Made of Fe-based nanocrystalline ribbons, with intermediate saturation flux density, high permeability and low loss, cores can effectively reduce the current error and phasic error of transformers. The saturation flux density (B_s) of nanocrystalline alloys is between ferrite and permalloys. And the lower B_s value, the more beneficial to reduce the instrument safety factor. When there is breakdown in the system, big current passes the current transformer, and cores is easier to be saturated, the secondary current is no longer rise in proportion, thus the secondary instruments is protected. The permeability of nanocrystalline alloys is close to permalloy, and an order of magnitude higher than silicon steel, therefore suitable for being used as transformer cores with high accuracy.