Multilevel inverter technology has emerged recently as a very important alternative in the area of high-power medium-voltage energy control. This paper presents the most important topologies like diode-clamped inverter (neutral-point clamped), capacitor-clamped (flying capacitor), and cascaded multicell with separate DC sources. Emerging topologies like asymmetric hybrid cells and soft-switched multilevel inverters are also discussed. This paper also presents the most relevant control and modulation methods developed for this family of converters: multilevel sinusoidal pulsewidth modulation, multilevel selective harmonic elimination, and space-vector modulation. Special attention is dedicated to the latest and more relevant applications of these converters such as laminators, conveyor belts, and unified power-flow controllers. The need of an active front end at the input side for those inverters supplying regenerative loads is also discussed, and the circuit topology options are also presented. Finally, the peripherally developing areas such as high-voltage high-power devices and optical sensors and other opportunities for future development are addressed.

Multilevel inverters: a survey of topologies, controls, and applications

This paper presents a technology review of voltage-source-converter topologies for industrial medium-voltage drives. In this highly active area, different converter topologies and circuits have found their application in the market. This paper covers the high-power voltage-source inverter and the most used multilevel-inverter topologies, including the neutral-point-clamped, cascaded H-bridge, and flying-capacitor converters. This paper presents the operating principle of each topology and a review of the most relevant modulation methods, focused mainly on those used by industry. In addition, the latest advances and future trends of the technology are discussed. It is concluded that the topology and modulation-method selection are closely related to each particular application, leaving a space on the market for all the different solutions, depending on their unique features and limitations like power or voltage level, dynamic performance, reliability, costs, and other technical specifications.

Multilevel Voltage-Source-Converter Topologies for Industrial Medium-Voltage Drives

The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.

Operation, Control, and Applications of the Modular Multilevel Converter: A Review

The authors discuss high-voltage power conversion. Conventional series connection and three-level voltage source inverter techniques are reviewed and compared. A novel versatile multilevel commutation cell is introduced: it is shown that this topology is safer and more simple to control, and delivers purer output waveforms. The authors show how this technique can be applied to either choppers or voltage-source inverters and generalized to any number of switches.<<ETX>>

Multi-level conversion : high voltage choppers and voltage-source inverters

Multilevel converters are considered today as the state-of-the-art power-conversion systems for high-power and power-quality demanding applications. This paper presents a tutorial on this technology, covering the operating principle and the different power circuit topologies, modulation methods, technical issues and industry applications. Special attention is given to established technology already found in industry with more in-depth and self-contained information, while recent advances and state-of-the-art contributions are addressed with useful references. This paper serves as an introduction to the subject for the not-familiarized reader, as well as an update or reference for academics and practicing engineers working in the field of industrial and power electronics.

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Multilevel Converters: An Enabling Technology for High-Power Applications

This paper presents a simple carrier symmetric method for the voltage balance of flying capacitors in flying-capacitor multilevel inverters. The carrier-redistribution pulsewidth-modulation (CRPWM) method was reported as a solution for the voltage balance but it has a drawback at the transition of voltage level. To achieve the voltage balance of flying capacitors, the utilization of each carrier must be balanced during a half-cycle of the switching period such as phase-shifted PWM. However, the CRPWM method causes the fluctuation of flying-capacitor voltages because the balanced utilization of carriers is not achieved. Moreover, it does not consider that the load current change has an influence on flying-capacitor voltages by assuming that the current flows into the load. Therefore, the charging and discharging quantities of flying-capacitor voltages do not become zero during the switching period. To overcome the drawbacks of CRPWM, it is modified by the technique where carriers of each band are disposed of symmetrically at every fundamental period. Firstly, the CRPWM method is reviewed and the theory on voltage balance of flying capacitors is analyzed. The proposed method is introduced and is verified through simulation and experimental results.

A symmetric carrier technique of CRPWM for voltage balance method of flying-capacitor multilevel inverter

The multi-level inverter system is very promising in AC drives, when both reduced harmonic content and high power are required. But, as the output voltage level increase, the number of using the semiconductor device is increased. In case of multi-level inverter system, the loss of semiconductor devices cannot be analyzed by conventional methods. The reason is that the loss of each the semiconductor device is different from one another unlike 2-level and the diode clamped inverter system needs extra clamping diode. In this paper, a simple and accurate method of computing conduction and switching loss is proposed for multi-level inverter system. The validity of the proposed method is proven for 3-level and 4-level diode clamped inverter system.

The analysis of conduction and switching losses in multi-level inverter system

The carrier-based space vector pulse width modula- tion (SVPWM), which is considered as simple and efficient PWM technology, can be also used in multilevel inverters. The method was originally designed for the two-level inverter and developed to the diode-clamped multilevel inverter structure. However it may be noted that it also cause bad switch utilization in cascaded mul- tilevel inverter. This paper introduces an improved carrier-based SVPWM scheme, which is fully suitable for cascaded multilevel in- verter topologies because it can achieve the optimized switch uti- lization through the redistribution of the triangular carrier waves considering leg voltage redundancies while having the advantages of the conventional carrier-based SVPWM. Using experimental re- sults, the superior performance of new PWM method is shown.

An Improved Carrier-Based SVPWM Method Using Leg Voltage Redundancies in Generalized Cascaded

The carrier-based space vector pulse width modulation (SVPWM), which is considered as simple and efficient PWM technology, can be also used in multilevel inverters. The method was originally designed for the two-level inverter and developed to the diode-clamped multilevel inverter structure. However it may be noted that it also causes bad switch utilization in cascaded multilevel inverters. This paper introduces an improved carrier-based SVPWM scheme, which is fully suitable for cascaded multilevel inverter topologies because it can achieve the optimized switch utilization through the redistribution of the triangular carrier waves considering leg voltage redundancies while having the advantages of the conventional carrier-based SVPWM. Using experimental results, the superior performance of new PWM method is shown.

An improved carrier-based SVPWM method using leg voltage redundancies in generalized cascaded multilevel inverter topology

This paper proposes a new solution using a carrier-based PWM method to solve the most serious problem of flying capacitor multi-level inverters, that is the unbalance of capacitor voltages. The voltage unbalance occurs due to the difference of each capacitor's charging and discharging time applied to the flying capacitor inverter. The new solution controls the variation of capacitor voltages into the mean '0' during some period by means of new carriers using the leg voltage redundancy in the flying capacitor inverter. The solution can be easily expanded to multi-level inverters. Also, this method can make the switching loss and conduction loss of devices equal by the use of leg voltage redundancy. First, this paper examines the unbalance of capacitor voltage and the conventional theory of self-balance using phase-shifted carriers. Then, a new method that is suitable for flying capacitor inverters is explained. Simulation results verify the proposed method.

Dae-Wook Kang

Papers

A symmetric carrier technique of CRPWM for voltage balance method of flying-capacitor multilevel inverter

The analysis of conduction and switching losses in multi-level inverter system

An Improved Carrier-Based SVPWM Method Using Leg Voltage Redundancies in Generalized Cascaded

An improved carrier-based SVPWM method using leg voltage redundancies in generalized cascaded multilevel inverter topology

The carrier-based PWM method for voltage balance of flying capacitor multilevel inverter