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

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

This work is devoted to review and analyze the most relevant characteristics of multilevel converters, to motivate possible solutions, and to show that we are in a decisive instant in which energy companies have to bet on these converters as a good solution compared with classic two-level converters. This article presents a brief overview of the actual applications of multilevel converters and provides an introduction of the modeling techniques and the most common modulation strategies. It also addresses the operational and technological issues.

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The age of multilevel converters arrives

This paper gives an overview of medium-voltage (MV) multilevel converters with a focus on achieving minimum harmonic distortion and high efficiency at low switching frequency operation. Increasing the power rating by minimizing switching frequency while still maintaining reasonable power quality is an important requirement and a persistent challenge for the industry. Existing solutions are discussed and analyzed based on their topologies, limitations, and control techniques. As a preferred option for future research and application, an inverter configuration based on three-level building blocks to generate five-level voltage waveforms is suggested. This paper shows that such an inverter may be operated at a very low switching frequency to achieve minimum on-state and dynamic device losses for highly efficient MV drive applications while maintaining low harmonic distortion.

Medium-Voltage Multilevel Converters—State of the Art, Challenges, and Requirements in Industrial Applications

Multilevel converters are considered today as the state-of-the-art power-conversion systems for high-power and power-quality demanding applications. This paper presents a tutorial on this technology, covering the operating principle and the different power circuit topologies, modulation methods, technical issues and industry applications. Special attention is given to established technology already found in industry with more in-depth and self-contained information, while recent advances and state-of-the-art contributions are addressed with useful references. This paper serves as an introduction to the subject for the not-familiarized reader, as well as an update or reference for academics and practicing engineers working in the field of industrial and power electronics.

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Multilevel Converters: An Enabling Technology for High-Power Applications

Multilevel Converters: a Survey

In this paper, a new pulsewidth modulation (PWM) inverter topology suitable for medium-voltage (2300/4160 V) adjustable-speed drive systems is proposed. The modular inverter topology is derived by combining three standard three-phase inverter modules and a 0.33-pu output transformer. The output voltage is high-quality multistep PWM with low dv/dt. Further, the approach also guarantees balanced operation and 100% utilization of each three-phase inverter module over the entire speed range. These features enable the proposed topology to be suitable for powering constant-torque, as well as variable-torque type loads. Clean power utility interface of the proposed inverter system can be achieved via an 18-pulse input transformer. Analysis, simulation and experimental results are shown to validate the concepts.

A new medium voltage PWM inverter topology for adjustable speed drives

In this paper a new PWM inverter topology suitable for medium voltage (2300/4160 V) adjustable speed drive (ASD) systems is proposed. The modular inverter topology is derived by combining three standard 3-phase inverter modules and a 0.33 pu output transformer. The output voltage is high quality, multistep PWM with low dv/dt. Further, the approach also guarantees balanced operation and 100% utilization of each 3-phase inverter module over the entire speed range. These features enable the proposed topology to be suitable for powering constant torque as well as variable torque type loads. Clean power utility interface of the proposed inverter system can be achieved via an 18-pulse input transformer. Analysis, simulation, and experimental results are shown to validate the concepts.

In this paper, a critical evaluation of the effect of voltage sags on adjustable-speed drives (ASDs) is presented. In particular, the DC-link voltage variation under voltage sag and its dependence on source impedance, DC-link inductance and output load is computed. It is shown that, for larger source impedance, the DC-link voltage variation under a voltage sag is also large and increases the susceptibility of an ASD and may result in a nuisance trip. The results from the analysis are plotted in per-unit quantities and serve as a design guide to assess ASD performance for a variety of sags. In order to improve the performance of ASDs, this paper proposes an integrated boost converter approach. This approach provides ride-through to critical ASD load during voltage sags without any additional energy storage device. Upon detection of a voltage sag, the boost converter operates with suitable duty ratio and maintains the DC-link voltage within acceptable limits. This prevents nuisance tripping and facilitates continuous operation of critical ASD load at rated torque. The proposed integrated boost converter does not introduce any additional semiconductors in the series path of the power flow and is low in cost. A commercially available 480 V 22 kVA ASD is modified with the integrated boost converter approach, and details are discussed. Analysis, simulation, and experimental performance of the ride-through approach are presented.

Effect of voltage sags on adjustable-speed drives: a critical evaluation and an approach to improve performance

Voltage sags are a common occurrence in industrial power distribution systems. Although a typical sag may last only 5-20 cycles, a voltage magnitude lower than 20% of its rated value can trip an adjustable speed AC drive (ASD). Such a nuisance tripping of a continuous industrial process can be very costly. In this paper, a low cost approach to improve the performance of an ASD under voltage sag and short term interruptions is presented. The approach consists of a low cost modification (addition of three diodes and an inductor) to the front end diode rectifier topology. This modification along with the dynamic-braking IGBT (Q/sub db/) control (a standard component in a ASD) is shown to provide ride-through capability for voltage sags. Further, it is shown that with the addition of a diode and battery, the ride-through capability can also be extended to short-term power interruptions. The IGBT Q/sub db/ is suitably controlled in the event of a sag to maintain rated DC-link voltage in a closed loop, thus avoiding any nuisance tripping or momentary speed fluctuations. A 460 V, 10 hp commercially available ASD is modified with the proposed approach. Analysis, design and simulation results are discussed. Experimental results illustrating the performance of the ASD with the proposed ride-through topology for a wide range of voltage sag conditions are presented.

Byeong Ok Woo

Papers

Multilevel Converters: a Survey

A new medium voltage PWM inverter topology for adjustable speed drives

A new medium voltage PWM inverter topology for adjustable speed drives

Effect of voltage sags on adjustable-speed drives: a critical evaluation and an approach to improve performance

A low cost approach to improve the performance of an adjustable speed drive (ASD) under voltage sags and short-term power interruptions