模块化多电平换流器分极控制策略

现有模块化多电平换流器(MMC)主流控制为基于dq旋转坐标系的直接电流控制方式,该控制将MMC的上、下桥臂施行对称统一的控制,导致换流器直流侧必须严格对称运行,对此文中提出了MMC完整的换流站级分极控制策略,在保证联结变压器二次侧无直流偏置的前提下,有效地改善了MMC直流侧不对称运行时的运行特性。通过对MMC交流侧与直流侧间的功率传递关系的推导,设计了一种基于直接电流控制思路的MMC分极控制策略,该控制策略可以灵活、独立地控制上、下桥臂分别输出的有功功率和无功功率,兼具一定的环流抑制效果;提出电压偏置率定义,通过对控制指令的修正与配合,可以在一定换流器结构下实现联结变压器二次侧无直流偏置的前提下MMC-HVDC系统直流侧的不对称运行,且同时可以明显降低直流侧不对称故障时直流母线的过电压水平;最后基于PSCAD/EMTDC搭建了两端11电平MMC-HVDC系统模型,仿真结果验证了所设计分极控制策略的正确性,以及对于桥臂环流的抑制效果、对于直流侧不对称运行特性改善的有效性。

Separate DC Polar Control for Modular Multilevel Converters

The existing control for modular multilevel converter(MMC)is mainly direct current control based on the rotating dq coordinate system by controlling the upper and lower arms together and symmetrically. This requires strictly symmetrical operation on the DC side of MMC. A separate DC polar control for MMC is proposed to effectively improve the operating characteristics of the DC asymmetric operation. The circuit of MMC is redefined and remolded to get the relation for the power transmitted from AC to DC side. Based on the power relation, the separate DC polar control designed controlling the arm current, active and reactive power of upper and lower arms respectively, with the effect of arm circulation current suppression. And a definition of the voltage offset rate is proposed. With some corrections to the control and MMC configuration, DC asymmetric operation for MMC can be realized by the separate DC polar control, the over voltage level of DC asymmetric fault also can be apparently reduced. Time-domain simulation studies based on PSCAD/EMTDC are performed to verify the proposed control scheme and its effectiveness.