电网不平衡工况下模块化多电平矩阵变换器控制策略

模块化多电平矩阵变换器(modular multilevel matrix converter,M3C)是一种可直接实现交交功率变换的新型高压大功率拓扑，在低频输电、大功率异步电机调速及低频海上风电送出等领域具有应用前景。由于2种频率的功率耦合作用，M3C桥臂电容电压在电网电压不对称时容易失稳。为此，文中首先对不平衡输入工况下M3C桥臂功率进行了计算，推导并总结了2种不同功率平衡方法下桥臂间的功率分配规律。在此基础上，研究低频环流对桥臂功率的影响，在保证系统总有功功率平衡的前提下，提出基于低频环流的M3C桥臂电容电压平衡控制策略，避免了网侧负序电流的引入；在不平衡工况下，通过桥臂电容电压闭环控制和功率直接补偿实现电容电压的快速平衡。所构造的低频环流仅在换流器内部流动，不影响M3C输入输出侧的解耦运行。最后在MATLAB中搭建了220 kV/400 MW M3C系统模型，验证了所提控制策略的有效性。

Control strategy of the modular multilevel matrix converter under unbalanced grid condition

Modular multilevel matrix converter (M3C) is a new topology of AC-AC power converter,which has very broad application in low frequency transmission system,high-power asynchronous motor speed regulation and low offshore wind power transmission. Due to the power coupling effect of the two frequencies,the capacitive voltage of M3C bridge arms is prone to instability when the grid voltage is asymmetric. Firstly,the arm power of M3C under unbalanced input conditions is calculated,and the power distribution law among the arms of two different power balance method is summarized. On this basis,the influence of low-frequency circulating currents on the arm power is analyzed. Under the premise of ensuring the balance of input and output power of the system,a capacitor voltage balance control strategy based on the construction of circulating currents is proposed,which avoids the introduction of negative sequence current on the grid side. Under unbalanced conditions,the capacitor voltage can be quickly balanced by the capacitor voltage closed-loop control and direct power compensation strategy. The constructed low-frequency circulating currents only flow inside the converter,and will not affect the decoupling operation of M3C input and output sides. The effectiveness of the proposed control strategy is verified by a 220 kV,400 MW M3C system implemented in MATLAB.