Comparison of Backflashover Performance of a Fully Composite Pylon with Internal and External Grounding Down-leads

Transmission towers made of composite material with aesthetic appearance are designed and proposed for the increasing installation of overhead lines. From the aspect of lightning protection, it is desirable to connect a grounding down-lead to bring the ground potential to the shield wires, either inside the hollow body of composite pylons or outside. This paper aims to compare the back flashover performance of these two grounding approaches. The mutual capacitances between the down-leads and phase conductors are simulated and calculated by FEM tools. A dynamic corona model is used to simulate the transient response of the down-leads. Without considering the corona, external down-lead has a slightly lower surge impedance and mutual capacitances, but the two grounding approaches give the close back flashover rates. It is stressed that the surge corona effect is necessary for modelling thin-wire grounding devices, otherwise, the back flashover rate can be overestimated by 20 % to 30 %. If the corona effect is considered, the back flashover rate provided by adopting internal down-leads is a bit higher, because the corona expansion can be hindered in the cross arm.