变压器外部故障切除后差动保护误动原因及防止对策

针对近年来相继出现的变压器差动保护在外部故障切除后发生误动的现象，研究了电流互感器(current transformer，CT)饱和以及Y/D接线变压器补偿方式对差动保护的影响。通过理论分析与仿真研究发现，变压器外部故障切除后CT发生局部暂态饱和时，由于只传变工频周期分量，误差很小，不足以引起差动保护误动。如果变压器一侧CT发生超饱和，而另一侧CT能正确传变时，由于两侧CT传变特性不一致可能引起差动保护误动；另外变压器Y侧电流的相位补偿也容易引起差动保护误动。通过对保护误动时的动作轨迹的分析，提出利用分区延时法以防止差动保护误动。通过大量的仿真实验，证明该方法既能保证差动保护在内部故障时的速动性，也能保证在外部故障切除后不误动。

Mal-operation of Transformer Differential Protection After Removal of External Fault and Countermeasure

Mal-operation of the transformer differential protection will occasionally take place after the removal of the external fault. The physical phenomena of the current transformer (CT) saturation and the phase compensation scheme of the transformer with wye-delta connection are studied in this paper. Based on the theoretical analysis and simulation research, this paper points out that CT partial transient saturation after the removal of transformer external fault only transforms the fundamental frequency component with a small transformation error, and won’t result in the mal-operation of differential protection. If CT on one side of the transformer becomes ultra-saturated and CT on the other side is in the linear region, the inconsistent transformation of CT will result in the mal-operation of differential protection. In addition, the phase compensation of the transformer wye-side current will also result in the mal-operation of differential protection. Through the analysis of the mal-operation point, a method of different time delays in different areas is presented to prevent the differential protection from mal-operation. Simulation results show that the new method will guarantee the fast operation of the differential protection in the case of an internal fault while avoiding the mal-operation after removal of the external faults.