Leader propagation models of ultrahigh-voltage insulator strings based on voltage/time curves under negative lightning impulses at high altitude

Lightning protection analysis is the foundation of estimating the lightning protection behaviors of transmission lines. The lightning flashover criteria of insulators/gaps are an important part of such analysis. One lightning flashover criterion of an insulator/gap is the leader propagation model recommended by CIGRE; however, this model does not consider altitude or longer gaps/insulators (>5 m). In this paper, the 50% disruptive-discharge voltage U50% and 50% voltage/time curve for an air gap of 7.1 m configured by a composite V-string and composite I-string of 7.1 m were measured under negative lightning impulses at the National Engineering Laboratory for Ultrahigh-Voltage Engineering Technology (Kunming), at an altitude of about 2100 m. The insulator types and configurations are similar to those used in the +800kV Ultrahigh-voltage Direct Current (UHVDC) power transmission project. The experimental result shows that the 50% disruptive voltage U50% of the V-string is 7% higher than that of the I-string, and the best-fitting constants of the new leader propagation model of the V-string are different from those of the I-string. The two 50% voltage/time curves were compared with the voltage/time curves predicted by the leader propagation model recommended by CIGRE. The best-fitting constants in leader propagation models are proposed to be used for long air gaps of the V-string insulator and longer I-string composite insulator at high altitude. The new models are verified for a 1.9 m air gap in parallel with a 2.04 m composite insulator and 2.04 m I-string composite insulator.