Abstract: With the complexity of the user circuit and the increasing of the load, the capacity of some current transformers can not meet the requirements of normal operation under the influence of short-circuit current, the saturation of Ta is the saturation of magnetic flux in the TA core. Because the magnetic flux density is proportional to the induced potential, the induced potential of the secondary circuit is large if the impedance of the secondary load of Ta is large, or in the same load impedance, the greater the secondary current, the greater the induced potential, both of which will make the magnetic flux density in the core, the magnetic flux density to a certain value, TA saturation. When TA is seriously saturated, the primary current becomes the excitation current, the secondary induced current is zero, the current flowing through the current relay is zero, the protection device will refuse to move. In this paper, the effect of TA saturation on line protection is further understood by analyzing a case of main transformer skip trip caused by TA saturation.