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

Neutral-point-clamped (NPC) inverters are the most widely used topology of multilevel inverters in high-power applications (several megawatts). This paper presents in a very simple way the basic operation and the most used modulation and control techniques developed to date. Special attention is paid to the loss distribution in semiconductors, and an active NPC inverter is presented to overcome this problem. This paper discusses the main fields of application and presents some technological problems such as capacitor balance and losses.

A Survey on Neutral-Point-Clamped 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

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 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

High-power inverters for traction drives employ in the upper range of stator frequency special pulse control methods, optimized synchronous pulsewidth modulation (PWM), mainly in combination with rotor-flux-oriented control schemes, and direct self-control (DSC) which encompasses motor control and pulse pattern generation. The well-known symmetrized sinusoidal PWM is only to be used in the lower speed range. This paper presents and compares exemplarily the stationary behavior of these two control methods; criteria are inverter peak current, motor harmonic losses, torque ripple, and DC-link harmonics. For these low switching frequencies DSC shows, in spite of its simplicity, a very good overall performance, mainly by avoiding the imperfect use of switching frequency by the synchronous pulse patterns.

Direct self-control and synchronous pulse techniques for high-power traction inverters in comparison

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

The aim of this paper is to spread the sliding-mode control design technique on the new class of the ac drives: an induction motor fed by a three-level voltage-source inverter (3LVSI). A comprehensive investigation of possible drive ldquovariable structuresrdquo was carried out. Based on this analysis, an original two-step design procedure allows the use of the ldquoclassicalrdquo result of sliding-mode theory for the real variable structure system with more than 2m variable structures ( m is the control space order). Based on this design procedure, the original control algorithm that includes a choice condition for 3LVSI input dc voltage and a switch table for the 3LVSI semiconductor switches are designed. The performance of the considered control structure has been examined by simulation.

Sliding-Mode-Based Control for a Three-Level Inverter Drive

A highly dynamic control scheme with very low torque ripple-direct self control (DSC) with torque hysteresis control-for very high-power medium-voltage induction motor drives fed by a double three-level inverter (D3LI) is presented. In this arrangement, two three-level inverters that are connected in parallel at their DC sides are feeding the open motor windings. The DSC, well known from two- and three-level inverters, is adapted to the D3LI and optimized for a minimum torque ripple. An 18-corner trajectory is chosen for the stator flux of the induction machine since it is approaching the ideal circle much better than the hexagon known from DSC for two-level inverters, without any detriment to the torque ripple. The machine and inverter control are explained and the proposed torque quality and dynamics are verified by measurements on a 180-kW laboratory drive.

Direct self control with minimum torque ripple and high dynamics for a double three-level GTO inverter drive

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