Summary The modular multilevel converters (MMCs) are frequently preferred for medium and high power energy conversion systems thanks to their modular structure and high quality output waveform. The remarkable studies related to the MMC have been carried out in recent years, which are mainly focused on its topology, control, or application. In this paper, MMC circuit topologies consisting of traditional submodule cells and new proposed configurations are introduced, and their control process, modulation techniques, and application areas are discussed in order to provide the readers the current state of the art of MMC technology. A wide part of this article is devoted to demonstrate the recent contributions arising in MMC studies. In this context, this paper not only outlines the circuit topology, control objectives, and applications of the MMC but also presents a comprehensive review, principally in terms of the latest developments of it. Ultimately, detailed proposals and new possible topics are provided to drive future expectations of MMC technology.

Summary Balanced operating conditions of photovoltaic (PV)‐connected cascaded H‐bridge multilevel inverters (CHB‐MLIs) require equal number of series and parallel arrays, equal amplitude modulation indices, and same environmental conditions such as temperature and irradiance for each H‐bridge (cell). Unbalanced operating conditions arise when at least one of these requirements are not met and/or a DC fault occurs. Phase‐shifted pulse width modulation (PS‐PWM) is the most widely used technique for CHB‐MLIs. However, under unbalanced operating conditions, harmonics that appear around the effective switching frequency (fsw) of each cell fail to eliminate each other when PS‐PWM is used, and consequently, undesired harmonics are generated at the multiples of fsw on the output voltage of CHB‐MLI. In this paper, a fault tolerant adaptive phase‐shifted pulse width modulation (FTA‐PS‐PWM) method is proposed for any number of cells to overcome this problem. For this reason, a mathematical representation of the harmonics appearing around the multiples of fsw on the output voltage of CHB‐MLI is suggested. In this analysis, on/off instants of the power semiconductors are investigated in frequency domain for a single carrier period. Based on derived formulations, phase angle of each cell is calculated at each sampling time using DC voltages and amplitude modulation indices of all cells in FTA‐PS‐PWM. In order to validate the usefulness of the proposed switching strategy, a three‐phase, PV‐connected CHB‐MLI is established in simulation environment and a prototype of a single‐phase CHB‐MLI is built. It is shown that FTA‐PS‐PWM reduces output voltage harmonics effectively in unbalanced operating conditions including DC faults for different number of cells. Moreover, FTA‐PS‐PWM is also compared with previous methods in the literature and compliance of output harmonics with international standards is examined for various cases. The results reveal that FTA‐PS‐PWM generally provides better results in terms of individual harmonics and total harmonic distortion of output voltages of CHB‐MLI and satisfies the requirements of the standards.

The expanding share of renewable energy sources (RESs) in power generation and rise of electric vehicles (EVs) in transportation industry have increased the significance of energy storage systems (ESSs). Battery is considered as the most suitable energy storage technology for such systems due to its reliability, compact size and fast response. Power converters are vital for the integration of batteries into power grid and EVs as they play an active role in both power conversion and battery management. Multilevel converters (MLCs) are types of power converters and attract widespread interest due to their improved power quality, reliability and modularity. There are two main challenges in MLC based battery storage systems (BSSs) which are selecting a proper MLC topology and balancing state‐of‐charges (SOCs) of batteries. Although some research has been carried out on either MLCs or SOC balancing, no single study exists which presents a comprehensive review on MLC based BSSs for large‐scale grid and EV applications. This paper begins by reviewing several major battery storage technologies that are utilised in MLC based BSSs. Later on, a systematical review of commonly used and recently proposed MLC topologies for BSSs are provided along with different control schemes for MLCs by specifically focusing on SOC balancing techniques. Finally, potential challenges and suggestions for future improvement of MLC based BSSs are addressed.

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The modular multilevel converter (MMC) topology is gaining more interest because of its modular design, high efficiency, and scalable voltage levels in medium- and high-power industrial applications, where the nearest level modulation (NLM) method is frequently preferred. In this paper, a novel NLM method is proposed with increased output voltage quality for MMC topology. In conventional NLM (C-NLM), output voltage is obtained as N + 1 levels, where N is the number of submodules (SMs) per arm. The output voltage is increased to 2N + 1 levels by the proposed NLM method without using any additional SMs. The proposed NLM method is based on the offset term injection, which is optimally determined in terms of the best output performance of MMC. Also, trapezoidal reference signal is used instead of sinusoidal reference, which provides better output voltage quality and controls the modulation process. The proposed NLM method presents simple implementation as in the C-NLM, and it is implemented to the upper and lower arms of the MMC; then, arm voltages are successfully controlled. Furthermore, output voltage returns the value of zero in the C-NLM process for low-modulation-ratio applications in relatively small amount of SM usage of MMC design. However, the proposed NLM method gives promising results instead of zero voltage. In order to validate the superior performance of the proposed NLM method, a comparative study is presented with C-NLM and third-harmonic injected NLM method in terms of total harmonic distortion (THD) and magnitude of the output voltage and current. THD of output waveforms of MMC is significantly reduced, and DC voltage utilization is remarkably increased, thanks to the proposed NLM method. In addition, capacitor voltage balancing for the proposed NLM method is accomplished to keep the capacitor voltage of each SM of MMC constant. Simulation results are presented to verify the effectiveness of the proposed NLM method under various case studies. Finally, experimental validation is carried out using a field programmable gate array (FPGA)-based hardware implementation on the laboratory prototype to show the applicability of the proposed NLM method.

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