To further optimize the modular multilevel converter(MMC)sub-module control strategy

especially the added ability to balance the performance of sub-module capacitor voltage fluctuations

the second harmonic of bridge arm circulating current

sub-module insulated gate bipolar transistor(IGBT)switching frequency and algorithmic computation

a kind of strategy based on the linear optimal solution is promoted to control the multilevel converter sub-module capacitor voltage balance. First

the inconsistency between sub-module capacitor voltage fluctuations and IGBT switching frequency in the sub-module is described. The analysis has also proved the existence of a nonlinear relationship between sub-module capacitor voltage fluctuation and second harmonic of the bridge arm circulating current and the necessity of finding an appropriate algorithm for them to be in an optimal situation at the same time. Then

for this goal

combined with a control algorithm proposed in literature

a strategy using an additional sub-module regulator is proposed to optimize the conventional algorithm and the control flow algorithm and its advantages are given. Finally

the conventional control strategy

the strategy proposed in literature

the proposed strategy and another control strategy which uses different quantities of additional sub-module regulator are compared by a dynamic simulation test. Experiment data indicate that the proposed strategy can achieve linearly best between sub-module voltage fluctuations

second harmonic of bridge arm circulating current and sub-module IGBT switching frequency.