子模块混联MMC-HVDC系统直流侧短路故障电流抑制方法

考虑到目前直流输电系统故障难以快速清除且故障后系统重启的复杂度较高，基于全桥及半桥混联的模块化多电平换流器系统，提出一种直流双极短路故障电流抑制方法。首先根据系统调制度裕量计算得到用于抑制故障电流的抑制电压。然后进行判断，当故障电流大于电流上限值时，控制每相级联电压为负的抑制电压；当故障电流小于电流下限值时，控制每相级联电压为正的抑制电压；当故障电流位于电流上、下限值之间时，控制每相级联电压为零，此时系统等效为两个并联的无功补偿器，可以为电网持续提供无功支撑。该方法能够在系统不闭锁的情况下快速将故障电流抑制在给定的范围内，算法简单、容易实现。最后基于PSCAD/EMTDC搭建了MMC-HVDC双端系统仿真模型，验证了所提出的故障电流抑制方法的正确性和有效性。

A DC pole-to-pole fault current suppression strategy of the half-and full-bridge based cell-hybrid modular multilevel converter

Given the current difficulties to remove faults in DC transmission system and high complexity for the system to restart after faults, a method to suppress the DC pole-to-pole fault current is proposed for half- and full-bridge based cell-hybrid modular multilevel converter (MMC). Firstly, the suppression voltage used to suppress the fault current is computed according to modulation margin of system. Then considering the fault current, if it is larger than the upper limit value, the phase cascaded voltage is controlled as a negative suppression voltage. If the fault current is smaller than the lower limit value, the phase cascaded voltage is controlled as a positive suppression voltage. If the fault current is between them, the phase cascaded voltage is controlled as zero, and the valve is equal to two STATCOMs in parallel which can provide reactive power for the grid. This method is simple to carry out and can limit the fault current in a given period quickly by not blocking the valve. Finally, a two-terminal MMC-HVDC transmission system is constructed in PSCAD/EMTDC, and simulation results show that the method mentioned above is correct.