Influence of a grading ring on AC flashover voltage of an EHV post insulator covered with ice

In this paper, we present preliminary results based on experimental and numerical investigation concerning the influence of a grading ring on the wet-grown ice accumulation on two units of an EHV post station insulator, and on its critical flashover voltage during the corresponding melting period. Flashover voltage was determined during the melting period where a conductive water film is present at the ice surface. In parallel with experimental tests, numerical simulations using the finite element method (FEM), implemented by the commercial software Comsol Multiphysics¿, were performed to study the influence of the grading ring on the potential and electric field distributions along the two insulator units at the beginning of the ice accumulation period. The numerical simulations show that the electric field strength in the vicinity of the HV electrode is drastically reduced by the presence of the grading ring. This leads to a decrease of the voltage drop along the first air gaps formed by icicles during the ice accretion which results in a more uniform ice distribution without air gaps. This was confirmed by experimental investigation which reveals that adding a grading ring affect the ice accumulation process by avoid air gap formation close to the HV electrode without affecting the critical flashover voltage of the EHV station insulator covered by ice.