The invention discloses a
frequency reduction control method of a modular multilevel converter containing auxiliary sub-modules. Each bridge arm of the modular multilevel converter topology is formedby connecting N half-bridge sub-modules, one auxiliary sub-module and one bridge arm
inductor in series, the auxiliary sub-module adopts a full-bridge structure, and the
capacitor voltage of the auxiliary sub-module is controlled to be 1 / 2 of that of the half-bridge sub-modules. A nearest level approximation modulation mode is adopted for the half-bridge sub-modules of each bridge arm, and the auxiliary sub-modules are input only when the input number of the bridge arm half-bridge sub-modules is smaller than or equal to a preset value; The forward input and the reverse input of the auxiliary sub-modules need to be determined according to the deviation between the actual values and the reference values of the
capacitor voltages of the auxiliary sub-modules in the current state and the current directions of the bridge arms. According to the control method, the auxiliary sub-modules connected in series can only be input in part of time periods, the
switching frequency of the auxiliary sub-modules is effectively reduced while the output
level number of the MMC is increased and the output
voltage harmonic distortion rate of the modular multilevel converter is reduced, and the loss of the
system is reduced.