基于超高压自耦变压器中性点零序电流的方向比较 纵联保护的研究

随着电力系统向超高压方向发展,自耦变压器容量越来越大,而其零序阻抗越来越小,导致保护上虽然零序电流达到定值。但零序电压很小,不满足灵敏度要求,零序方向元件误动和拒动,从而零序方向纵联保护无法投入。但超高压自耦变压器中性点所产生的数值很大的零序电流的作用一直被忽略,没有被有效利用。据此提出利用自耦变压器中性点零序电流来代替保护上两侧零序电压的基于自耦变压器中性点零序电流的方向比较纵联保护方案。该方案以自耦变压器中性点零序电流为参考量,与两侧保护处的零序电流进行比相,以两者相位差作为方向判据,判断自耦变压器两侧故障方向,进而根据纵联方向比较原理比较两侧故障方向来易区分内部故障和外部故障。该方案能够消除保护处零序电压对零序方向元件方向判据的不利影响,有效地解决了目前保护处零序电压太小而引起的传统零序方向纵联保护拒动和误动的难题。经动模实验数据和数字仿真验证了该方案对接地故障具有灵敏度高,选择性好,能够正确地反应变压器各种轻微匝间故障,判据不受故障侧电流互感器(CT)饱和影响等优点。

A novel scheme for UHV auto-transformer protection based on zero-sequence current longitudinal comparison principle

With the development of ultra-high voltage (UHV) power system, the capacity of auto-transformer gets lager and its zero-sequence impedance gets smaller accordingly. Therefore, the zero-sequence directional relay may have poor sensitivity due to the small zero-sequence voltage, although the zero-sequence current threshold is reached. The large value of zero-sequence current of the neutral point in auto-transformer is not utilized effectively. In view of this problem, the relationship of zero-sequence current phases between the neutral points on high voltage side and medium voltage side is analyzed, and a new auto-transformer pilot protection scheme based on the zero-sequence current comparison is proposed. With the zero-sequence current of neutral point as reference, the fault direction can be determined by comparing the phase shift between zero sequence current of high voltage side, medium voltage side and the reference current. So the internal fault and external fault can be discriminated. The proposed scheme eliminates the adverse effects yielded by zero-sequence voltage effectively. The results of dynamic simulation experiment and digital simulation show that the proposed directional relay can detect turn-to-ground and turn-to-turn fault reliably and sensitively even with the serious saturation of current transformer.