基于行波固有频率一、二次频差的HVDC输电线路纵联保护方法

A HVDC Transmission Line Pilot Protection Method Based on Frequency Difference Between Dominant Natural Frequency and Secondary Natural Frequency of Traveling Wave

行波固有频率与输电线路的故障距离和边界条件有关,直流输电线路两端边界条件相同,仅利用直流线路两端固有频率主频差的纵联保护在线路中点存在保护盲点。提出一种基于行波固有频率一、二次频差的高压直流输电线路纵联保护方法,利用线路两端固有频率主频差构造主判据,利用一、二次频差构成辅助判据。通过对主判据、辅助判据门槛值的整定,解决了固有频率主频差保护在直流线路盲点发生故障时拒动的问题,增强了保护的可靠性。同时,利用故障后故障极电流变化程度较大的特点,定义了两极电流突变量积分绝对值的比值,构建了区内故障选极判据。PSCAD/EMTDC仿真结果表明,该保护方法适用于任意直流线路边界条件,仅利用线路两端电流信息即可准确、快速地识别区内外故障和故障极。

Natural frequencies of traveling waves are related to fault distance and boundary conditions. The boundary conditions of DC transmission line are the same on both ends. So the pilot protection method only based on dominant natural frequency difference between the two ends of transmission line has a problem that a blind zone exists at the middle of the line. In this paper, a HVDC transmission line pilot protection method based on frequency difference between dominant natural frequency and secondary natural frequency of traveling wave is proposed. The method uses the dominant natural frequency difference to build the main protection criterion and uses the frequency difference between dominant and second natural to build the auxiliary criterion. Through setting the threshold values of the main and auxiliary criteria, refusal-operation is avoided when faults occurred in blind zone of protection based on dominate natural frequency, and the reliability of protection is enhanced. Meanwhile, fault current feature that current measured at fault polar changes more dramatically when fault occurred is used to form the polar selecting criterion by defining the ratio of integral absolute value of fault current component. PSCAD/EMTDC simulation indicates that the protection method is valid for all DC line boundary conditions, in which only current information on both terminals of the line is utilized to discriminate between internal and external faults, and identify the fault line accurately and quickly.