A New Method to Evalute Length Ratio of Positive and Negative Leaders in Breakdown Process of Composite Insulator String

Abstract

It is important to evaluate the lightning performance of transmission lines, in which the flashover model of insulators plays a crucial role. The leader progression model (LPM) was widely used to simulate the insulator flashover process, and its implementation starts with the assumption that the positive and negative leaders develop with an equal length at both sides. However, some experiments [6], [9] provided evidences that this assumption may be not always true. To figure out the length ratio of the positive and negative leaders, the breakdown experiments of a 220kV composite insulator string suspended on an actual tower were carried out under the positive short tail lightning impulse <tex>$(\pmb{T}_{\mathbf{f}}/\pmb{T}_{\mathbf{t}}=\mathbf{0.6}/\mathbf{8.6}\ \mu\mathbf{s})$</tex>. A new method, which could overcome the shooting speed limit of the camera, was proposed to distinguish the attachment point of positive and negative leaders. Experimental results indicate that the length ratio of the positive leader to the negative leader is not equal to 1, and increases with the applied voltage. A linear regression model was used to identify the correlation between the length ratio and <tex>$\mathbf{U}_{\mathbf{P}}/\mathbf{U}_{\mathbf{50}\%}$</tex>, where <tex>$\mathrm{U}_{\mathrm{P}}$</tex> is the voltage amplitude and <tex>$\mathbf{U}_{\mathbf{50}\%}$</tex> is 50&#x0025; breakdown voltage.