Abstract: To the question that the output distortion of zero sequence current transformer makes the relay protection equipments inaccurate, the zero sequence current transformer which is jamproof and can measure accurately under the complex electromagnetic environment. The electromagnetic in iron core is balanced by burning the main winding and balance winding. Use the silicon steel sheets as shielding layer of the transformer to shield the out electromagnetic interference. Then the problem that the output distortion of zero sequence current transformer makes the relay protection equipments inaccurate caused by the different positions of the primary conductor in the core of the zero sequence current transformer and the closed electromagnetic field Source is solved.