基于换流站不同出线低频暂态能量比值的多端柔直电网线路保护方案

针对限流电抗器安装在换流站出口，基于线路边界元件特性的多端柔直电网线路保护难以适用的问题，提出了基于换流站不同出线低频暂态能量比值的多端柔直电网线路保护方案。首先，通过分析故障后模块化多电平换流器(modular multilevel converter, MMC)在直流侧呈现的阻抗频率特性，推导出实际频率大于谐振频率时MMC等效阻抗呈感性。然后，通过分析母线处电压行波折射系数的幅频特性可知，折射过程会对故障电压行波中的低频分量具有较明显的衰减作用，并以此为依据分析区内外故障时线路两侧换流站不同出线低频暂态能量比值的差异，可利用此差异识别故障。最后，PSCAD/EMTDC的仿真结果表明，所提保护方案能够可靠识别故障，不依赖线路边界元件，且具有一定的耐过渡电阻能力。

Protection scheme for a multi-terminal flexible DC grid line based on low frequency transient energy ratios of different outgoing lines at the converter station

The multi-terminal flexible DC grid line protection based on the characteristics of line boundary elements is hard to apply to the power grid when a current limiting reactor is installed at the outlet of the converter station. Thus a multi-terminal flexible DC grid line protection scheme based on low frequency transient energy ratios of the different outgoing lines of the converter station is proposed. First, through analyzing the impedance frequency characteristics of an MMC on the DC side after a fault, it is concluded that the MMC exhibits an inductive characteristic when the actual frequency is greater than the resonant frequency. Then through analyzing the variation rule of the voltage traveling wave refractive index at the bus with frequency is analyzed, we conclude that the low-frequency components with frequencies greater than the resonant frequency have obvious attenuation characteristics after being refracted by the bus. Then the difference in the low-frequency transient energy ratio of different outgoing lines of the converter station during forward and reverse faults is analyzed based on this conclusion. The fault direction can be determined via this difference. Finally, simulation results of PSCAD/EMTDC show that the proposed protection scheme can reliably identify fault directions without relying on line boundary elements, and it has a certain ability to withstand transition resistance.