# A Review of Modular Multilevel Converters for Stationary Applications
## 6 Conclusions  
The MMC is expected to be a preferable choice in the medium and high-voltage power applications with intrinsic advantages such as scalable multilevel output voltage, low harmonic content of output voltage and output current, modular and flexible design, improved efficiency, and redundancy. As an application-oriented topology, it is predictable that the MMC will be guided to be more customized and well-adapted in the specific application area in terms of power transmission and quality improvement. This paper reviews the MMC with respect to the submodule and overall topologies, mathematical modeling and control methods, modulation techniques, and power losses with the incorporation of WBG technology. For submodule topology, the HBSM, owing to its simple architecture and low cost, maintains the domination in commercial utilization among a variety of submodule configurations. Newly developed submodule circuits could be investigated along with a comparative study to compete and balance between module size and cost as well as switching losses and fault tolerance. New advanced topologies could be explored to output better performance and meet different load requirements with the overall structure of the MMC, especially under fluctuating or unbalanced load. With respect to MMC control, the output voltage and current control under different grid conditions, submodule balancing control, and circulating current control are discussed. The challenging issues include the submodule balancing control, circulating current control, multiple variables' simultaneous control, and the complexity of consequent control. Nonlinear and predictive control strategies could be promising alternatives compared to conventional control methods. Modulation techniques are reviewed and classified according to switching frequency with decent  
Figure 19: Comparative performance of using Si and SiC semiconductors at different switching frequencies and load (100 kW) and input voltage per MMC HBSM (300 V): (**a**) Semiconductor power losses; (**b**) MMC efficiency.  
application area and implementation effort. Power losses are investigated with the incorporation of WBG technology by using different modulation methods and with different switching frequencies. The comparison of performance indicates that PSC-PWM has better output performance while generating higher power losses compared to PD-PWM and SAM-PWM, and the WBG semiconductors have superior performance regarding reduced power losses and higher power efficiency, especially at the high switching frequency. The incorporation of WBG technology will facilitate the MMC application with further advantages of high-voltage and high-power operations, low power losses, high efficiency, improved reliability, and reduced module size and cooling system. Finally, this review research focuses on the topology, modeling, control, and modulation techniques of the MMC for stationary applications, and future work will survey the MMC for vehicular applications.  
[PERSON] reviewed and wrote the first version of the paper; [PERSON] contributed to modeling the simulation and editing the paper; [PERSON] reviewed the MMC control and improved the structure of the paper; [PERSON] and [PERSON] contributed by analyzing the results and editing the paper; [PERSON] reviewed and edited the manuscript and also supervised this research study. All authors have read and agreed to the published version of the manuscript.  
This study was funded by China Scholarship Council (CSC).  
[PERSON] was sponsored by China Scholarship Council. The authors also acknowledge Flanders Make for the support of this research study.  
The authors declare no conflict of interest.