Evaluation of the Electric Stress on an Insulating Down-Conductor Caused by Lightning Strikes Through ATP-EMTP Simulations

Electrically insulated lightning protection systems, LPS, utilize insulating stand-offs and insulating down-conductors to maintain the required separation distance from grounded objects. An equivalent separation distance offered by these insulating components can be determined through comparative tests, such as those suggested in IEC TS 62561-8:2018, where crossed-conductors air gaps under negative standard lightning impulse voltages are used as reference. However, in service the insulating components of an electrically insulated LPS are stressed by non-standard fast-front overvoltages related to lightning return-stroke currents. In this study the electric stress experienced by the insulating components of an electrically insulated LPS utilizing a single air terminal, connected to the grounding system via an insulating down-conductor, due to direct negative lightning flashes is assessed through ATP-EMTP simulations. The ground resistance of the LPS grounding system, the length and the resistance of the semiconductive screens of the insulating down-conductor, as well as the characteristics of the lightning return-stroke current were considered as parameters affecting the fast-front overvoltages. For an evaluated electrically insulated LPS the critical lightning currents causing failure of internal and external insulation can be determined; thus, creepages and clearances, as well as the dielectric strength of internal insulation ensuring a safe design against lightning overvoltages can be determined.