A Simplified Dynamic Surge Impedance Model with Corona Effect for Grounding Down-Leads of Composite Pylons

Design of a novel 400 kV fully composite pylon utilizes external down-leads to bring grounding potential to earth wires. In lightning transient studies, the corona effect on the thin-wire down-leads should not be neglected. A simplified dynamic surge impedance model for thin-wire vertical conductors considering voltage-dependent surge corona is proposed. The model describes the relationship between corona development and the surge overvoltage, it focuses on the macro effect of corona on the surge response of conductors, and it ignores the micro plasma dynamics inside the corona. Higher applied lightning current peaks will cause a longer corona radius and cause more reduction in overvoltage. If the front time of applied lightning impulse current increases, the maximum corona radius increases first and then decreases, because the corona development is dominated by expansion velocity and critical streamer-tip electric field in succession. The dynamic surge impedance model shows a reduction of the overvoltage at the top of the conductor by 40% approximately compared with a constant surge impedance model without considering corona, which shows it is necessary to consider the corona effect in the lightning transients research of thin-wire grounding devices.