中文摘要：

模块化多电平换流器(MMC)在中高压直流输电中得到了广泛的研究和应用。三相不平衡交流电网下的功率波动引起MMC三相桥臂能量不均衡,导致内部环流的产生。针对三相不平衡直流输电系统,对MMC内部环流的产生机理进行了理论推导和分析,并根据内部环流的特点,将比例—积分控制与重复控制相结合,提出了一种基于正、负序双同步旋转坐标系下复合控制的环流抑制方法。该方法同时适用于三相平衡和三相不平衡系统。同时,为了提高三相不平衡条件下直流输电系统的波形质量,引入了基于dq解耦控制的负序电流补偿方法。仿真结果证明,无论交流系统侧三相是否平衡,该控制策略对环流都具有良好的抑制效果,同时能够确保系统交流侧的三相电流保持平衡。

英文摘要：

The modular multilevel converter (MMC) has been widely researched and applied in high-voltage direct-current (HVDC) transmission.MMC three-phase bridge arm energy imbalance is caused by power fluctuations in the three-phase unbalanced alternating current(AC) grid,which generates internal circulating current.In light of the three-phase unbalanced DC transmission system,the internal circulating current mechanism of MMC is derived and analyzed.According to the characteristics of internal circulating current,a compound control method combining proportional-integral (PI) control and repetitive control is proposed for circulating current suppression based on positive and negative sequence in a double synchronous rotating coordinate system.This method is suitable for both three-phase balanced and three-phase unbalanced systems.Meanwhile,in order to improve the waveform quality of the direct current(DC)transmission system under the three-phase unbalanced condition,the negative sequence current compensation method based on dq decoupling control is introduced. The simulation results have verified that no matter whether the AC system three phases are balanced or not,the control strategy always has good performance in circulating current suppression,while ensuring the balance of the system”s three-phase AC. This work is supported by the State Key Program of National Natural Science Foundation of China(No.5 1237002).