中文摘要：

模块化多电平换流器（MMC）桥臂环流中的谐波直接影响MMC的工作特性和损耗。文中推导了MMC交流侧电流、桥臂电流和环流之间的内在联系,指出MMC在交流电网对称和不对称情况下桥臂环流中都含有二倍频谐波。为了对MMC谐波环流进行有效抑制,提出了一种基于比例-积分-谐振（PIR）控制的MMC新型环流控制器,无需二倍频负序坐标变换和相间解耦。最后,在PSCAD/EMTDC中搭建了包含该PIR环流控制器的双站MMC仿真模型,并与旋转坐标系下的环流控制器在交流电网对称和不对称情况下进行了对比仿真分析,仿真结果验证了所提出的PIR环流控制器的有效性。

英文摘要：

The system losses and working performance of the modular multilevel converter（MMC）are directly affected by the harmonics in the circulating current.The inherent relation between the AC-side current, the leg current and the circulating current is first directly derived, which reveals that there always exists double-frequency harmonics whether under balanced or unbalanced grid conditions.Then, a novel circulating controller based on proportional-integral-resonant（PIR）control is proposed for effectively suppressing the harmonics in the circulating current, without the double-frequency negative-sequence transformation and the phase decoupling control.A MMC-HVDC simulation of a two-station model containing the PIR is created based on PSCAD/EMTDC.Simulation results have verified the availability of the control strategy when compared to the results from the circulating controller under double fundamental-line frequency negative sequence rotational frame.