Abstract: There are a large number of sub-modules in MMC-HVDC project. Nearest level modulation(NLM) modulation algorithm is widely used in the project. The voltage sharing sequence of sub-modules is carried out in combination with the current direction of bridge arm and the voltage of sub modules, so as to determine the number of sub-modules to be switched. Based on the existing modulation and voltage sharing strategy, it is inevitable that sub-modules will have high switching frequency or even ultra-high switching frequency in the process of project implementation. Firstly, the phenomenon and mechanism of high frequency in engineering are analyzed, and the influences of high frequency on converter valve sub-module driver, power device loss, junction temperature and diode reverse recovery characteristics are analyzed. Then, combined with the high-frequency withstand capability of power devices and the reliability of protection, a two-stage inverse-time-limit sub-module high-frequency protection strategy is proposed, and the protection setting method and basis are established. The simulation verifies the effectiveness of the strategy and the protection strategy function test is carried out. The strategy has good real-time performance and reliability, and has no impact on the normal operation of the system. Finally, the strategy will be applied to improve the reliability of MMC-HVDC project in operation.