含混合型MMC的四端柔性直流电网改进自适应重合闸策略

柔性直流电网是进行大规模新能源远距离架空线传输的重要技术手段。但架空线比直流电缆更容易发生瞬时性短路故障，待故障消失后需迅速重合闸。若直流电网发生永久性故障，常规基于换流器主动信号注入的故障性质判别方法易对直流电网的稳定运行产生较大干扰。针对上述问题，提出一种基于混合型模块化多电平换流器（MMC）主动信号注入的柔性直流电网改进型自适应重合闸方法，在混合型MMC极控制器中附加主动信号控制，使柔性直流电网在不中断功率传输的前提下实现故障性质辨识。此方法具备较强的耐过渡电阻能力，且不影响柔性直流电网功率传输的稳定性。在PSCAD/EMTDC搭建了混合型MMC四端柔性直流电网的电磁暂态模型，通过仿真验证了该方法在配备混合型MMC和机械式直流断路器的柔性直流电网中的有效性。

An Improved Adaptive Reclosing Strategy of Four-terminal Flexible DC Grid with Hybrid Modular Multilevel Converter

The flexible DC grid is an important technical measure for large-scale long-distance overhead line transmission of renewable energy. However, overhead lines are more prone to instantaneous short-circuit faults than DC cables, and they need to be reclosed quickly after the fault is cleaned. If a permanent fault occurs in the DC grid, the conventional fault identification method based on the active signal injection of the converter is likely to cause great interference to the stable operation of the DC grid. To solve these problems, an improved adaptive reclosing strategy with active signal injection of the hybrid modular multilevel converter (MMC) for flexible DC grids is presented. Active signal control is added to the hybrid MMC pole controller, so that the flexible DC grid can realize the identification of the fault properties without interrupting the power transmission. This method has strong resistance to transition resistance and does not affect the power transmission stability of the flexible DC grid. The electromagnetic transient model of the four-terminal flexible DC grid with hybrid MMC is built in PSCAD/EMTDC. The simulations verify the effectiveness of the method in the flexible DC grid equipped with hybrid MMC and mechanical DC circuit breakers.