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

Current-fed quasi-Z-source inverter has many advantages when it is used in the vehicle motor drive: it has buck-boost function with single stage; it has regeneration capability; it protects the open circuit by itself. This paper is to demonstrate that this inverter also has fast transient response that it only needs several switching cycles to transfer from motoring mode to regeneration mode, which makes it very suitable to be used for HEV or EV motor drive. Theory analysis, simulation results and experimental results are all given to demonstrate it.

Transient modeling of current-fed quasi-Z-source inverter

This paper presents a novel configuration of unified power flow controller (UPFC). The proposed configuration has two inverters connected face-to-face on the AC side instead of connected back-to-back through a common DC link. The new configuration has the following advantages: 1) there is no active power exchange between the two inverters and both inverters only provide or absorb reactive power; 2) cascade multilevel inverters can be used in this configuration, thus eliminating the conventional magnetic interface-zigzag transformers, and greatly reducing power losses; 3) the total required VA rating of the inverters can be reduced greatly over a wide load conditions; and 4) The new configuration with modular cascade multilevel inverters and their redundancy has greater flexibility to system design and realization of higher reliability.

A novel configuration of unified power flow controller

Switched-capacitor converter (SCC) features a compact solution due to its magnetic-less structure, which fulfills the high density requirement in low power applications. Compared to other SCC topologies, Fibonacci switched-capacitor converter (FSC) utilizes the least number of flying capacitors to perform any voltage conversion. This paper proposes a monolithic voltage-scalable FSC, which can be used in energy harvesting applications. The challenges to implement on-chip FSC are addressed. The parasitic charge loss is substantial in FSC. Therefore, a charge recycling technique exploiting the presence of parasitic capacitors in each other phase of FSC has been employed resulting in 27% reduction in the power loss, which improves the overall efficiency by 12.7%. For a proof of concept, the proposed converter is implemented in 0.5- $\mu \text{m}$ CMOS technology on 4-mm2 die area. The postlayout simulation is carried out using Virtuoso ADE. With 0.3-V input voltage, the system achieves 47.5% peak power efficiency with 5- $\mu \text{W}$ load.

A Monolithic Voltage-Scalable Fibonacci Switched-Capacitor DC–DC Converter With Intrinsic Parasitic Charge Recycling

In order to handle high voltage with limited voltage rating self turn-off devices, it is necessary to give a simple and reliable series connection technique of these devices, especially IGBTs for high voltage applications. This paper proposes a snubber circuit to guarantee the voltage balance for IGBTs in series connection both at static and dynamic transients. The proposed circuit features with simple control at the gate side, unlimited number of devices in series, high reliability and low loss. The simulation and experimental results verify these advantages. A 10A/3000V prototype with three series-connected IGBTs have been designed and implemented. It has exhibited excellent dynamic voltage balance and an accepted overshoot level with LC snubber circuits and auxiliary clamp circuits.

The voltage sharing of commercial IGBTS in series with passive components

SUMMARY

Current balancer-based phase leg paralleling provides a preponderant technology for the grid-tie inverter to extend current rating when transporting high wind power to the grid. However, the existing technology used one current balancer per phase output (one core per phase), so that three current balancers (three separate cores) were required for the three-phase system, which caused redundant large size. Also, the low-frequency circulating current components have to be controlled in order to avoid saturation of the current balancer designed for minimizing the switching frequency components of circulating current. This paper contributes in three aspects: (i) two multiphase coupling current balancers are proposed for two parallel configurations, only one single core is employed for the three-phase system, and the compactness and light weight are achieved with a very high permeability magnetic core and no air gap. (ii) The detailed current balancer design methods are proposed for both configurations, and the designed two prototypes compare two current balancers for application to two inverter configurations. (iii) A current closed-loop control is proposed to minimize the circulating current, which is verified by the simulated results. Copyright © 2013 John Wiley & Sons, Ltd.

Fang Zheng Peng

Papers

Transient modeling of current-fed quasi-Z-source inverter

A novel configuration of unified power flow controller

A Monolithic Voltage-Scalable Fibonacci Switched-Capacitor DC–DC Converter With Intrinsic Parasitic Charge Recycling

The voltage sharing of commercial IGBTS in series with passive components

Current balancer‐based grid‐connected parallel inverters for high power wind‐power system