混合型MMC降低子模块电容电压波动的优化策略

模块化多电平换流器(multilevel modular converter,MMC)在高压直流输电(high voltage direct current,HVDC)领域得到了广泛的应用。由半桥以及全桥子模块构成的MMC因具备主动使换流器直流侧输出极间零电压以适应短路故障条件的能力,引起了国内外学者的广泛关注。首先,从混合型MMC的开关函数角度出发,对理想情况下混合型MMC进行建模,建立了子模块电容电压基频、二倍频波动数学模型,并提出单位降容比的概念,研究了调制比对子模块电容电压波动的影响。其次,提出提高调制比的抑制子模块电容电压波动配合策略,有效降低子模块电容电压波动。在此基础上,提出基于三次谐波注入的新增半桥子模块数目优化方法,减少半桥子模块的新增数目,解决了单纯提高变比带来的全桥电容电压降落的副作用。最后,在PSCAD/EMTDC中搭建双端±160 kV混合型MMC的仿真模型,验证了所提降容策略的正确性和有效性。

Optimized Strategy for Reducing Capacitor Voltage Ripple in Hybrid Modular Multilevel Converter

Modular multilevel converter （MMC） has been widely used in high voltage direct current （HVDC）. The MMC， which consists of half bridge and full bridge submodules， has attracted the attention of domestic and foreign scholars because of the self-cleaning ability of DC short-circuit fault. According to the switching function of the hybrid MMC， the hybrid MMC is modeled under the ideal condition， and the fundamental frequency voltage fluctuation on the capacitor of the submodule is analyzed. A coordination strategy is proposed， which suppresses capacitor voltage fluctuation of sub-module by over modulation. On this basis， a new method for optimizing the number of half-bridge sub-modules based on third harmonic injection is proposed， which reduces the number of new added half-bridge sub-modules and alleviates the side effects of full-bridge capacitance voltage drop caused by simply increasing the modulation ratio. Finally， the correctness and validity of the proposed strategy are verified by simulation of a two-end ±160 kV hybrid MMC in PSCAD/EMTDC.