•Grounding impedance of the overhead line tower is frequency-dependent.•Flashover occurrence on the insulator string depends on the grounding impedance.•Model of the grounding is based on the impulse impedance measurements up to 410 kHz.•Grounding impedance is modeled in EMPT-type tool.•State space equations and rational approximation are used to model impedance. The paper demonstrates the importance of taking the frequency-dependence of the tower grounding impedance into account when studying the lightning performance of an overhead line located in an area with high soil resistivity. Towers’ impulse grounding impedance measurements and their rational approximations are used to construct a wideband model of the towers’ grounding electrodes in an EMTP-type software program. In order to examine the impact of tower grounding impedances, simulations of lightning strokes to the overhead line tower were performed using an EMTP-type tool in the following three different cases: constant grounding impedance of the tower hit by a lightning stroke, frequency-dependent tower grounding impedance and frequency-dependent tower grounding impedance for the tower equipped with a Line Surge Arrester (LSA) installed at the lowest phase. The simulations have shown the influence of the frequency-dependence of the tower grounding impedance on the level of the build-up voltage across the tower’s insulator string and consequently on the flashover occurrence in the observed configuration with a high soil resistivity.