Insulation Characteristics of 345-kV Transmission Towers

The impulse and switching surge flashover characteristics of 345-kV structures, consisting of a 15-to 20-unit string of insulators hanging from the crossarm of a steel tower, have been investigated. In all cases, the horizontal air gap between the conductor and the face of the tower was coordinated with the insulator string so that both discharge paths would have equal flashover voltages when tested with positive-polarity switching surge waves under dry conditions. New, weathered, and service-contaminated insulator units were used in this study. The switching surge flashover characteristics of insulator strings, as well as those of the horizontal air gap, are given for both positive and negative polarities and for both dry and wet conditions. The impulse flashover characteristics of insulator strings hanging from the tower crossarm are found to be equal to published catalogue data for both polarities, while the impulse flashover characteristics of the horizontal air gap are found to be about 95 percent of that of NEMA rod gap with the same spacing for the positive polarity and about 92 percent for the negative polarity. As the insulator string swings towards the tower, the flashover level of the 345-kV structure is determined by the discharge path having the lower flashover voltage.