Analysis of traction transformer differential protection misoperation and its solution

The analysis shows that the transient saturation of CT(Current Transformer) causes the misoperation of a traction transformer differential protection,which is verified by the CT test.There are different methods to reduce and avoid the influence of CT transient saturation.For external fault,asynchronism method can recognize CT transient saturation according to the different occurrence times of differential current variation and restraint current variation.But it is failed in this case because of the external connection conditions of CT.It is proposed to use the sums of 3-phase differential current variations and restraint current variations to recognize CT transient saturation and then block the differential protection.This criterion is validated by simulation.