Experimental Study of Critical Cascading Flashove on Insulator Assembly Using Model Arrangements (Part 1) : The Influence of a floating Electrode, Placed in the Air at Mid-Gap, on the Formation of a Flashover Path

Abstract

This paper describes the phenomena of cascading flashovers in model arrangements for experimental analysis of critical cascading flashovers in an insulator assembly used on high voltage transmission lines. The testing model used consisted of a 15 cm gap length of rod-rod electrode which was 1.0 cm in diameter with a hemispherical end, with a potentially floating electrode which was a copper wire of 0.16 cm diameter. The applied lightning impulse voltage of positive or negative polarity was much higher than the breakdown voltage when the flashover occurred for only a short time lag. The experimental results included V-t curve and 50% flashover voltage characteristics for a gap on the testing model, flashover path, pre-breakdown corona distributions, and cascading flashover rates. The flashover phenomena in the air gap with a floating electrode clarified that the cascading flashover rate remained at a constant value whether the floating electrode existed or not. The coronas from the floating electrode were generated only when the coronas from the rod electrode reached the floating electrode. In order to form a cascading flashover, it was necessary that the bright leader coronas from the rod electrode grow toward the floating electrode, causing the coronas from the floating electrode to generate and meet the other coronas growing from the rod electrode.