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

Cascaded multilevel inverters synthesize a medium-voltage output based on a series connection of power cells which use standard low-voltage component configurations. This characteristic allows one to achieve high-quality output voltages and input currents and also outstanding availability due to their intrinsic component redundancy. Due to these features, the cascaded multilevel inverter has been recognized as an important alternative in the medium-voltage inverter market. This paper presents a survey of different topologies, control strategies and modulation techniques used by these inverters. Regenerative and advanced topologies are also discussed. Applications where the mentioned features play a key role are shown. Finally, future developments are addressed.

A Survey on Cascaded Multilevel Inverters

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

A novel switched-capacitor inverter is proposed. The proposed inverter outputs larger voltage than the input voltage by switching the capacitors in series and in parallel. The maximum output voltage is determined by the number of the capacitors. The proposed inverter, which does not need any inductors, can be smaller than a conventional two-stage unit which consists of a boost converter and an inverter bridge. Its output harmonics are reduced compared to a conventional voltage source single phase full bridge inverter. In this paper, the circuit configuration, the theoretical operation, the simulation results with MATLAB/SIMULINK, and the experimental results are shown. The experimental results accorded with the theoretical calculation and the simulation results.

A Switched-Capacitor Inverter Using Series/Parallel Conversion With Inductive Load

This paper, along with an earlier published paper as Part 1, provides a comprehensive review of the state of the art of high-power converters (above 1 MW) for adjustable-speed ac drives. In this highly active area, different converter topologies have been developed for various drive applications in the industry. Due to its extensive coverage, the subject is divided into two parts: multilevel voltage source and current source converter topologies. This paper is focused on the second part and covers the current source converter technologies, including pulsewidth-modulated current-source inverters (CSIs) and load-commutated inverters. In addition, this paper also addresses the present status of the direct converter, which is also known as cycloconverter (CCV). This paper focuses on the latest CSI and CCV technologies and an overview of the commonly used modulation schemes. It also provides the latest technological advances and future trends in CSI- and CCV-fed large drives. This paper serves as a useful reference for academic researchers and practicing engineers in the field of power converters and adjustable-speed drives.

Current-Source Converter and Cycloconverter Topologies for Industrial Medium-Voltage Drives

High-power pulsewidth-modulated inverters for medium-voltage applications operate at switching frequencies below 1 kHz to keep the dynamic losses of the power devices at a permitted level. Also, the sampling rate of the digital signal processing system is then low, which introduces considerable signal delays. These have adverse effects on the dynamics of the current control system and introduce undesired cross coupling between the current components i/sub d/ and i/sub q/. To overcome this problem, complex state variables are used to derive more accurate models of the machine and the inverter. From these, a novel current controller structure employing single-complex zeros is synthesized. Experimental results demonstrate that high dynamic performance and zero cross coupling is achieved even at very low switching frequency.

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Design of fast and robust current regulators for high power drives based on complex state variables

Grinding mill drives play an important role in the mining industry. Close to 60% of the electrical energy consumed by modern concentrator plants goes to grinding mill drives. Different technologies are used in grinding systems presenting special characteristics from the electrical and process points of view. This paper presents a technical evaluation and practical experience of two different technologies used in high power grinding mill drives. The analysis is focused on the static power converter and associated control scheme required to drive the motor. Cycloconverters and load commutated inverters are analyzed in terms of the interaction with the power grid, the motor, and the required control scheme. The evaluation is supported with practical results obtained in different concentrator plant facilities.

Technical evaluation and practical experience of high power grinding mill drives in mining applications

Underground mining is characterized by space constraints forcing the use of long cables to feed large machines, where the application of variable speed drives can generate overvoltages at the machine terminals. This paper presents the analysis and mitigation of harmful overvoltages detected in inverter-fed 1400-hp 13.8-kV motors used for air ventilation in an underground mine. The possible causes of the overvoltages, namely wave reflection and resonance, are described theoretically and simulated. On-site direct voltage measurements at the 13.8-kV motor terminals confirmed that resonance was the dominant phenomenon. Based on this study, a suitable low-pass filter was designed, constructed, and installed. Finally, the adequate operation of such filter is demonstrated by further on-site measurements.

Resonances and overvoltages in a medium voltage fan motor drive with long cables in an underground mine

High-power converters for regenerative drives and distributed generation need a network-friendly operation, low harmonic conductive electromagnetic emission for low electromagnetic interference and high electromagnetic compatibility reaching high reliability and performance. Nevertheless, the converters must be controlled with low switching frequency in order to reduce the commutation losses. These two requirements can be satisfied if an optimal modulation strategy is used. The selective harmonic elimination (SHE) is one of the low-switching-frequency strategies most used today. However, this strategy only eliminates a reduced set of harmonic components from the input current. This paper presents a novel optimal modulation strategy whose objective is to reduce the total harmonic distortion of the input current. Six- and twelve-pulse three-level neutral-point-clamped pulsewidth modulation rectifiers are used in order to implement both modulation techniques. The results confirm the advantages of the proposed strategy, namely, less input current distortion and remarkable reduction of higher order harmonics compared with the SHE method, while keeping a low-switching behavior.

Network-Friendly Low-Switching-Frequency Multipulse High-Power Three-Level PWM Rectifier

Mining systems have complex power networks including high-power equipment, variable configuration and distributed harmonic injections. This paper presents the problem of resonance mitigation and dynamical behavior of a power system with harmonic filters (HF) for improving reliability in mining plants. It is known that the main objective of the harmonic filters is to provide power factor correction and limiting the harmonic distortions. Nevertheless, until now it is not well covered in the literature that HF also provides mitigation of impedance resonances. Simulation results obtained of the transient behavior analysis of industrial electrical system when harmonic filters are switched are presented, with good correlation between steady and transient behavior, supporting the results given by tools intended for stationary analysis of harmonics performance. Different operating conditions of an industrial power system given by variable topology, multiple harmonic injections and loading factor should be taken into account for the proper design of HF systems. The presented methodology supports the risk assessment, failure analysis and reliability improvement of industrial systems as well as the decision making process at the design stage of power systems

P. Newman

Papers

Design of fast and robust current regulators for high power drives based on complex state variables

Technical evaluation and practical experience of high power grinding mill drives in mining applications

Resonances and overvoltages in a medium voltage fan motor drive with long cables in an underground mine

Network-Friendly Low-Switching-Frequency Multipulse High-Power Three-Level PWM Rectifier

Resonance mitigation and dynamical behavior of systems with harmonic filters for improving reliability in mining plants