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

The use of distributed energy resources is increasingly being pursued as a supplement and an alternative to large conventional central power stations. The specification of a power-electronic interface is subject to requirements related not only to the renewable energy source itself but also to its effects on the power-system operation, especially where the intermittent energy source constitutes a significant part of the total system capacity. In this paper, new trends in power electronics for the integration of wind and photovoltaic (PV) power generators are presented. A review of the appropriate storage-system technology used for the integration of intermittent renewable energy sources is also introduced. Discussions about common and future trends in renewable energy systems based on reliability and maturity of each technology are presented

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Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey

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.

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Recent Advances and Industrial Applications of Multilevel Converters

Multilevel voltage source converters are emerging as a new breed of power converter options for high-power applications. The multilevel voltage source converters typically synthesize the staircase voltage wave from several levels of DC capacitor voltages. One of the major limitations of the multilevel converters is the voltage unbalance between different levels. The techniques to balance the voltage between different levels normally involve voltage clamping or capacitor charge control. There are several ways of implementing voltage balance in multilevel converters. Without considering the traditional magnetic coupled converters, this paper presents three recently developed multilevel voltage source converters: (1) diode-clamp, (2) flying-capacitors, and (3) cascaded-inverters with separate DC sources. The operating principle, features, constraints, and potential applications of these converters are discussed.

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Multilevel converters-a new breed of power converters

This paper presents a new multilevel converter topology suitable for very high voltage applications, especially network interties in power generation and transmission. The fundamental concept and the applied control scheme is introduced. Simulation results of a 36 MW-network intertie illustrate the efficient operating characteristics. A suitable structure of the converter-control is proposed.

An innovative modular multilevel converter topology suitable for a wide power range

In Part I of this paper, the self-voltage balancing nature of modular multilevel converters (MMCs) has been proven mathematically. To utilize this merit of MMCs, a novel modulation, namely Y-matrix modulation (YMM), is proposed to transform the math analysis of Part I into modulation practice. With the proposed YMM, MMCs have secured self-voltage balancing. Conventionally, either a complicated voltage balancing control, or extra components are embedded in MMCs to balance the capacitor voltage. Compared to conventional MMC capacitor voltage balancing strategies, YMM features simple algorithms and good reachability to high-level MMCs while maintaining the original half-bridge submodule topology. To simplify the analysis, YMM is introduced to two-level and three-level MMCs as examples. Then, the generalized YMM is derived, which is suitable for high-level MMCs. Several YMM based MMC case studies are provided for verification purposes.

A Modular Multilevel Converter With Self-Voltage Balancing Part II: Y-Matrix Modulation

Z-source converters have been developed as a new type of converter besides the other two kinds of traditional converters: voltage-source and current-source converters. Z-source converter has many inherent advantages and is suitable for many applications such as fuel cell system or motor drive because of its single stage structure. Present research works are mostly focused on using Z-source network as a dc-link. For ac/ac conversion, a diode-rectifier front end is needed and the Z-source network will be employed on the dc-link. In this paper, a new three-phase ac-ac Z-source converter is proposed to perform ac/ac conversion directly. Compared to the traditional ac-ac converter, the new ac/ac converter has the same number of devices. Analysis and simulation for this three-phase ac/ac Z-source converter are presented. A 1 kW prototype was built and the experimental results verify the analysis.

A new three-phase ac-ac Z-source converter

Active power filters have been used in practice to suppress the harmonic interference in power systems. To compensate for harmonic currents of loads active power filters are usually connected to power systems in parallel with the loads. These filters which are called shunt active filters here are very effective to loads that can be considered as current sources, such a thyristor rectifiers with large DC reactances. Many papers have covered the shunt active filters applied to these currentsource loads. No paper, however, has discussed characteristics of the shunt active filters when they are applied to voltage-source loads.On the other hand, since more and more diode rectifiers with capacitive DC filters are used recently, harmonics generated by which become an issue.The diode rectifier with capacitive DC filters behaves like a voltage source rather than a current source. When a shunt active filter is applied to such a diode rectifier, the current injected from the shunt active filter may flow into the diode rectifier. As a result, harmonics of the source current cannot be reduced effectively, and harmonic current flowing into the diode rectifier increases largely.This paper reveals the above-mentioned problem of shunt active filters analytically and experimentally. Then a series active filter is proposed to suppress harmonic current of the diode rectifiers. The features, operating conditions, and considerations of shunt active filters and series active filters are described analytically and demonstrated experimentally. Taking a diode rectifier with capacitive DC filter as a typical voltage-source load, compensation characteristics of shunt active filters and series active filters are discussed by experiment and simulation. The validity of the series active filters is illustrated experimentally.

Compensation Characteristics of Shunt Active and Series Active Power Filters

The emergence of silicon carbide- (SiC-) based power semiconductor switches, with their superior features compared with silicon- (Si-) based switches, has resulted in substantial improvement in the performance of power electronics converter systems. These systems with SiC power devices have the qualities of being more compact, lighter and more efficient; thus, they are ideal for high-voltage power electronics applications such as a hybrid electric vehicle (HEV) traction drive. More research is required to show the impact of SiC devices in power conversion systems. In this study, findings of SiC research at Oak Ridge National Laboratory (ORNL), including SiC device design and system modeling studies, are discussed.

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Testing, characterization, and modeling of SiC diodes for transportation applications

This paper explores control methods for voltage-fed quasi-Z-source matrix converter [1]. First voltage-fed quasi-Z-source matrix converter topology is compared with others in the family. It is a component less, size compact, high efficient, wide range buck-boost topology. Secondly, for shoot through control, a maximum constant boost control is presented to produce the lowest output harmonics and lowest switch voltage and current stress under a given voltage gain. For matrix converter control, a PWAM control has been presented to control either the rectifier side or inverter side of the equivalent circuit, in order to reduce the switching loss and achieve high efficiency. In addition, novel commutation and protection methods have been proposed for this new circuit. A prototype has been built in the lab using Fuji 18MBI200W-060A reverse-blocking matrix converter IGBT module. Simulation and experimental results have been conducted to verify the theoretical analysis.

Fang Zheng Peng

Papers

A Modular Multilevel Converter With Self-Voltage Balancing Part II: Y-Matrix Modulation

A new three-phase ac-ac Z-source converter

Compensation Characteristics of Shunt Active and Series Active Power Filters

Testing, characterization, and modeling of SiC diodes for transportation applications

Pulse-Width-Amplitude-Modulated voltage-fed quasi-Z-source direct matrix converter with maximum constant boost