The impact of electromagnetic interference on secondary systems in substations can be reduced by the installation of secondary equipotential grounding grid. A simplified grounding grid model based on CDEGS is established to study the effects of the connection mode between the secondary equipotential grounding grid and the main grounding grid on ground potential rise and current in the secondary equipotential grounding grid. By considering soil resistivity and current injection points, the characteristics of the grounding grid under power frequency current, lightning current, and high-frequency damped oscillation current are simulated and analyzed. It is found that when the injection current frequency is low, the ground potential difference and current inside the secondary equipotential grounding grid close to the fault point can be effectively reduced by the single-point grounding connection mode. When the injection current frequency is high, the multi-point grounding connection mode can effectively reduce ground potential rise and current. Besides, the ground potential rise and current of grounding conductors at different locations increase with increasing soil resistivity. When the current injection point is at the corner of the grounding grid, the ground potential difference and current inside the secondary equipotential grounding grid is significantly increased.