Numerical investigation of transient surge impedance of a vertical conductor over a perfect ground

This paper presents an investigation into transient surge impedance of a vertical conductor over a perfect ground during a lightning struck. An equivalent circuit approach is employed, and voltage on the conductor is directly obtained by using traditional circuit analysis techniques. The transient surge impedance is then computed with the instantaneous voltage and current obtained in the simulation, which is path-independent. It is found that surge impedance generally increases as time goes on, and is determined by the geometry of the conductor and the waveform of the injected surge current. The simulation shows that the transient surge impedance remains the same no matter how long the conductor is. It is affected by conductor diameter, and can be computed by adding an offset if the diameter is changed. The curve of transient surge impedance is significantly affected by the waveform of the surge current. However, it does not change with grounding resistance. The grounding condition does not affect the sure impedance at the top of the vertical conductor. A comparison with the approximate formulas of characteristic surge impedance is also made in the paper. These formulas generally have consistent offsets to the simulation result for the surge current with a unit step waveform. The difference, however, varies significantly with conductor height for the surge current with a ramp waveform or a long rising time.