Preface

Multilevel converters have gained much attention in recent years for medium/high-voltage and high-power industrial and residential applications. The main advantages of a multilevel converter over two-level converters include less voltage stress on power semiconductors, low dv/ dt, low common voltage, reduced electromagnetic interference, and low total harmonics distortion among others. Better output power quality is ensured with the increasing number of levels in the synthesized output voltage waveform. Several multilevel topologies have been reported in the literature, such as neutral point clamped (NPC), flying capacitor (FC), cascaded H-bridge (CHB), hybrid cascaded H-bridge, asymmetrical cascaded H-bridge, flying capacitor, modular multilevel converter (MMC), active neutral point clamped converter (ANPC), and packed U-cell-type converter, and various topologies based on reduced device count and reduced number of sources have also been proposed in the literature.

The multilevel converters have been commercialized in many standard and customized products such as mixers, blowers, pumps, compressors, conveyors, fans, crushers, rolling mills, extruders, grinding machines, solar photovoltaic systems, wind power conversion, traction and ship propulsion, electric vehicles, variable speed drives, static compensators (STATCOMS), high-voltage direct-current (HVDC) transmission, and hydro pump storage, to name a few. However, the multilevel converter, although a proven and enabling technology, still presents numerous challenges in topologies, modulation, and control and need-based applications. Since multilevel converters offer a wide range of possibilities, research and development in the area of multilevel converter topologies, modulation, and control and in various applications are still growing in depth and width. To further improve multilevel converters' energy efficiency, reliability, power density, and cost, many research groups across the world are working to broaden the application areas of multilevel converters and to make them more attractive and competitive compared to classic topologies. Multilevel converters represent a very important topic in modern power electronics, and many research groups are working on topologies, modulation techniques, and control strategies to improve the global behavior of the system.

This book, "Multilevel Converters", intended to provide a deep insight about multilevel converters' operation, modulation, and control strategies and the various applications of multilevel converters such as in variable speed drives, renewable energy generation, and power systems. The book will serve as a reference for academic researchers, university students, and practicing engineers who are working in the area of multilevel converters. This book contains 17 chapters on multilevel converter topologies, modulation, control, and applications and organized as detailed below.

Chapter 1 discusses the invention and applications of multilevel converters, which were created to overcome voltage limitations in semiconductor devices. The chapter's main objective is to comprehensively study multilevel inverters, including their advantages, drawbacks, and recent applications. The study begins with an examination of dual two-level inverter systems, highlighting their benefits, challenges, techniques for issue resolution, and various applications.

Chapter 2 highlights the historical development of multilevel inverters. It mentions the evolution of MLI configurations, including newer models with fewer switches that produce more voltage levels. The chapter emphasizes the industrial uses of MLI in on-grid renewable energy systems and discusses important MLI configurations such as flying capacitor MLI (FC-MLI), neutral point clamped (NPC) or diode clamped MLI (DC-MLI), and cascaded H-bridge MLI (CHB-MLI). Finally, it notes the attractiveness of multilevel technology for photovoltaic (PV) applications and provides an overview and classification of MLI topologies relevant to PV systems.

Chapter 3 provides a detailed explanation of both conventional and new multilevel converter topologies. It also covers various pulse width modulation techniques and discusses their applications in motor drives. To support the theoretical concepts, the chapter includes simulation and hardware results to validate the discussed findings.

Chapter 4 introduces a novel inverter modulation technique to generate 11 voltage levels at the inverter output. This modulation optimizes capacitor charging and discharging, reducing inrush current peaks significantly, improving reliability, and extending the inverter's lifespan, making it well-suited for solar PV applications even in challenging weather conditions. This innovative approach also reduces stress on switch voltage and current. To validate the effectiveness of this enhanced technology, comprehensive experiments are conducted using hardware-in-the-loop setups and simulation environments as well as rigorous testing in a laboratory prototype.

Chapter 5 discusses a four-level open-end winding induction motor (OEW-IM) drive controlled by a single inverter through space vector pulse width modulation (SVPWM). It employs two two-level voltage source inverters (VSIs) connected to each side of the OEW-IM. To achieve four-level inversion, the DC-link voltages of the dual-inverter system are maintained at a 2:1 ratio. The four-level OEW-IM drive faces two main challenges: 1) circulating zero sequence currents in the OEW-IM phase windings and 2) preventing overcharging of the lower DC-link voltage capacitor by the higher DC-link voltage capacitor. The latter issue is mitigated by providing independent DC power supplies to the dual-inverter system. The former challenge is addressed through a novel SVPWM scheme, which promises improved performance compared to existing SVPWM methods.

Chapter 6 discusses the use of field-oriented control (FOC) in induction motor drives, which is widely adopted for high-performance applications. FOC enhances motor drive performance by independently controlling the torque and field flux components, making the motor operation similar to a separately excited DC motor. This chapter specifically introduces a diodeclamped multi-level inverter for the closed-loop control of an induction motor drive using FOC. It covers the operation of the induction motor in both motoring and regenerating modes, powered by a diode-clamped three-phase three-level inverter circuit. The entire drive system is developed and analyzed using MATLAB/Simulink.

Chapter 7 focuses on the development of innovative topological designs for multilevel inverters that offer several key advantages, including a low component count, high voltage gain, cost-effectiveness, and minimized voltage stress on power switches. The study introduced five-level single-cell multilevel inverter (SCMLI) topologies designed to efficiently generate multilevel voltage waveforms, particularly suitable for medium- and high-power applications. Experimental testing has been conducted to validate the practical feasibility of each proposed topology. The results highlight several significant advantages, including reduced voltage stress, high voltage gain, the ability to operate at various power factors, reduced switching components, and high overall efficiency. The suggested topologies underwent both simulation in the MATLAB/Simulink environment and practical experimentation using the dSPACE-1104 controller, further reinforcing their viability and potential for practical implementation in real-world applications.

The switched-capacitor multilevel converters (SCMLCs) are gaining popularity for the integration of solar photovoltaic (SPV) systems into the grid. The current emphasis is on minimizing the number of components in SCMLCs, leading to cost savings and the creation of smaller converters. SCMLCs are particularly suitable for grid integration of low-voltage sources like SPV and fuel cells due to their ability to provide significant voltage boosting. In this context, Chapter 8 discusses a 13-level SCMLC topology for the grid-connected operation of SPV systems. This topology achieves threefold voltage boosting while utilizing a reduced component count, making it a promising solution for efficiently integrating SPV systems into the grid.

Chapter 9 discusses the multilevel inverters useful for solar photovoltaic (SPV) systems and provides a concise overview. The selection of suitable maximum power point tracking (MPPT) strategies for grid-connected SPV/hybrid systems with multilevel inverter has also been discussed in detail.

Chapter 10 discusses electric-drive-reconstructed onboard converter (EDROC), which can function in either the grid-to-vehicle (G2V) or the vehicle-to-grid (V2G) mode. It gives an extensive examination of the global implementation of power converters for electric vehicle systems and contemporary solutions for electric vehicle (EV) charging infrastructure.

In Chapter 11, the packed U-cell (PUC) inverter is highlighted as an innovative technology that achieves a seven-level voltage output using only six active switches, one isolated DC source, and a second capacitor source. However, maintaining a precise voltage ratio between the two capacitor sources is crucial. The chapter delves into the modulation and control of the PUC topology, focusing on its application in solar photovoltaic systems.

Chapter 12 proposes a reduced switch count-based multilevel converter for application in unified power quality conditioner (UPQC) in distribution power systems. The independent control of real and reactive power exchanged between the...