Application Of Multilevel Converter Research Articles

Multicarrier PWM techniques for three phase modular multilevel converter application in MRI systems

This research paper presents about the performance analysis of three phase modular multilevel converter with level shifted (phase disposition pulse width modulation) and phase shifted pulse width modulation (PWM) method. This research not only discusses the significance of different modulation procedures in the context of modular multilevel converters but also carefully evaluates each of their unique performance characteristics. This research is based on the fundamental creation of pulses and patterns that can only be acquired by comparing sinusoidal reference signals with triangular carrier signal. Both levels shifted (phase disposition) and phase shifted PWM procedures are used to calculate the phase output voltage of a converter at 2N+1 level. The output waveform is crucially shaped by these modulation techniques, which also ensure effective energy conversion and lower harmonic content. On the basis of the output voltage waveform's quality, the comparative research is conducted and simulated results were presented.

Structures, Submodule Topologies and Control Strategies for Modular Multilevel Converter

Global energy demand is rapidly increasing, and the limitations and disadvantages of traditional energy sources are becoming more evident. Traditional energy sources are finite, and their sustainability is facing significant challenges. Compared to traditional multi-level converters, modular multi-level converters (MMC) adopt a modular design that divides the entire converter system into several independent sub-modules, each responsible for converting specific voltage levels. This article provides an overview of the theoretical aspects, topology structures, basic operation principles, and output characteristics of multi-level converters. This paper reviews the characteristics and application ranges of different topology structures, highlighting the differences in response speed, output quality, and control complexity. This paper also discusses the advantages and limitations of multi-level converters in various application scenarios. Explored the application of multi-level converters in wind power grid integration and electric vehicle charging systems, emphasizing their energy-saving and environmentally friendly features. In conclusion, this paper emphasizes the critical role of multi-level converters in power systems and future energy applications, stressing the importance of their reliability and promising future developments.

Cascaded H-Bridge Multilevel Converter Applied to a Wind Energy Conversion System with Open-End Winding

With the growing expansion of renewable sources around the world, wind energy is among those that stand out. With the advances of technology, wind turbine projects have considerably increased their power, reaching higher power, mainly for offshore installations. One of the main challenges is the power converters, more specifically the semiconductor components, which have limited voltage and current capabilities. Thus, the concept of multilevel converters emerged, increasing the voltage levels and thus carrying higher power levels. In addition to the application of multilevel converters, it is possible to increase the voltage and power levels employing an open-end winding (OEW) connection to the generator. In this context, the present work investigated the application of a multilevel converter (three-level cascaded H-bridge back-to-back) driving a squirrel-cage induction machine in an open-end winding configuration, connected to a wind energy conversion system (WECS). The analysis of the proposed system was developed through dynamic simulation of a 1.67 MW WECS, using PLECS software, including the modeling of the main system components: generator, power converters, system control, filter, and grid connection. The results show that the objective of obtaining a 5-level behavior in the output voltage is achieved by using the OEW connection. Furthermore, a low harmonic content is achieved in the machine current as in the current injected into the grid. In addition, it is possible to verify the power distribution between the converters, demonstrating that converters with smaller power can be combined to reach higher WECS power.

Design of Active Fault-Tolerant Control System for Multilevel Inverters to Achieve Greater Reliability With Improved Power Quality

Modern renewable energy systems mostly utilize multilevel converter applications for improved power quality and grid synchronization purposes. A multilevel converter is an electrical device that may offer several amounts of voltage levels at the output in order to make the output more comparable to a pure sine wave. The integrity of the multilevel converter depends on the reliability of individual switches such that the converter will collapse in case of faults in these switches. In this paper, a novel 7-level Fault-Tolerant Cascaded H-Bridge Multilevel Inverter (FT-CHB-MLI) has been proposed that offers high reliability with improved power quality. A dedicated Fault Detection and Isolation (FDI) unit has been built to diagnose the faulty switch and replace it with a standby redundant switch. Total harmonic distortion and the determination of a normalized output voltage factor are employed for fault diagnosis. The Phase Disposition Pulse Width Modulation (PD-PWM) technique has been utilized for switching due to its superior performance as compared to other conventional techniques. This early fault detection method not only identified the issues but also performed preventative actions to keep the system healthy and stable. The proposed system was tested on the MATLAB / Simulink environment to verify its performance. The simulation results demonstrated that the THD has been reduced to almost 18% with a significant increase in reliability with advanced fault-tolerant architecture consisting of FDI units. The reliability analysis was carried out using Markov chains that also showed its increased reliability. A comparison of the proposed work with literature was also carried out to demonstrate its superior performance with increased reliability.

Multilevel Converter Applications in the Area of Renewable Energy, More-Electric Propulsion, Electric Vehicles and Power Grid Integration

The papers in this special section focus on Multilevel converter applications in the area of renewable energy, electric propulsion, electric vehicles and power grid integration. Today, power electronics converters have shifted into a decisive part of our ways of living, noticeably for the profitable utilization of renewable energy sources (RESs). Electric network grid, even standalone type, “more-electric” propulsion, electric vehicles, space applications, military, medical, process/heavy industries, etc., are all affected by converters. In the last few years, we have seen a rising demand for the grid-connected multilevel converter (MC) structures, precisely with RES integration.

Model Predictive Control of Modular Multilevel Converters Using Quadratic Programming

The finite control set-model predictive control (FCS-MPC) has been adopted as an excellent choice for the applications of multilevel converters during the last two decades for its salient performance. However, in the case of modular multilevel converters (MMCs), a high amount of calculation is always involved in the implementation, making the FCS-MPC less suitable especially for an MMC with a high number of submodules. To cope with the issue, this article proposes an MPC technique for the MMC with a very low calculation cost. In each sampling period, the arm voltage references of each phase are determined analytically by solving a constrained quadratic programming problem formulated from the cost function. Both a rigorous and simplified procedure is provided to solve the optimization problem. Then, the four nearest candidates around the arm voltage references are evaluated, leading to a proper selection of arm voltage levels. Several experimental tests on an MMC prototype are carried out to validate the effectiveness of the proposed method. Results show that compared with the conventional FCS-MPC method which evaluates all voltage-level combinations, the proposed scheme presents an apparent advantage in terms of calculation cost while achieving similar performance.

Suboptimal multirate MPC for five-level inverters

The application of multilevel converters to renewable energy systems is a growing topic due to their advantages in energy efficiency Regarding its control, model predictive control (MPC) has become very appealing due to its natural consideration of discrete inputs, its optimization capability, and the present-day availability of powerful processing hardware. The main drawback of MPC compared to other control techniques in this field is that the control input is held constant during the sampling period, and it is usually difficult or even impossible to reduce this sampling period because of hardware limitations. For this reason, a multirate MPC algorithm is proposed, which allows to change the control input several times within the sampling period. The optimization problem is simplified and made suboptimal to substantially decrease computational burden. This approach is tested in simulation on a three-phase, five-level diode-clamped converter (DCC) operating in inverted mode with a three-phase resistive load. Results show significant reduction in harmonic distortion at the cost of an increase in the number of commutations with respect to a standard MPC operating at the same sampling period.

Application of Multi-Level Converter for Fast Current Control in Small-Scale DC Power Network

DC microgrids have been emerging as next-generation small-scale electric power networks, where the line impedance is very low. This phenomenon causes large currents in the microgrids, even for a slight change in voltage; therefore, it is critical for a power flow controller to have faster transient response and precise power flow control. In this study, multi-level converters are applied as the power flow controllers to realize high-speed and high-precision power flow control in a dc microgrid. The output filter can be small, as a multi-level converter is used. This paper also presents the design of the output LC filter of a multi-level converter to satisfy a requirement of current ripple. We experimentally verified that a multi-level converter with a smaller filter can realize high-speed and high-precision power flow control for low line impedance conditions compared with the conventional two-level converters.

Voltage Jump Suppression and Capacitor Voltage Fluctuation Analysis for a Four-Level Hybrid Flying Capacitor T-Type Converter

In low and medium voltage power conversion systems, multilevel converters are becoming more and more attractive due to improved power density. However, the complexity of topology and control is a big challenge for the application of multilevel converters. In this paper, a four-level (4L) hybrid flying capacitor (FC) T-type converter has been researched in detail. The topological advantage of the converter is displayed in comparison to existing four-level converters. According to the feature of the topology, the operating status has been analyzed and the reason for the voltage jump is researched in detail during the dead-time period. A strategy to reduce voltage jump by adjusting the switching states has be presented. The FC voltages can be balanced by selecting the appropriate switching states. The relationships between the fluctuations of FC voltages and the modulation index and power factor (PF) have been analyzed by simulation results. The performance of the 4L converter has been investigated in MATLAB/Simulink as well as on a down-scaled laboratory prototype.

A Developed Two-Leg Ladder Multilevel Converter Structure

The applications of multilevel converters (MCs) in industry have been increased because of their advantages such as high quality output waveform and lower harmonic distortions. This paper proposes an improved two-leg ladder topology for MC. For generating all levels at output voltage waveform, two methods are investigated for selecting the values of dc sources. The suggested structure generates a large number of levels at output voltage waveform with the least number of power electronic components such as insulated gate bipolar transistors, gate driver circuits, dc voltage sources and anti-parallel diodes in comparison with other similar topologies. Also, the magnitude of blocked voltage by switches is low. Power losses analysis on the proposed topology is provided. It is shown that the number of on-state switches in the presented structure is less than other similar topologies, which causes the voltage drop and power losses of proposed topology to be reduced. To show the merits of the proposed structure, comparison results are provided with other structures. To validate the analytical results of proposed topology, an experimental work for a 9-level converter and the simulation results for a 25-level converter are provided. PSCAD/EMTDC software is used for simulation works.

Modular Multilevel Converter With Magnetically Coupled Branch Inductors

The modular multilevel converter is a well-known modular multilevel topology. This paper proposes a novel branch inductor coupling and compares it to the center-tapped inductor and the standard noncoupled inductor topology. The proposed coupling has a significant influence on the inductor and converter dimensioning. Its properties make it especially well suited for use in single-to-three-phase ac/ac modular multilevel convert-er applications.

Current-Fed Multilevel Converters: An Overview of Circuit Topologies, Modulation Techniques, and Applications

Multilevel converters (MLCs) have emerged as standard power electronic converters in high power as well as quality demanding applications. They are classified into current-fed MLCs and voltage-fed MLCs. Voltage-fed MLCs have widely researched whereas the current-fed MLCs are the recent topic of research. Based on the principle of duality between voltage and current sources, several current-fed MLCs analogous to voltage-fed MLCs have been identified. Current-fed MLCs offer several advantages in terms of high power capability, transformerless operation, short-circuit protection, and excellent quality of output current waveform. The goal of this paper is: 1) to present review of circuit topologies, modulation schemes, and applications of current-fed MLCs; and 2) to review an emerging low-device switching frequency modulation technique known as synchronous optimal pulsewidth modulation for current-fed MLCs. The circuit configuration and advantages of each topology along with various modulation techniques are discussed in detail. Compared to voltage-fed MLCs, the operation of current-fed MLCs need to satisfy additional switching constraints. A survey of classical methods for realization of these operational constraints has been done and a new generalized method has been proposed. Finally, future scope of research has been presented to encourage further development of topologies and modulation techniques for current-fed MLCs.

Design methodology for very high insulation voltage capabilities power transmission function for IGBT gate drivers based on a virtual prototyping tool

For the operation of the semiconductor components in medium voltage converter or multilevel converter applications (where the DC bus voltage up to several ten kilovolts), their gate driver systems require very high galvanic insulation voltage capabilities. Hence, this study details a design methodology to optimise a power supply function for insulated gate bipolar transistor gate drivers with high insulated voltage capabilities. To achieve this, a dc-to-dc full-bridge series–series resonant converter with a high air gap pot core planar transformer is selected. This air gap length and the dielectric material define the high insulation voltage capability. The objective of the study is to optimise the geometric elements of the transformer and its associated electrical components in the proposed converter. Magnetic and electrostatic modellings are provided. Maximise the converter efficiency (η con) is the main objective of the proposed optimisation design. Thus, the optimisation results are proposed: the converter efficiency (η con) as function of converter output power (P out). Finally, to validate the authors’ proposed methodology, the experimental results are compared with the simulation ones. Then, the results of an improved optimisation design are provided.

A Survey of Low Switching Frequency Modulation Techniques for Medium-Voltage Multilevel Converters

Multilevel converters (MLCs) have emerged as standard power electronic converters for medium-voltage high-power industrial applications. Owing to dominating device switching losses in high-power applications, it is preferable to use low device switching frequency (LDSF) modulation techniques. Then, it is possible to achieve higher device utilization, higher converter efficiency, and reduced cooling requirements. However, there exists a tradeoff between device switching frequency and harmonic distortion of converter output currents. Therefore, the main challenge for LDSF modulation techniques is to minimize the harmonic distortion of the output currents. The goal of this paper is to provide a review of various LDSF modulation techniques proposed in the literature and also discuss in detail about one of the emerging LDSF control techniques known as synchronous optimal pulsewidth modulation. Finally, challenges to LDSF modulation techniques for emerging multilevel topologies and future trends in applications of MLCs are discussed to motivate further research, to enhance the proposed LDSF techniques, and to explore for new alternatives.

Cascade‐multi‐cell multilevel converter with reduced number of switches

Recently applications of multilevel converters have been pointed out because of some advantages include high-quality output waveform, reduced harmonic distortion and lower electromagnetic interference. However, some drawbacks such as increased number of components like switches and dc power supplies and complex pulse width modulation control method are associated with these converters. In this study, a new cascade multilevel converter is introduced named `cascade-multi-cell' converter. First, the basic constitutive of the proposed multilevel converter has been explained. Then, symmetric and asymmetric configurations are presented. The number of switches and gate driver circuits in the proposed structure are less than the conventional cascade converter. Therefore its output voltage levels will be increased. Also, the number of on-state switches is reduced in presented topology, and so the conductive losses will be reduced. The peak inverse voltage of the proposed converter is equal with the traditional cascade converter. Finally, simulation and experimental results are presented. The presented results show the validity and effectiveness of the proposed multilevel structure.

Analysis, Simulation &Comparison of Various Multilevel Inverters Using Different PWM Strategies

Multilevel inverters are developed as an extension to the conventional inverters. They are widely used for high power applications as they cause very less disturbances and have the possibility to function at lower switching frequencies than ordinary two-level inverters. This paper compares different topologies of inverters .The comparison is done with respect to Total Harmonic Distortion. This paper presents a brief account on the most important topologies like diode-clamped inverter (neutral-point clamped), capacitorclamped (flying capacitor), and cascaded H-bridge with separate dc sources. It also presents the most relevant control and modulation methods developed for this family of inverters namely sinusoidal pulse width modulation and space-vector modulation. Multilevel inverter promises a lot of advantages over conventional inverter especially for high power applications. Main advantages are that output waveforms were improved since multilevel inverter produces nearly sinusoidal output voltage waveforms, hence the THD will be low. Multilevel converter applications focus on industrial medium-voltage motor drives, utility interface for renewable energy systems, UPS, flexible AC transmission system, and traction drive systems.

Compensation of Nonlinearities in Diode-Clamped Multilevel Converters

The application of multilevel converters for exciting permanent-magnet machines with low-phase inductance to dc-link voltage ratios facilitates a reduction in high-frequency switching harmonics. However, converter nonlinearities and, in-particular, on-state device voltage drops, create additional low-frequency harmonics. This paper therefore proposes a generic compensation scheme to accommodate the effects of such converter nonlinearities and, in doing so, improve the harmonic quality of the machine phase currents. Experimental results gathered from a prototype five-level diode-clamped converter validate the benefits of the proposed scheme by showing quantitative reductions in low-frequency harmonics.

Recent Advances and Industrial Applications of Multilevel Converters

Multilevel converters have been under research and development for more than three decades and have found successful industrial application. However, this is still a technology under development, and many new contributions and new commercial topologies have been reported in the last few years. The aim of this paper is to group and review these recent contributions, in order to establish the current state of the art and trends of the technology, to provide readers with a comprehensive and insightful review of where multilevel converter technology stands and is heading. This paper first presents a brief overview of well-established multilevel converters strongly oriented to their current state in industrial applications to then center the discussion on the new converters that have made their way into the industry. In addition, new promising topologies are discussed. Recent advances made in modulation and control of multilevel converters are also addressed. A great part of this paper is devoted to show nontraditional applications powered by multilevel converters and how multilevel converters are becoming an enabling technology in many industrial sectors. Finally, some future trends and challenges in the further development of this technology are discussed to motivate future contributions that address open problems and explore new possibilities.

New approach in back-to-back m-level diode-clamped multilevel converter modelling and direct current bus voltages balancing

Multilevel power converter structures have been introduced as the solution for high-power high-voltage applications and also for grid interface connection of renewable energy sources systems, where they have several advantages, namely low distortion voltages and currents and low switching losses resulting in higher efficiency. As a consequence of increasing interest on the multilevel converter applications, accurate models of these power converters are essential to be used in computer simulation studies. This study presents a new systematic modelling approach suitable to obtain global generalised models for back-to-back m-level diode-clamped multilevel converters connecting AC loads. Using the proposed converter modelling, an advantageous generalised method to balance the (m−1) DC bus capacitors, based on the DC bus average power flow is also presented. The generalised proposed model is particularised to the back-to-back multilevel structure for m=5. This back-to-back converter is studied working with bidirectional power flow, connecting an induction machine (in motor or self-excited generator mode) to the power grid as a flexible AC transmission system.

The age of multilevel converters arrives

This work is devoted to review and analyze the most relevant characteristics of multilevel converters, to motivate possible solutions, and to show that we are in a decisive instant in which energy companies have to bet on these converters as a good solution compared with classic two-level converters. This article presents a brief overview of the actual applications of multilevel converters and provides an introduction of the modeling techniques and the most common modulation strategies. It also addresses the operational and technological issues.