Possible Impacts of Earthing Path Impedance on Lightning Return Stroke Currents

摘要

Effects of the impedance of lightning earthing path on lightning currents are investigated by computer simulation. Distributed-circuitry transmission line models with adequate values of inductance, resistance and capacitance are chosen to represent lightning channels, upward connecting leaders, structures and lightning protection systems (LPS). Both the current and voltage as a function of time and height of the lightning channel are simulated. Results show that for a lightning stroke without an upward connecting leader, the large lightning stroke current is firstly generated at the channel base and then propagates upward as its amplitude decreases. For the case with an upward leader, however, the return stroke current appears firstly at the connecting point of the upward- and downward- leaders, and then propagates bidirectionally down and up as its amplitude decreases. The existence of an upward leader makes both the lightning current and the rate of change of the current at the channel base smaller than with no an upward leader. For a LPS with larger resistance and inductance, the lightning current through it would be smaller but the larger voltage across the LPS would increase the risk of flash over. LPS with large capacitance, however, can make the lightning currents at the channel base smaller without increasing the risk of flash over. The results suggest that a LPS with higher resistance or inductance cannot limit the lightning current effectively, but a LPS with larger capacitance such as the Faraday cage can.