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 compact nX dc-dc converter-based PV power system is proposed to perform the power electronic interface with small size, light weight, low cost, high efficiency, and suitable for high temperature of solar power. The 1-kW 8X dc-dc converter prototype is built to investigate its performances, including size, weight, voltage gain, and efficiency. The results verify the 8X dc-dc converter best suitable for PV power system, due to its small size (7 inches length by 3 inches width), light weight, and high efficiency (no need for special heat sink) larger than 98%, and the magnetic-less design provides an excellent candidate in application to high temperature of solar power.

A compact nX DC-DC converter for photovoltaic power systems

This paper proposes the use of the Z-source inverter with grid connected wind turbine generation systems. The maximum boost control is used to boost up the output voltage with lower voltage stress across the IGBTs. The grid-connected control method is proposed for the system. The two independent control variables, the shoot-through duty ratio and the modulation index, are used to control the proposed system in order to provide the required voltage and power. The simulation and experimental results show that the system and control method effectively produce the desired voltage and precisely deliver the produced power to the utility grid, even though the input voltage can be lower or higher than the output voltage.

Z-source inverter with grid connected for wind power system

A new type of three-phase quasi-Z-source indirect matrix converter QZS-IMC is proposed in this paper. It uses a unique impedance network for achieving voltage-boost capability and making the input current in continuous conduction mode CCM to eliminate the input filter. The complete modulation strategy is proposed to operate the QZS-IMC. Meanwhile, a closed-loop DC-link peak voltage control strategy is proposed, and the DC-link peak voltage is estimated by measuring both the input and capacitor voltages. With this proposed technique, a high-performance output voltage control can be achieved with an excellent transient performance even if there are input voltage and load current variations. The controller is designed by using the small-signal model. Vector control scheme of the induction motor is combined with the QZS-IMC to achieve the motor drive. A QZS-IMC prototype is built in laboratory, and experimental results verify the operating principle and theoretical analysis of the proposed converter. The simulation tests of QZS-IMC based inductor motor drive are carried out to validate the proposed converter's application in motor drive. Copyright © 2013 John Wiley & Sons, Ltd.

A novel quasi-Z-source indirect matrix converter

This paper discusses islanding control of power electronics interfaced distributed generation (DG) systems in microgrids. Particularly, the following topics will be addressed: microgrid sytem configurations and features, grid-connected system control mode, transient analysis during power outage, islanding detection and load shedding, standalone system control mode, synchronization in reconnection. Some simulation and experimental results are also shown in this paper to illustrate the above mentioned issues.

Islanding control of DG in microgrids

This paper presents a double-wing multilevel modular capacitor-clamped (DW-MMCCC) dc-dc converter. Compared to the traditional single-wing (SW) MMCCC with the same voltage conversion ratio, the capacitor voltage stress of DW-MMCCC is reduced by half. And the switch and capacitor current stress, the switch voltage stress of the SW-MMCCC and the DW-MMCCC are the same. When the conversion ratio is equal to N, the total device and capacitor number of the even conversion ratio DW-MMCCC is 3N-2 and N, which is the same with the corresponding SW-MMCCC. The total device and capacitor number of odd conversion ratio DW-MMCCC is 3N-1 and N+1 respectively. By using the distributed stray inductance existing in the circuit, zero current switching (ZCS) can be achieved for all the switches of DW-MMCCC. And by using multi-phase DW-MMCCC, the input current ripple and input capacitor size can be reduced significantly. Compared with the traditional multilevel dc-dc converter, the power density of DW-MMCCC could be increased by about ten times. A 100 W three-phase 3X-ZCS-DW-MMCCC prototype has been built and tested. The simulation and experimental results are provided to verify the validity of the proposed converter.

Fang Zheng Peng

Papers

A compact nX DC-DC converter for photovoltaic power systems

Z-source inverter with grid connected for wind power system

A novel quasi-Z-source indirect matrix converter

Islanding control of DG in microgrids

A double-wing multilevel modular capacitor-clamped dc-dc converter with reduced capacitor voltage stress