The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.

Operation, Control, and Applications of the Modular Multilevel Converter: A Review

Modular multilevel converters have several attractive features such as a modular structure, the capability of transformer-less operation, easy scalability in terms of voltage and current, low expense for redundancy and fault tolerant operation, high availability, utilization of standard components, and excellent quality of the output waveforms. These features have increased the interest of industry and research in this topology, resulting in the development of new circuit configurations, converter models, control schemes, and modulation strategies. This paper presents a review of the latest achievements of modular multilevel converters regarding the mentioned research topics, new applications, and future trends.

Circuit topologies, modeling, control schemes, and applications of modular multilevel converters

Modular multilevel converter (MMC) is one of the most promising topologies for medium to high-voltage high-power applications. The main features of MMC are modularity, voltage and power scalability, fault tolerant and transformer-less operation, and high-quality output waveforms. Over the past few years, several research studies are conducted to address the technical challenges associated with the operation and control of the MMC. This paper presents the development of MMC circuit topologies and their mathematical models over the years. Also, the evolution and technical challenges of the classical and model predictive control methods are discussed. Finally, the MMC applications and their future trends are presented.

Evolution of Topologies, Modeling, Control Schemes, and Applications of Modular Multilevel Converters

The power converters based on a modular structure have gained importance for electric transportation applications, including electric vehicles (EVs), EV charging stations, railway traction, and electric ships. Modular multilevel converters (MMCs) are recognized as one of the promising topologies to improve the energy conversion efficiency and fault-tolerant ability of power conversion systems. This paper aims at a review of latest research contributions regarding the evolution of circuit topologies, operational challenges, control schemes, and fault-tolerant strategies of the MMC. Finally, the current state of the art, opportunities and the future perspective of MMC in electric transportation applications are addressed.

Modular Multilevel Converters for Transportation Electrification: Challenges and Opportunities

Traction inverter, as a critical component in electrified transportation, has been the subject of many research projects in terms of topologies, modulation, and control schemes. Recently, some of the well-known electric vehicle manufacturers have utilized higher-voltage batteries to benefit from lower current, higher power density, and faster charging times. With the ongoing trend toward higher DC-link voltage in electric vehicles, some multilevel structures have been investigated as a feasible and efficient option for replacing the two-level inverters. Higher efficiency, higher power density, better waveform quality, and inherent fault-tolerance are the foremost advantages of multilevel inverters which make them an attractive solution for this application. This paper presents an investigation of the advantages and disadvantages of higher DC-link voltage in traction inverters, as well as a review of the recent research on multilevel inverter topologies for electrified transportation applications. A comparison of multilevel inverters with their two-level counterpart is conducted in terms of efficiency, cost, power density, power quality, reliability, and fault tolerance. Additionally, a comprehensive comparison of different topologies of multilevel inverters is conducted based on the most important criteria in transportation electrification. Future trends and possible research areas are also discussed.

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A Review of Multilevel Inverter Topologies in Electric Vehicles: Current Status and Future Trends

The choice of the converter topology feeding the electric machine of the main propulsion of electric ships plays an important role for the feasibility of an all electric ship concept. The medium-voltage machine has to be speed-adjustable in the whole speed range at high power and maybe it is connected by a long cable to the inverter. In other high power industrial drives with long cable length, e.g. in fan motor drives in underground mines, overvoltages at the machine terminals are observed, resulting in premature motor insulation failure. For that reason a multilevel topology for the used converter is indispensable. State of the art are multilevel converters as Diode-Clamped Converter, Imbricated-Cell Converter or Stacked H-Bridge Converter with a low number of levels limited by the restrictions in practical construction. In contrast the Modular Multilevel Converter allows a nearly unlimited cascading of its sub-modules and therewith of voltage levels at the same complexity in construction. The challenge of using this topology as drive converter is the AC fluctuation of the sub-module voltages rising with sinking output frequency, controllable only by special control schemes and by a special load characteristic of the machine. This paper shows the dimensioning of a 17-level Modular Multilevel Converter feeding a SIEMENS H-COMPACT PLUS 3.9-MW 4.16-kV machine. The advantages and the challenges of the modular multilevel topology are presented and a characteristic diagram of the drive is shown. Simulation results document the chosen control strategies in the full operation range.

Modular Multilevel Converter for propulsion system of electric ships

For medium-voltage drives multilevel converters are advantageous to reduce the insulation stress of the windings of the machine. State of the art are multilevel converters as Diode Clamped Converter, Imbricated Cell Converter or Stacked H-Bridge Converter with a low number of levels limited by the restrictions in practical construction. The Modular Multilevel Converter is not limited in the number of levels. Its modularity allows a nearly unlimited cascading of sub-modules and therewith of voltage levels at the same complexity in construction. The challenges of this topology used as drive converter are the AC fluctuations of the sub-module voltages rising with sinking output frequency. This paper derives analytically equations for the module-voltage fluctuations for two different controls using the converter in the whole speed range. The results are analysed and compared to simulation results.

Analysis of the module-voltage fluctuations of the Modular Multilevel Converter at variable speed drive applications

Electric vehicles (EV) pose specific challenges to the used machine, the inverter and the control scheme. High torque at low speed combined with very high maximum speed enforce a wide field-weakening range. In addition the battery voltage varies as a consequence of state of charge and of acceleration or deceleration load current. Especially in case of rapid changes from acceleration to deceleration the voltage changes very quickly. Safety-relevant features like slip-slide control, electronic stability programs or emergency brake assistance require fast torque control in the whole speed range. Stator-flux-oriented Indirect Stator-Quantities Control (ISC) satisfies these criteria with excellent performance, limited only by the physical bounds of available voltage and switching frequency. Moreover it does not require any voltage control margin in field-weakening range. ISC is in practical use in many railway applications with induction machines. The paper introduces the control scheme and demonstrates its abilities by simulations using an induction machine dimensioned for use in an electric vehicle. As the suggested machine does not exist physically, measurements are performed at a test bench for railway application, emulating EV conditions with very wide field weakening by reduction of the DC-link voltage. ISC can be applied to induction machine and permanent-magnet synchronous machine drives as well.

Stator-flux-oriented control with high torque dynamics in the whole speed range for electric vehicles

Modern electrical induction machine drives demand highest torque dynamic with well-defined guidance of the stator flux at the same time. The stator-flux-oriented Indirect Stator-Quantities Control (ISC) satisfies these criteria in the full operating range. Actual control systems with high processing power are able to utilize all advantages of ISC.

Indirect Stator-Quantities Control as benchmark for highly dynamic induction machine control in the full operating range

Highly dynamic stator-flux-oriented controls need an exact model-based observer of the induction machine to predict the non-measurable fluxes and to calculate the speed in sensorless operation. Inverter voltage errors cause inaccuracy and instability in the observer. Therefore an offline-measurement procedure without any additional measurement devices is presented, to identify the error voltages. An online correction is introduced and verified by measurement results.

Correction of inverter voltage errors for model-based induction machine control

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