Through the description of the generating mechanism of the zero sequence current transformer, this paper points out that the different core saturation of each phase of transformer is the fundamental causes of the zero sequence components. Based on EMTDC/PSCAD, the effect of residual magnetism, closing angle and the neutral grounding mode of transformer on the maximum of fundamental component of magnetizing inrush zero sequence component is analyzed. Results show that when the residual magnetism is maximum, adjusting the closing angle can obtain the maximum of zero sequence current fundamental component. In addition, it is found by simulation that there is disoperation risk in the line zero sequence overcurrent accelerating protection caused by the larger inrush current zero sequence component. Improving the reliability of protection by increasing the zero sequence current acceleration setting value or twice harmonic restraint principle is suggested.