The modular multilevel converter (MMC) is extensively utilized in medium and high-voltage engineering applications due to its modular structure and flexible scalability. However

the power devices and capacitors within the submodules (SMs) are susceptible to failure under high current and voltage conditions

which can compromise the stable operation of the MMC. When an SM is removed due to faults

the asymmetric operation of the MMC can progressively deteriorate its operational status

potentially resulting in system downtime. Fault-tolerant control methods have been extensively studied as effective means to ensure the continuous and reliable operation of MMCs. This paper begins by introducing the MMC topology and analyzing the characteristics of asymmetric operation. It clarifies the primary effects of asymmetric operation on circulating current

DC voltage

and other operational states. Secondly

based on the control objectives and specific execution methods of MMC fault-tolerant operation techniques

a review of both domestic and international literature on fault-tolerant operation methods is conducted. The strengths and weaknesses of various types of fault-tolerant operation methods are summarized. Finally

the primary future research direction for fault-tolerant operation methods in MMCs is discussed. This focuses on key challenges and concepts that need to be addressed

including neutral point offset

fundamental frequency injection

and sub-module voltage control

aiming to enhance voltage utilization and minimize losses.