Influence of the linear non-uniformity of pollution layer on the insulator flashover under impulse voltage - estimation of the effective pollution thickness

This paper is aimed at the influence of the linear non uniformity of pollution layer on critical current I_cri and critical flashover voltage V_cri of polluted insulator under lightning impulse voltage. It is shown that the experimental values of I_cri and V_cri of uniformly and non-uniformly polluted insulator depends on the configuration of pollution and the polarity of voltage. A mathematical model enabling to compute the flashover discharge characteristic constants (n and N) and the critical thickness of pollution layer and thence the critical characteristics of flashover (current and voltage) is presented. It´s also shown that n and N are not static and they depend on the parameters of the equivalent electrical circuit, thermal characteristics of discharge and the condition of propagation. The computed critical voltages are found in a good accordance with the experimental ones while the computed critical currents are higher than the experimental ones. The recalculated critical currents and critical voltages assuming that only a part of the thickness of pollution layer contributes to the conduction phenomena at the interface between the discharge and electrolyte well match to the experimental values. Thus there is an effective thickness of pollution layer which varies linearly with the resistivity of pollution layer.