一种MMC子模块电容电压波动最小化容值优化方法

模块化多电平变换器MMC(modular multilevel converter)广泛应用于直流配电系统。MMC由多个子模块组成,减小子模块电容可以降低子模块成本,提高变换器功率密度。分析了共模电压、环流和调制比对电容电压波动的影响,利用电容电压波动的解析表达式,通过注入合适幅值和相位的三次共模电压和二、四次环流,选取最优调制比,使目标函数电容电压纹波达到最小化,从而在一定电容电压波动率下使其电容取值达到最小化。采用所提方法可以使子模块电容容值的需求降低至常规方法的22%,变换器功率损耗降低至常规方法的87%,而开关管电流应力与现有方法相比只增加0.5%。RT-LAB实时仿真结果验证了所提理论的正确性和可行性。

Capacitance Optimization Method for Minimizing Submodule Capacitor Voltage Fluctuation of Modular Multilevel Converter

Modular multilevel converters (MMC) have been widely applied to DC power distribution systems. An MMC is composed of multiple submodules, and the decrease in submodule capacitance can reduce cost and improve the converter''s power density. In this paper, the effects of common-mode voltage, circulating current and modulation index on capacitor voltage fluctuation are analyzed. Using the analytical expression of capacitor voltage fluctuation, the objective function of capacitor voltage ripple is minimized by injecting third-order common-mode voltage and second- and fourth-order circulating current with appropriate amplitude and phase and selecting the optimal modulation index. Therefore, the capacitance can be minimized at a certain capacitor voltage fluctuation rate. Using the proposed method, the submodule capacitance requirement is reduced to 22% of that when using the conventional method and the converter''s power loss is reduced to 87%, while the transistor current stress only increases by 0.5% compared with that obtained using the existing method. The real-time simulation results based on RT-LAB verify the correctness and feasibility of the proposed theory.