With the intelligentization upgrade of power substations, a large amount of secondary equipment has been deployed dispersedly near the high-voltage primary equipment. The enclosures of these secondary devices are electrically connected to the grounding grid underground. When a fault current injection causes a potential rise in the grounding grid, there is a potential difference between any two points on the grounding grid conductors. For two pieces of secondary equipment located at different positions and connected by a secondary cable, the potential difference can induce common mode and differential mode disturbance voltages at the ports of the secondary cable, which may interfere with the normal and stable operation of the secondary system. In this article, the grounding grid of a 220-kV substation is modeled and the corresponding potential difference is calculated by means of CDEGS software. Also, the effects of the grid density and size, soil resistivity, positions of current injection, and potential observation on the potential difference are investigated. The results are helpful for guiding grounding grid design and evaluating the influence of ground potential difference on secondary equipment.