The reliable operation of Modular Multilevel Converter (MMC)

as the commonly used grid connection scheme of large scale PV collection system

is the key prerequisite for achieving the applications of large-scale PV collection dc systems. In recently years

many researches have been performed to enhance reliability of MMCs by optimizing the thermal distribution. However

such related researches as taking the MMC as the research object

summary and analysis of its electrothermal characteristic and corresponding optimization method are still lacked. Therefore

the thermal distribution characteristics of MMCs and their influencing factors are analyzed in this paper

and the latest progress of the electrothermal optimization

adjustment and control methods is summarized. Firstly

the thermal distribution characteristics between the inside of the MMC sub-model and sub-model inside the same arm and the reasons for the difference are analyzed. Then

the basic principles and control objectives of different MMC electrothermal regulation methods are introduced and their pros and cons are also discussed. Finally

the main challenges of different improvement methods and research directions in the future are analyzed so as to provide a reference for the research of thermal distribution improvement and reliability enhancement of MMC.