模块化多电平变换器模型及稳态特性研究

Research about the Model and Steady-state Performance for Modular Multilevel Converter

模块化多电平变换器(Modular Multilevel Converter,MMC)以其高度模块化、高效率、低失真和高可靠性的特点在中高压大容量应用领域有着广阔的前景。本文以上下桥臂总体电容电压之和、上下桥臂总体电容电压之差、桥臂环流以及输出电流为状态变量建立了MMC的四阶大信号模型,并在大信号模型的基础上研究了MMC的稳态特性,得到了环流、桥臂电流以及上下桥臂平均电容电压的精确解析表达式。研究表明,环流主要含有直流分量和二次谐波分量,上桥臂电容电压与下桥臂电容电压的偶次谐波同相,奇次谐波反相,电容电压直流分量与稳态额定值略有差异。最后搭建了三相48模块的MMC仿真平台及实验样机,仿真波形和实验波形均与理论解析表达式高度吻合,验证了所提四阶模型及稳态解析表达式的准确性。

Modular multilevel converter(MMC) is a potential topology for the medium or high voltage and high power applications due to the modular structure, high efficiency, low distortion and high reliability. In this paper, a four order large signal model is built with the state variables of the summation of all the capacitor voltages, the difference between the total voltages of the upper arm and lower arm, the circulating current and the output current. Based on the model, the steady-state performance can be evaluated and the accurate steady-state expressions of the circulating current, the arm current and the average capacitor voltage can be obtained. It can be inferred that the circulating current is mainly composed of the DC component and the second harmonic. For the upper capacitor voltages and the lower capacitor voltages, the even harmonics are with the same phase and the odd harmonics are with the inverse phase. Furthermore, the DC bias of the capacitor voltage is a little different from the rated value. At last, a three-phase 48 sub-modules platform is built to verify the accuracy of the four order model and the steady-state expressions. The simulation and experimental results are highly matched with the theoretical expressions.