There may be high-amplitude zero-sequence inrush generated by a high voltage built-in high-impedance transformer in the process of no-load closing. The zero-sequence inrush will distribute in the line and may cause the maloperation of the line zero-sequence over current protection. In addition, the above phenomenon exists when a spare transformer is put into operation, and the automatic device transfer operates unsuccessfully. In order to solve this problem, the characteristics of zero-sequence inrush are studied based on the formula of zero-sequence inrush in this paper. The dead angle and second harmonic of zero-sequence inrush are calculated and analyzed. On this basis, the feasibility of a dead angle and second harmonic breaking criterion to identify zero-sequence inrush is analyzed. The results show that the dead angle of zero-sequence inrush reduces so significantly compared with phase inrush that it cannot be regarded as a characteristic to identify zero-sequence inrush. The second harmonic component increases obviously, so the second harmonic breaking criterion is an advisable scheme to ensure that automatic device transfer is put into operation successfully. Finally, a simulation test based on PSCAD/EMTDC verifies the accuracy of the analysis results. This work is supported by the National Natural Science Foundation of China (No. 51677108) and the Science and Technology Project of State Grid Ningxia Electric Power Co., Ltd. (No. SGNX0000DKJS1900578).