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

A new space vector modulation (SVM) strategy is proposed for quasi-Z-source indirect matrix converter (QZS-IMC) to reduce common-mode voltage and to ensure a high voltage transfer ratio. QZS-IMC's common-mode voltage is analyzed, and the subdivided sectors are employed to rearrange switching sequence in the inverter stage, which can reduce common-mode voltage and power loss. The proposed modulation method mitigates the peak value of common-mode voltage by 42% and reduces the switching losses in the inverter stage. Experimental results verify the proposed method.

A new space vector modulation strategy to reduce common-mode voltage for quasi-Z-source indirect matrix converter

This paper proposes an integrated unified power flow controller (UPFC) with only nine IGBTs to improve grid compliance of renewable (i.e., wind and solar) power generation systems. The integrated UPFC has two AC outputs, which are served as an AC injected voltage source (DC-AC inverter) and an AC current source (AC-DC rectifier), respectively. The modulation of the UPFC is developed to provide sufficient voltage for power flow control as well as minimizing system rating. Then operating range of the developed UPFC is analyzed with a given system power rating. Furthermore, voltage oriented control of the UPFC is introduced to achieve independent control of real and reactive power. A 2-MVA simulation model is built to demonstrate the operating performance of the proposed UPFC configuration.

Nine IGBTs based UPFC topology and control for renewable power integration

A novel virtual inertia control utilizing cascaded multilevel inverter (CMI) based STATCOM for wind power generation is presented in this paper. Although wind power is a promising renewable energy source, its intermittent character has caused some difficulties for grid-connected and islanded operation. As wind plants (WP) installations are at an emergent pace, they must also contribute to maintain stability and reliability of the power system. However, the rotating mass of variable-speed wind turbines is decoupled from the grid frequency. Thus, it does not inherently exhibit an inertial frequency response (IFR) unless controlled for that specific purpose. In this paper, the comparison between traditional synthetic inertia control (SIC) and proposed novel virtual inertia control (VIC) is demonstrated. A Matlab/Simulink model of DFIG-based WP equipped with CMI-based STATCOM is presented. Both theoretical and simulated results are provided.

STATCOM-based virtual inertia control for wind power generation

This paper proposes an optimized fundamental frequency modulation (FFM) for cascaded multilevel inverter (CMI) based transformer-less unified power flow controller (UPFC). Compared to traditional FFM techniques, the proposed method features superior power sharing between H- bridge modules, thereby leading to better equalization of dc capacitor currents and voltage ripples. It also retains all the advantages of the traditional FFM such as low total harmonic distortion (THD) of output voltage and high efficiency. The proposed method is developed for a 2 MVA transformer-less UPFC system, nevertheless, it also can be applied to other applications such as CMI based static synchronous compensators (STATCOM), static synchronous series compensator (SSSC) and motor drives. I. INTRODUCTION

Optimization of Fundamental Frequency Modulation for Cascaded Multilevel Inverter Based Transformer-less UPFC

The 2ω harmonic component on the dc link side of the H-bridge inverters has long been a thorny problem, which requires a huge dc link capacitor bank to absorb this 2ω low frequency current ripple in order to maintain the dc link voltage ripple under a tolerable value. Till now, very few papers have discussed about this topic. This paper presents a simple 3rd harmonic injection method for the cascaded H-bridge multilevel inverters for photovoltaic systems. This approach achieves a 50% to 75% reduction of the dc link capacitance, without adding any extra components or increasing the control complexity. Theoretical analysis and simulation results are demonstrated to prove the effectiveness.

Fang Zheng Peng

Papers

A new space vector modulation strategy to reduce common-mode voltage for quasi-Z-source indirect matrix converter

Nine IGBTs based UPFC topology and control for renewable power integration

STATCOM-based virtual inertia control for wind power generation

Optimization of Fundamental Frequency Modulation for Cascaded Multilevel Inverter Based Transformer-less UPFC

Minimizing DC capacitance requirement of cascadeded H-bridge multilevel inverters for photovoltaic systems by 3 rd harmonic injection