本文分析了电力系统输电线路二次交流电压回路在中线断线后导致零序方向不正常,在发生接地故障时很可能导致零序方向保护不正确动作,分析了现场中线断线后的典型特征是在UA、UB和UC电压回路中均会出现较大的三次谐波,提出了线路保护装置利用三次谐波检测TV二次回路中线断线的方法措施。实践证明,该方法有效的避免了线路零序保护的不正确动作。 This paper analyzes that the secondary AC voltage loop of transmission line in power system leads to abnormal zero-sequence after the middle line is disconnected, which may lead to incorrect protection of zero-sequence direction in the event of a ground fault. The typical features after on-line neutral break It is the third harmonic that appears in the UA, UB and UC voltage loops. The method of detecting the middle line breakage of TV secondary circuit by the third harmonic detector is proposed. Practice has proved that this method effectively avoids the line zero sequence protection of the incorrect action.