Balance Control Strategy for Sub-module Capacity Voltage of MMC Based on Linear Optimization

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.