical modeling of flying-capacitor stacked multicell multilevel converters

Purpose – This study aims to propose a mathematical model for stacked multicell converters (SMCs), to be exploited in the analytic determination of natural voltage balancing dynamics of the flying-capacitor (FC) stacked multicell multilevel converters, i.e. investigations of the start-up behavior, dynamic response, and natural voltage balancing phenomenon. Design/methodology/approach – The crux of the proposed strategy is based on the closed-form analytic solution derivation for the switching functions used in the switching of the SMCs operated under phase disposition (PD) and phase shifted carrier (PSC) pulse width modulation (PD-PSC-PWM) technique. Hence, the suggested approach develops an analytic solution for the Fourier series and associated Fourier coefficients pertinent to the switching functions of the SMCs by obtaining the switching instants of the PD-PSC-PWM modulator in terms of Kapteyn series when the frequency of the triangular carrier waveform (fc) and that of the sinusoidal reference waveform (fr) have an integer ratio, i.e. f c · f r −1=k, k∈N. Findings – This approach results into a model, first order differential equation based model, which can be readily developed for the SMCs with any number of levels expediting the investigation of their performance. Furthermore, by an experimental scrutiny conducted on a 4×2-cell-nine-level topology of an SMC, it is inferred that under PD-PSC-PWM modulation technique, FC voltages balance naturally for higher number of stacks and cells, therefore the natural balancing exist for high-level SMCs. Research limitations/implications – Despite the sophistication of the proposed methodology and mathematical model, this study presents an alternative approach with high potential of applicability for derivation of the multilevel converter mathematical model exploiting the Kapteyn (Bessel-Fourier) series. Practical implications – Numeric computation results of the proposed analytic model for the SMCs and the simulation results as well as investigational measurements taken from 2×2-cell-five-level and 4×2-cell-nine-level experimental set-ups are presented in order to substantiate the suggested approach, derived model, and verification of natural balancing. Originality/value – This article and its innovations are original.

New mixture of hybrid stacked multicell with half‐cascaded converter to increase voltage levels

This study presents a new topology of hybrid stacked multicell (SM) converter which generates high number of output voltage levels with fewer number of power electronic switches. The proposed topology is composed of three parts including traditional SM converter, half-cascaded converter and H-bridge converter. In other words, this converter is the mixture of H-bridge converter and SM converter series with half-cascaded converter. Number of output voltage levels and the rootmean- square value in proposed converter is four times than that of traditional flying capacitor multicell and SM converters. Moreover, it improves output voltage frequency spectrum and decreases the number and rating of components, stored energy and rating of flying capacitors which are available in the proposed converter. For switching this converter, phase-shifted pulse-width modulation method has been used. Therefore the self-balancing trait of the flying capacitor converter is preserved in presented converter. The suggested topology is simulated using MATLAB/SIMULINK software, and simulation results are presented to indicate the well-performance of the novel converter. In addition, the experimental results of proposed topology prototype have been presented to validate its practicability.

Analytical modelling of single‐phase stacked multicell multilevel converters exploiting Kapteyn (Fourier–Bessel) series

This study proposes a mathematical model for stacked multicell (SM) converters (SMCs), to be exploited in the analytic determination of the natural voltage balancing dynamics of the SMCs, that is, investigation of the start-up behaviour, dynamic response and natural voltage balancing phenomenon. The crux of the proposed strategy is based on the closed-form analytic solution derivation for the switching functions used in the switching of the SMCs operated under phase disposition (PD) and phase-shifted carrier (PSC) pulse-width modulation (PD–PSC-PWM) technique. Hence, the suggested approach develops an analytic solution for the Fourier series and associated Fourier coefficients pertinent to the switching functions of the SMCs by obtaining the switching instants of the PD–PSC-PWM modulator in terms of ‘Kapteyn series’ when the frequency of the triangular carrier waveform (f c) and that of the sinusoidal reference waveform (f r) have an integer ratio, that is, f c f r −1 = k, k ∈ ℕ. This strategy results into a model (‘first-order differential equation based model’) which can be readily developed for the SMCs with any number of levels expediting the investigation of their performance. Numeric computation results of the proposed analytic model for the SMCs and simulation results as well as measurements taken from an experimental set-up are presented in order to validate the suggested approach and derived model.

Elimination of DC voltage sources and reduction of power switches voltage stress in stacked multicell converters: analysis, modeling, and implementation

SUMMARY Multilevel converters have been widely used in medium-voltage high-power applications. One of the typical breeds of multicell converters is stacked multicell converter (SMC). This paper presents an improved configuration for SMCs, accompanied by its analysis, modeling, and implementation. The main advantages of the proposed converter, in comparison with the conventional one, are that the numbers of required dc voltage sources are halved in the improved topology and also the voltage rating of half of the power switches are decreased by 50% which results in reducing the cost, size, and weight of the SMCs effectively. This progress is achieved by connecting the upper stacks to the lower stacks back to back while the number of high-frequency power switches and flying capacitors (FCs), voltage ratings of FCs, as well as the number of high-frequency switchings during a full cycle are kept constant. The control methodology is based on the phase-shifted carrier sinusoidal pulse width modulation technique; therefore, the well-known natural balancing phenomenon of FC voltages, one of the critical advantages of SMCs, is maintained in the proposed converter. This paper also presents an analytic model to study and analyze the dynamic of FC voltages in the proposed SMC. Numerical analysis results of the derived analytic model, simulation results, and, moreover, measurements taken from the laboratory prototypes are presented in order to validate the effectiveness and advantages of the proposed configuration as well as its control strategy and analytic model. In addition, the comparison of the proposed converter with other types of multilevel converters is carried out to highlight the advantages of the proposed structure. Copyright © 2013 John Wiley & Sons, Ltd.