When lightning current enters a substation grounding system, the resulting nonuniform ground-potential rise and transient electromagnetic field can jointly couple into secondary control cables connecting primary equipment to the protection room. This paper establishes a three-dimensional model that includes homogeneous soil, a copper-bar main grounding grid, the secondary equipotential grid in the protection room, the main control and communication building, cable trenches, representative grounded metallic enclosures of primary equipment, and a two-core shielded control cable. A transient source in Schematic excites the three-dimensional field model, which is coupled to a Cable Studio multiconductor transmission-line model and terminal networks to form a field-line-circuit computational chain from the lightning source, grounding-current propagation, and spatial field to the cable and its terminations. With the lightning location and cable route fixed, six cases are defined by combining three shield-grounding configurations with two differential-mode intercore terminal impedances at the protection-room end. The results show that lightning current propagates along the main grid and equipment-grounding branches, producing nonuniform transient surface-current and magnetic-field distributions in the secondary-equipment area. The shield-grounding topology substantially changes the shield return path and current envelope; when a direct grounding path exists at the primary-equipment end, the P7 current reaches the order of 10³ A. For the same grounding topology, changing the protection-room intercore termination from 50 Ω to 10 kΩ does not change the overall shield-current order of magnitude or decay trend. Under the symmetric two-core condition, the two node voltages are strongly synchronous, indicating a pronounced common-mode component and relatively weak differential-mode conversion. The findings provide a reference for shield-grounding design, terminal-boundary analysis, and joint lightning-disturbance modeling of substation secondary control cables.

substation, lightning electromagnetic disturbance, secondary control cable, cable-shield grounding, differential-mode terminal impedance, field-line-circuit co-simulation.