目前高压直流输电线路利用行波波头的上升速率构成保护判据，由于长距离线路频率相关特性对高频信号的衰减作用较强，导致保护的灵敏性不足。利用零极点法分析线路故障暂态信号的频率特性，发现故障电气量的高频分量中包含位置主频和边界主频。高频暂态量由边界主频和位置主频共同作用产生。区内、外故障的特征差异取决于反射系数频率特性的通带和透射系数频率特性的阻带，因此特征频带应为透射系数阻带的截止频率与反射系数通带的最高频率间的频带。由PSCAD仿真结果证明，区内、外故障时电流在此特征频带内差异明显。%Currently, most of the transient protections for high voltage direct current （HVDC） transmission lines utilize the rising rate of traveling wave to constitute the protection criterion. This approach lacks sensitivity information because of the significant attenuation to the high-frequency signal due to the frequency-dependent characteristics of long lines. This paper employs pole-zero method to analyze the frequency characteristics of the transient signal for faulty HVDC transmission line. It is discovered that the transient signals are consist of position main frequency components, which is generated by fault position, as well as boundary frequency components, which is generated by the boundary filters. The different characteristics between internal and external faults are mainly determined by the pass frequency band of the reflection coefficient and the choke frequency band of the refraction coefficient. Therefore, the feature band for distinguish internal and external fault is from the cutoff frequency of the stop band to the highest frequency of the pass band. Due to the zeros of the voltage in the feature band, the fault information in voltage will be counteracted. However, the differences of the current in the feature band are significant for there are no zero points in feature band. The simulation results by power system computer aided design （PSCAD） associated with the typical HVDC transmission system show that the differences of the current between internal and external faults are quite obvious.
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