By analyzing the behavior of a 220 kV transmission line’s pilot direction-protection, it was found that directional element based on variation of power-frequency rejected action when single-phase line-broken earth fault happened on the line. Based on the two-port network theory, the formula of directional element is deduced in case of such fault. The results show that when the system M and N are connected by a single transmission line, the variation directional protection can properly act when such fault happens on the line; when the system M and N are connected by electrical ring, the directional element may be judged as reverse direction fault owing to the different circuit parameters. Finally, the results of PSCAD simulation further validate the principle defect that the directional element may reject action when such fault happens in electrical ring system.%通过分析某220 kV输电线路单相断线接地复故障时线路纵联方向保护的动作行为，发现了变化量方向元件在本次故障时判为反方向而拒绝动作的情况。采用双口网络理论推导了变化量方向元件在线路发生单相断线接地复故障时的计算公式。分析结果表明：当双端系统通过单回线路连接时，线路两侧变化量方向保护均能正确动作；当双端系统存在环并等值线路时，变化量方向元件因环并等值线路参数的不同可能出现判为反方向故障的情况。最后，通过PSCAD仿真分析，进一步验证了变化量方向元件在环并系统线路发生故障时可能拒绝动作的原理性缺陷。
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