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Un-accurate modeling of power system may cause to under or over-design the insulation, which in turn, leads to increase in the investment and/or maintenance cost of the network protection against lightning overvoltages. The grounding system is an effective parameter on the magnitude of the lightning overvoltages and insulation coordination of high voltage transmission lines (TLs). This paper presents a comprehensive evaluation of the effect of grounding electrode impedance on the estimation of overvoltages caused by lightning strokes, in which both the vertical and non-vertical strokes are considered. In the presented study, static and wide-band model of the tower-footing grounding system is adopted assuming the soil electrical parameters to be either constant or frequency dependent. Also, the effect of modeling type of grounding system is considered on the TLs' reliability indices. The study was performed on a typical 400-kV transmission line which is modeled in EMTP-RV. From the simulation results, it is found that the outage rate of TLs is markedly affected by the model of the tower footing grounding system. This effect is more pronounced when the soil electrical parameters are frequency dependent. It is also concluded that, in addition to the lightning overvoltages, the unavailability of the TLs is significantly affected by wide-band model of grounding system, non-vertical lightning strokes and soil resistivity.

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