In view of the event that an interphase fault of the 10 kV line leads to the malfunction of the 110 kV main transformer′s differential protection

the paper analyzes the reasons of the main transformer′s differential speed protection malfunction when the fault is outside the protected area and the current transformers on both sides are in transient saturation. And the solution measures of prevent CT transient saturation and the saturation criterion of the main transformer protection device are put forward.