AbstractSteel is widely used as grounding material in China. The resistivity and permeability of steel are higher than those of copper. If a grounding grid is very large, it is a problem for steel grounding grid to have great potential difference within the grounding grid. Based on field-circuit coupling theory and method of moment, a numerical calculation method of steel grounding grids is presented in this paper. The resistance and self-inductance of conductors as well as mutual inductance between conductors are taken into account. The nonlinear characteristic of steel permeability saturation is also taken into account. The impedance of a large substation grounding grid is analyzed based on the method presented in this paper. The results show that the grounding impedance is decreased as the short circuit current increases. Thus, the grounding impedance with great failure current is lower than measured grounding impedance with measurement current from several amperes to tens of amperes.