500-kV Line Design: I - Insulation Characteristics of Towers

The results of tests using a full-scale transmission line for the insulation design of the first 500-kV transmission line in Japan (the Boso Line of Tokyo Electric Power Company) are described. Having clarified the switching surge and impulse flashover characteristics of V-string insulator and strain insulator assemblies, installed on a simulated tower with a long gap of three meters or more, insulation was designed which withstands a switching surge of 2.5 per unit and is capable of unbalanced insulation between vertically arranged double circuits. The flashover path, predischarge, etc., of the switching surge was also investigated.