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.

The permanent magnet synchronous generator (PMSG) is dominantly used in the present wind energy market. Reflecting the latest wind energy market trends and research articles, this study presents a survey on important electrical engineering aspects for PMSG‐based megawatt‐level wind energy conversion systems (WECSs). A comprehensive analysis on power converter topologies for wind turbines (WTs), grid integration of wind farms, digital control schemes, fault‐ride‐through compliance methods, and future trends is presented. The updated market share, technology trends, WT products information, in‐depth technical analysis, and promising research works highlighted in this study will help the reader to understand the state‐of‐the‐art and emerging technologies for PMSG‐based WECS.

Arm voltage and submodule (SM) capacitor voltage balancing is a key factor for the safe and reliable operation of modular multilevel converters (MMCs). The arm voltage balancing is achieved through a zero-sequence voltage controller in carrier pulse-width modulation (CPWM). In this study, a dual space-vector pulse-width modulation (SVPWM) technique is proposed for an MMC, which eliminates the external controller for arm voltage balancing. In this approach, the three-phase top and bottom arms are independently controlled using SVPWM. In addition, the capacitor voltage balancing can be achieved using redundant switching vectors. However, this will increase the computational load on the space-vector modulator. Therefore, an external capacitor voltage-balancing approach is proposed to minimize the computational complexity. The proposed approach uses the direction of load current instead of the arm current in SM selection process. As such, the required number of current sensors is reduced to 50% in a three-phase system. The proposed modulation and voltage-balancing approach are simulated and experimentally verified on the MMC system with three-level flying capacitor (3L-FC) SMs. Simulation and experimental results show the successful balancing of the arm voltage and SM capacitors voltage.

The high-power voltage source inverters are switched at low frequency to reduce the losses in the inverter. The discontinuous pulsewidth-modulation (PWM) sequences reduce the switching frequency to two-thirds compared to the conventional continuous PWM sequences. At low switching frequency, the output will be rich in harmonics, and synchronization is a must to avoid subharmonics. This paper presents four basic types of synchronized discontinuous PWM (DPWM) sequences that ensure synchronization, three-phase symmetry, and half-wave symmetry. The proposed synchronized DPWM sequences are verified through simulation and experiment on a constant- v/f open-loop induction motor drive. The performance of the proposed DPWM algorithms is studied in the entire modulation region, including the overmodulation region, and is compared with that of the conventional synchronized space vector PWM. A different type of DPWM with low common-mode voltage is suggested for low-modulation region and verified experimentally.

The modular multilevel converter (MMC) has several submodules in cascade. To control the submodule capacitors voltage, a new generalized dynamic voltage balancing algorithm along with a simple pulse width modulator structure is proposed in this paper. With proposed pulse width modulator structure, the dynamic voltage balancing algorithm can be easily implemented with any type of carrier based pulse width modulation (including level-shifted and phase-shifted pulse width modulation) scheme without any modifications. This method does not require the sorting technique for selection of submodule capacitors voltage. The performance of the proposed voltage balancing algorithm is presented for MMC with three-level flying capacitor submodules (MMC-3L-FC). The proposed method is also applicable to the conventional two-level half bridge submodules as well. The effectiveness of the proposed voltage balancing algorithm is verified with the phase-shifted carrier-based pulse with modulation scheme. The results obtained from the MATLAB/SIMULINK simulations on 6-kV/ 2.5-MW system and the dSPACE DS1103-based laboratory prototype of 208-V/3-kVA MMC-3L-FC system are in close relationship, and thus, the proposed methodology is validated.