Abstract: Current transformer saturation caused by in-area fault rejection, out-of-area fault misoperation in the operation of the power grid is present, the occurrence of such faults on the grid and the impact on users is also relatively serious, especially 66 kV and below the voltage level current transformer. Due to manufacturing, cost and other reasons, TA saturation phenomenon is more common than 110 kV and above voltage levels. This case are in-zone fault rejection and out-of-zone fault false operation caused by a 110 kV user with 35 kV switchgear fault and two sets of 35 kV current transformers saturated in succession , so that half of the plant shut down until the restoration of normal power supply.