电流差动保护在柔直接入的交流电网中适应性分析及改进措施研究

为适应新能源规模化接入电网，柔性直流输电技术应用日益广泛，我国电网已初步成为交直流混联电网。交流系统发生故障时，柔直系统提供短路电流的特征与传统同步发电机相比发生显著变化。首先分析故障特征的变化，柔直侧由于故障电流幅值受限，且负序阻抗无穷大，导致故障电流的幅值和相角与交流侧不同。分析了输电线路电流差动保护由于区内故障时两侧电流夹角增大，导致电流差动保护的动作量减少、制动量增加，导致灵敏度下降甚至拒动。在此基础上，提出了差动保护判据的改进方案：分别利用零序电流、负序电流和正序电流突变量构成辅助判据，提高区内接地故障和相间短路时电流差动保护的灵敏度，从而提高了差动保护在含柔直接入交流电网中动作的可靠性。最后，通过仿真验证了所提方案的可行性。

Adaptability analysis of current differential protection in an AC power grid withan MMC-HVDC and improvement measures

To adapt to the large-scale access of new energy to the power grid, an MMC-HVDC is increasingly widely used, and China''s power grid has initially become an AC-DC hybrid power grid. When the AC system fails, the characteristics of short-circuit current provided by an MMC system change significantly compared with a traditional synchronous generator. First, this paper analyzes the change of fault characteristics. Because of the limited amplitude of fault current and infinite negative sequence impedance on the MMC side, the amplitude and phase of fault current are different from those on the AC side. The paper finds that when there is an increase of the current angle on both sides in the case of a fault in the transmission line, the differential current will be reduced and the braking current will be increased. Also the sensitivity of protection will be reduced or even the protection will not be activiated. This paper proposes an improved scheme for the differential protection criterion: the zero-sequence current, the negative-sequence current and the positive-sequence current mutation are respectively used to form auxiliary criteria to improve the sensitivity of the current differential protection during ground faults and phase-to-phase short-circuits. This improves the reliability of the differential protection in the AC power grid with an MMC system. Finally, the feasibility of the proposed scheme is verified by simulation.This work is supported by the Key Project of National Natural Science Foundation of China (No. 51637005).