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 method for independent real and reactive power flow control without sensing receiving end voltage in a power flow controller (PFC) (228) includes calculating a first reference phase angle, calculating a first reference voltage, modifying the first reference phase angle calculated using a first phasor modifier, calculating a first reference current for a first terminal (1), calculating a second reference phase angle for current through the first terminal (1), calculating a second reference voltage across a second CMI (234) by subtracting voltages at the first terminal (1) and a second terminal (2), and controlling the first CMI (230) and the second CMI (234) for controlling the power flow through the PFC (228).

Method for independent real and reactive power flow control using locally available parameters

Analytical modeling of resonant converters in frequency domain for wireless power transfer: Continuous current mode (CCM) operation

A full-bridge dc-dc converter employing diode rectifier in the output experiences severe voltage overshoot and oscillation problem across the diode rectifier caused by the interaction between the junction capacitance of the rectifier diode and the leakage inductance of transformer. The pronounced reverse-recovery current of high power diodes enforces these issues significantly by increasing power loss and voltage overshoot. Conventional energy recovery clamping circuits suffer from high voltage overshoot if the input voltage of converter is wide. In this work, a novel energy recovery clamp circuit is proposed to overcome this problem. Performance of the proposed circuit is verified both theoretically and experimentally with a 70 kW dc-dc converter.

An improved energy recovery clamp circuit for PWM converters with a wide range of input voltage

A delta (A) -configured auxiliary resonant snubber inverter is developed to overcome the voltage floating problem in a wye (Y)-configured resonant snubber inverter. The proposed inverter is to connect auxiliary resonant branches between phase outputs to avoid a floating point voltage which may cause over- voltage failure of the auxiliary switches. Each auxiliary branch consists of a resonant inductor and a reverse blocking auxiliary switch. Instead of using an antiparalleled diode to allow resonant current to flow in the reverse direction, as in the Y -configured version, the resonant branch in the A-configured version must block the negative voltage, typically done by a series diode. This paper shows single-phase and three-phase versions of A- configured resonant snubber inverters and describes in detail the operating principle of a single-phase version. The extended three-phase version is proposed with nonadjacent state space vector modulation. For hardware implementation, a single-phase 1-kW unit and a three-phase 100-kW unit were built to prove the concept. Experimental results show the superiority of the proposed topology.

http://ieeexplore.ieee.org/iel1/28/10824/00502162.pdf

A Delta-Configured Auxiliary

High-frequency transformers (HFTx) are typically one of the lossiest, bulkiest and heaviest components in a power electronics system with galvanic isolation; for this reason, many literatures are saturated with research in HFTx size and loss optimization. These approaches are limited to the transformer design and manufacturing, and typically rely on complex algorithms. In this study, a circuit-based approach to reduce the transformer overall losses through specifically addressing the core loss, is proposed. This approach allows the implementation of a simple control strategy to reduce the HFTx core loss by transferring a desired RMS voltage (thus supplying a desired power to the load) from its primary to secondary side with low peak AC flux density. It is also independent of the HFTx design and manufacturing, thus can be implemented in a system that is already built. However, the control strategy can still be taken into consideration as part of the design stage of the system so as to build a transformer as small as possible without much sacrifice in its losses. A Z-source-inverter-based isolated power converter is used in this research to validate the control strategy, but the same concept can be implemented with any kind of converter that has control over the voltage shape across the transformer. Theoretical analysis shows that the converter’s overall efficiency can be high using the proposed control strategy, especially at light load conditions. Tradeoffs of implementing such kind of control strategy are also addressed in this paper.

Fang Zheng Peng

Papers

Method for independent real and reactive power flow control using locally available parameters

Analytical modeling of resonant converters in frequency domain for wireless power transfer: Continuous current mode (CCM) operation

An improved energy recovery clamp circuit for PWM converters with a wide range of input voltage

A Delta-Configured Auxiliary

Control Strategy for Core-Loss Reduction in High-Frequency Transformers