同塔双回线纵向故障特征分析及零序方向保护改进

同塔双回线在发生不对称纵向故障时,由于负荷电流的转移,健全线路上会流过较大的不平衡电流,可能导致健全线路上带零序电压补偿的纵联零序方向元件误动。分析了同塔双回线纵向故障时零序、负序分量的特征,阐释了单相故障跳闸后健全线路上带零序电压补偿的零序方向保护误动的机理,并且提出了利用单回线负序分量信息的保护改进方法。在零序电流达到门槛值而零序电压不足时,引入负序电流门槛来判断是否启动零序电压补偿,并结合带负序电压补偿的负序方向元件来判断故障方向。仿真结果表明,所提方法在相邻线路发生不对称纵向故障时能有效闭锁本线路的方向保护,防止保护误动作;在本线路靠近对侧发生接地故障,本侧零序电压较低的情况下改进的零序方向保护有较高的灵敏度,能够可靠动作。

Characteristic analysis on longitudinal fault of dual-circuit transmission line and improvement of zero-sequence directional protection

When asymmetric longitudinal fault occurs in a dual-circuit transmission line, the shift of load current may produce large unbalance current in the healthy line, leading to the improper operation of pilot zero-sequence directional element with voltage compensation. The characteristics of zero-and negative-sequence components are analyzed for the longitudinal fault of dual-circuit transmission line, the improper operation of the pilot zero-sequence directional protection with voltage compensation of the healthy lines after the trip against single-phase grounding fault is explained, and an improved protection method based on the single-line negative-sequence component information is proposed. When the zero-sequence current reaches the threshold and the zero-sequence voltage is lower, the negative-sequence current threshold will be used as the startup criterion of zero-sequence voltage compensation and the negative-sequence directional element with voltage compensation is integrated to judge the fault direction. Simulative results show that, the proposed method can effectively block the directional protection of own line to avoid its improper operation when the adjacent line has an asymmetric longitudinal fault; the improved zero-sequence directional protection has higher sensitivity to ensure the reliable operation when a grounding fault occurs near the opposite end of own line and the zero-sequence voltage of own line is lower.