Contents I. Introduction
The lightning impulse performance of grounding system plays an important role in the safe and reliable operation of power system. As already evidenced in [1] [2]–[3] [4], the characteristic of grounding systems subject to high impulse current is dramatically different from that at low frequency. If a high impulse current, such as lightning current, is injected into the grounding system, because of the high impedance in the grounding conductor at high frequency, the electric field surrounding the grounding electrodes near the current injected point may exceeds the critical breakdown value and soil will be ionized. The affected portion of the soil will become a good conductor, which is equivalent to expanding the radii of the electrodes. Thus, the ground impedance of buried electrodes will decrease during soil ionization. However, because this soil ionization phenomenon is too difficult to be simulated, for example the parameters are not only time varying but also frequency varying, when the grounding systems are analyzed, the dynamic and nonlinear ionization phenomenon is often omitted [5]–[6] [7].
