柔直送出下大规模光伏送端电网的故障特征受双端电力电子设备的控制策略和系统拓扑形式影响显著，传统继电保护的适应性受到挑战。综合考虑换流器的故障控制策略、送端电网的拓扑、变压器接线方式和故障位置，从设备建模和系统建模两个方面建立了光伏送端电网交流送出线路的故障等值模型。通过分析系统的复合序网，提取了不对称故障下线路的差流特征和故障距离阻抗特征。基于柔直侧的故障特征，提出了光伏送端电网的不对称故障距离保护方案：对于接地短路故障，利用序分量特征计算故障距离阻抗，在原理上不受过渡电阻影响；对于不接地短路故障，基于阻抗复平面法利用故障相差流特征计算故障距离。PSCAD/EMTDC 仿真结果表明，该方案仅利用了柔直侧的单端量信息，能够准确计算故障距离阻抗，可靠地区分区内外故障，提高了双端弱馈场景中距离保护的耐受过渡电阻能力。

The fault characteristics of large-scale photovoltaic （PV） sending-end grid under VSC-HVDC are significantly affected by the control strategy and system topology form of double-end power electronic equipment, and the adaptability of traditional relay protection is challenged. Considering the fault control strategy of the converter, the topology of the sending-end grid, the transformer wiring mode and the fault location, a fault-equivalent model of the AC transmission line of the PV sending-end grid in terms of both equipment modeling and system modeling is established. By analyzing the composite sequential network of the system, the differential current characteristics of asymmetrical faults and fault distance impedance characteristics are extracted. Based on the fault characteristics of the VSC-HVDC side, asymmetric fault distance protection schemes for PV sending-end grid are proposed. For the distance protection scheme of grounded short-circuit fault, the fault distance impedance is calculated by using the sequential component characteristics, which is not affected by the transition resistance in principle. For the distance protection scheme of the ungrounded short-circuit fault, the fault distance is calculated based on the impedance complex-plane method by utilizing the fault-phase differential-current characteristics. Based on the simulation results of PSCAD/EMTDC, the proposed schemes can accurately calculate the fault distance impedance by utilizing only the single-ended quantity information of the VSC-HVDC side, reliably distinguish between internal and external faults, and improve the ability of the distance protection to tolerate the transition resistance in the double-ended weak-feeding scenario.