基于桥臂基波平均开关函数的MMC模型在直流电网仿真中的应用

Application of MMC model based on arm fundamental wave average switching function in DC grid simulation

在直流电网的研究中,模块化多电平换流器(MMC)的建模起着至关重要的作用。鉴于此,提出了一种基于桥臂基波平均开关函数的MMC行为模型,相比于已有的以桥臂为基本建模单元的MMC模型,该模型将MMC的解锁运行状态与闭锁运行状态结合为一个整体,且模型主电路中只采用了受控电压源、不变电阻以及二极管等静态元件,进而可以在现有的仿真软件上完成模型的图形化搭建而无需重新进行元件的编码与数值计算处理;同时,在所提出的模型中,由于桥臂基波平均开关函数可以直接从控制系统中得到,而无需通过调制系统,因此简化了整个模型的搭建与调试。通过与详细模型的对比实验,验证了所提行为模型的精确性与快速性。采用上述建模方法,对一个四端金属回线接地真双极直流环网的直流故障特性进行了仿真分析与研究。仿真结果表明金属回线的阻抗及接地电极的位置与阻抗都会影响接地性直流故障的故障电流,但对于双极故障则不会产生任何影响;而直流电抗器的使用,则会对双极故障具有"改善"作用,但随着直流电抗器值的增加,"改善"作用的提高度越来越不明显。

MMC （Modular Muhilevel Converter） model plays a vital role for DC power grid, so a MMC behavior model based on the arm fundamental wave average switching function is proposed. Compared with the existing MMC model using the arm as the basic modeling unit,the proposed model combines unlock state and lock state of MMC as a whole, and the main circuit only includes the controlled voltage source, constant resistor and diodes and other static components,so this model can be completed in the existing simulation software without recoding and the numerical calculation. At the same time, the average switching function of the arm can be obtained directly from the control sys- tem without passing through the modulation system in the proposed model, thus the construction and debugging of the whole model are simplified. Compared with the detailed model through experiment, the accurancy and rapidity of the proposed model are verified. Then the DC fault characteristics of four-terminal metallic return grounding double-pole DC ring network are simulated and analyzed based on the proposed model. The simulative results show that the impedance of the metallic return and the location and impedance of grounding electrode may affect the grounding fault current, while have no influence on the bipolar fault current. The DC reactor has an ＂improvement＂ effect on the bipolar fault,but the ＂improvement＂ effect is less obvious with the increase of DC reactor value.