This work of this paper is about a large crossing project with hybrid voltage , so there is no experience about it. Therefore，it is necessary to carry out special lightning protection research on the large crossing project to provide technical support for engineering design and construction.  It is obvious that the large crossing project has the characteristics of high tower, long span length, and hybrid voltage erection. Based on these factors, this paper uses ATP software to model the project and obtain the corresponding lightning withstand level and lightning shielding failure proof level. Subsequently, the shielding failure trip-out rates of the DC part and AC part of the AC / DC common-frame tower are obtained by the improved electro-geometrical model. According to the safety regulations of large crossing projects, this paper converts the counterattack trip-out rate and shielding failure trip-out rate to the safety years in order to pass assessment. At the same time, in the process of lightning protection calculation, considering the specialty of large crossing engineering, in order to ensure the safe of the project, this paper adopts lightning protection measures including but not limited to modifying the protection angle of lightning line or adding hanging arrester, and then calculates and analyzes its lightning protection effect for large crossing project.

safety years,transmission lines,electrical geometry model,shielding failure trip-out rate,large crossing