MMC的电容电压纹波效应及其对换流器优化设计的影响分析

Analysis of Ripple Effect of Submodule Capacitor Voltage and Its Influence on Optimal Design of MMC

模块化多电平换流器(modularmultilevelconverter,MMC)子模块上存在电容电压波动,是影响MMC分析和设计的重要因素。现有研究通常只关注电容电压纹波给器件电压应力和桥臂内部环流带来的影响,很少关注其对MMC交、直流端口输出特性的影响。揭示了电容电压纹波对交流输出电压和电容电压直流分量的影响机理,提出了基于标幺值的纹波效应偏差解析计算模型,可以根据运行工况准确求解出交流参考电压和子模块电容电压直流分量。在交流输出侧,计及纹波效应偏差影响可以扩大MMC线性调制区,提高MMC交流侧额定电压,降低桥臂额定电流。对于子模块电容电压,纹波效应给电容电压直流分量带来的偏差起到了降低电容电压峰值的作用,可以降低所需电容值。计及纹波效应偏差影响的参数优化可以使MMC的成本、体积和损耗都得到相应的下降,尤其是可以使电容用量得到大幅下降,使MMC的体积和成本得到显著优化。数字仿真结果验证了所提出的纹波效应理论及优化设计方法。

The voltage fluctuation of the submodule capacitors in the modular multilevel converter(MMC) is an important factor affecting the analysis and design of the MMC.The existing researches only focus on the effect of the capacitor voltage fluctuation on the voltage stress and the internal circulating current, but seldom on the systematic and accurate analysis of the output voltages of the AC and DC ports of the MMC. In this paper, an analytical per-unit model of the ripple effect deviation is proposed, and a method to accurately calculate the AC-side reference voltage and the DC component of the submodule capacitor voltage is proposed according to the operating conditions. The ripple effect deviation is taken into account to accurately calculate the linear modulation region and the peak value of the capacitor voltage. On the AC side, the ripple effect deviation has a favorable effect on expanding the linear modulation region of the MMC, so the rated current of the arm is reduced by increasing the rated voltage on the AC side of the MMC. In terms of the submodule capacitor voltage, the ripple effect deviation reduces the peak value of the capacitor voltage and thus decreases the required capacitance value. The optimization of the above parameters will reduce the cost, volume and loss of the MMC, especially the capacitance usage. It has a significant impact on the decrease of the volume and cost of the MMC. The digital simulation results verify the proposed ripple effect theory and the optimization design method.