A Parametric Study on the Critical Lightning Currents Causing Flashover to the Overhead Lines of a ±533 kV HVDC Transmission System

An ATP-EMTP simulation model for the evaluation of the lightning performance of the overhead lines of the ±533 kV Cahora-Bassa HVDC transmission system is presented. This model is used for the estimation of the minimum lightning currents causing flashover of the positive and negative pole insulators due to lightning strikes to shield wires (backflashover) and pole conductors (shielding failure flashover). A parametric analysis is conducted to investigate the influence on the critical lightning currents of several factors, namely, the span length, sag of pole conductors and shield wires, number of sub-conductors per bundle, tower ground resistance, soil resistivity and the pole operating voltage and polarity. It is shown that the minimum shielding failure flashover current of the HVDC lines is significantly affected by the polarity of the pole operating voltage only; the variation due to the other investigated parameters is found to be lower than about 10%. The minimum backflashover current, in addition to the polarity of the pole operating voltage, is influenced considerably by span length, lightning strike location along the span and tower ground resistance, thus also soil resistivity.