MMC接入的交流侧线路故障特征及故障分量方向元件的动作特性

高压大容量柔性直流系统经模块化多电平换流器（MMC）接入交流电网时，MMC的控制策略将导致其故障出力有别于传统同步发电机。基于MMC的控制策略和柔性直流系统的接地方式，研究了MMC接入的交流侧线路发生故障后的电压、电流相量特征，并分析了其对故障分量方向元件的影响。理论分析结果表明，在交流侧线路发生故障后，公共耦合点（PCC）电压相位可能发生较大变化，进而影响MMC输出正、负序电流的实际相位；零序方向元件仅适用于联接变压器采用Y0/d接线的场景，取决于柔性直流系统的接地方式；正序突变量、负序方向元件计算的阻抗角受电网故障穿越需求所决定的MMC控制系统d、q轴电流参考值的影响显著，可能导致MMC侧方向元件在正向线路故障时和电网侧方向元件在背侧系统故障时的误判。理论分析结果与典型工程PSCAD模型的仿真相吻合。

Fault characteristics of AC line connected to MMC and operating characteristics of fault component based directional relayings

When high-voltage and large-capacity flexible direct current(DC) system is connected to alterna-ting current(AC) grid via modular multilevel converter(MMC),the control strategy of MMC leads to diffe-rent fault characteristics from conventional synchronous generator. Based on the control strategy of MMC and the grounding method of flexible DC system, the characteristics of voltage and current phasors under faults occurring at AC line connected to MMC are investigated, and the impact on fault component based directional relaying is analyzed. The phase angle of point of common coupling(PCC) voltage may change significantly after AC fault and have an impact on the actual phase angle of positive-and negative-sequence currents output from MMC. Zero-sequence directional relaying is only available when the winding connection of converter transformer is Y0/d, which depends on the grounding method of flexible DC system. The impedance angle calculated by positive-sequence fault component based directional relaying and negative-sequence directional relaying are affected by the d and q axis current reference values of MMC control system decided by grid fault ride through requirements, which may lead to the misjudgment of directional relayings at MMC side when forward fault occurs and the misjudgment of directional relayings at grid side when a backward fault occurs. Theoretical analysis is verified by PSCAD simulation based on ty-pical MMC-HVDC project.