三相串联 MMC 直流换流站在交流故障下的控制策略研究

摘要

Three-phase series-connected MMC (SC-MMC) is constructed by connecting three single-phase MMC in series. Compared with conventional three-phase parallel-connected MMC, the number of semiconductor switches is reduced by one third with the assumption to withstand the same DC voltage. However, the operation of SC-MMC under unbalanced grid conditions remains a key issue. This paper derives the AC power balance equations based on stationaryα-β frame, which equalizes the AC power of each phase to balance the DC-side voltages and enables to adjust the transmitted power according to the severity of AC faults. In addition, this paper presents the method based on stationaryα-β frame to protect SC-MMC from over-current. Besides, the overall control scheme based on the proportional resonant controller is proposed. Finally, the SC-MMC model is established and the simulation of single-line and double-line to ground faults are both performed in PSCAD/EMTDC. The simulation result proves that SC-MMC has very good AC fault ride-through ability with the proposed control strategy.%三相串联 MMC 将三个单相 MMC 串联，在承受相同直流电压的情况下，相比传统的三相并联 MMC，可以减少三分之一的开关器件，造价更低，但三相串联 MMC 在不平衡交流电压下的控制保护是一个难题。推导了静止α-β坐标系下的交流侧有功功率平衡方程，通过平衡各单相 MMC 交流侧的有功功率实现交流故障下的直流侧均压，该策略可以根据故障的严重程度自动调整换流器交流故障下的功率输送。此外，提出了静止α-β坐标系下防止换流器过电流的参考电流整定方法。构建了基于比例谐振控制器的总体控制框图。最后，在 PSCAD/EMTDC中搭建了三相串联 MMC 换流站仿真模型，对交流电网单相和两相接地故障进行了仿真验证。仿真结果表明：三相串联 MMC 换流站在所提的控制策略下具有良好的故障穿越能力。