中文摘要：

介绍了青藏直流联网工程试运行期间发生的两次直流闭锁故障,分析了事故原因,并说明了相应的直流谐波保护修改措施。在此基础上介绍了换流器直流侧低次非特征谐波产生的机理以及直流谐波保护的逻辑原理。通过PSCAD/EMTDC软件对各类可能在直流线路中产生低次谐波的故障进行仿真,明确了各类故障在直流线路上产生谐波的次数以及含有率。最后提出：在原有谐波保护改进的基础上,令青藏直流新增的150Hz谐波保护,作为阀短路保护的后备保护。仿真证明此措施提高了工程的稳定性,也拓展了150Hz保护这一新型保护的适用范围。

英文摘要：

In this paper,two blockings failures of QinghaiTibet HVDC project during its pilot operation are introduced,their causes are analyzed,and improved measures of relative DC harmonic protection are discussed.Then the causing mechanism of low-order harmonic in DC side and the logic mechanism of DC harmonic protection are introduced.The simulations for all kinds of low-order harmonic faults in DC transmission lines are carried out by using PSCAD/EMTDC software,and the harmonics frequency and magnitude of all faults have been determined.In the end,based on the improvement of Qinghai-Tibet HVDC project,a kind of new 150 Hz harmonic protection is applied as the backup protection of valve short circuit protection.Simulations show that proposed measure can improve the stability of the project,which widens the application area of this new 150 Hz protection.