中文摘要：

在分析直流线路两端特定频率电流波形特征的基础上,提出了一种新的直流输电线路纵联保护方案。对直流滤波器进行阻抗–频率特性分析,发现滤波器在特定频率点阻抗值接近于零,即滤波器对该频率电流具有良好的滤波效果,正常运行时直流线路两端特定频率电流几乎为零。故障时,由于系统阻抗特性改变,线路两端特定频率电流显著增加。通过对直流线路谐波等值网络进行分析,发现区内故障时,线路两端特定频率电流都由直流母线流向线路;整流侧区外故障时,直流线路整流端的特定频率电流由直流母线流向线路,而逆变端则由线路流向直流母线;逆变侧区外故障时,与整流侧区外故障情形相反。特定频率电流方向一致时波形匹配程度高,而当方向相反时,波形匹配程度低,利用该特征构成直流线路区内、外故障判据。针对现行直流线路电流差动保护的缺陷,提出了一种改进的直流线路后备保护方案。大量仿真结果表明,该保护方案原理简单,能可靠、准确识别直流线路区内、外故障,且具有较高的过渡电阻能力。

英文摘要：

In this paper a novel pilot protection principle for HVDC transmission lines is proposed based on waveform characteristics analysis of specific current at both terminals of DC line. The impedance value of DC-filter at specific frequency is found nearly zero with spectrum analysis. Namely, there is no specific frequency current detected by DC shunts during normal operation, while the current will increase significantly when fault occurs due to system impedance change. Equivalent network for DC system at specific frequency is analyzed and it is found that when internal faults occur, the direction of specific current detected at DC line ends is from DC bus to DC line, while the direction of specific current detected at rectifier side is from DC line to DC bus and direction of specific current detected at inverter side is from DC bus to DC line during external faults at rectifier side. The situation of external fault at inverter side is opposite to that of faults at inverter side. The direction difference of specific current at DC line ends leads to significant difference in waveform matching, which can be used to identify the fault location （external or internal）. An improved backup protection scheme for HVDC transmission line is also proposed to overcome the shortage of the existing current differential protection. Simulation results show that the proposed protection scheme can identify internal and external faults reliably, and owns ability to respond to faults with high transition resistance.