基于虚拟阻抗的多落点混合级联直流系统故障电流抑制方法

多落点混合级联直流系统存在特有的模块化多电平换流器(MMC)功率盈余问题。当受端交流系统发生短路故障时，MMC过流、过压将引起MMC阀组闭锁，进一步可能导致系统功率中断。多落点混合级联直流系统整流侧采用电网换相型换流器(LCC)、逆变侧采用LCC与多台MMC级联。针对该系统提出一种适用于受端交流系统故障的故障电流限制方法，在逆变侧MMC控制中引入虚拟阻抗降低故障电流，无需额外添加设备。对虚拟阻抗的控制引入、计算以及投入实现过程进行了详细阐述，并在PSCAD/EMTDC中搭建模型进行仿真分析。结果表明，所设计的虚拟阻抗控制器可以实现故障电流的有效抑制，并防止功率倒送，从而实现混合级联直流系统的交流故障成功穿越和功率可靠传输。

Fault current suppression method of multi-point hybrid cascaded DC system based on virtual impedance

There is a unique power surplus problem of modular multilevel converter(MMC) in multi-point hybrid cascaded DC system. When a short circuit fault occurs in the receiving end AC system, the MMC overcurrent and overvoltage will cause the MMC valve blocked, which may further cause system power interrupted. The rectifier-side of multi-point hybrid cascaded DC system adopts the line commutated converter(LCC),and the inverter-side adopts the cascade connection of LCC and multiple MMCs. A fault current suppression method suitable for fail occurred at receiving end AC system is proposed. The fault current is reduced by introducing a virtual impedance in the MMC control at inverter side without additional equipment. The control introduction, calculation and input realization process of virtual impedance are elaborated in detail, and a model is built in PSCAD/EMTDC for simulation analysis. The results show that the designed virtual impedance controller can realize the fault current suppression effectively and prevent power reversal, so as to achieve the successful ride-through of AC faults and reliable power transmission of hybrid cascaded DC system.