基于等效序阻抗差异特征的低频输电线路差动保护优化方案

低频输电线路故障时，两侧短路电流均由模块化多电平矩阵变换器提供，两侧短路电流相角均受控且幅值相当，导致差动保护灵敏度严重下降。分析了低频输电系统定电压控制侧和功率控制侧的等效正、负序阻抗特征，指出因控制策略不同，系统故障时两侧等效正、负序阻抗特征存在明显差异。分析了低频输电线路区内和区外故障两侧保护装置测量到的等效序阻抗之间的差异性和相似性，基于此特征构建了两侧等效序阻抗差异指标，提出了基于等效序阻抗差异指标的差动保护制动系数优化方法。仿真结果表明，所提方案能够显著提升低频输电线路区内故障时差动保护的动作速度和动作灵敏性。

Optimization scheme for differential protection of low frequency transmission line based on equivalent sequence impedance difference characteristics

During the fault of low frequency transmission line(LFTL),the short circuit currents on both sides of the line are provided by a modular multilevel matrix converter. The phase angle of the short circuit currents on both sides is controlled and the amplitude is equivalent, resulting in a serious decrease in the sensitivity of differential protection. The characteristics of equivalent positive-and negative-sequence impedances on the voltage control side and power control side of low frequency transmission system are analyzed. It points out that due to different control strategies, there are significant differences in the characteristics of equivalent positive-and negative-sequence impedances on both sides of the system during faults. Then, the difference and similarity between the equivalent sequence impedances measured by the protection relays on both sides of the LFTL under internal and external faults are analyzed. Based on these features, an indicator for the difference of equivalent sequence impedances on both sides is constructed. A method for optimizing the braking coefficient of differential protection using the indicator is proposed. The simulative results show that the proposed scheme can significantly improve the operation speed and sensitivity of differential protection in case of faults in the LFTL area.