Electric field & voltage distribution along non-ceramic insulators

Porcelain and glass insulators have been used for over a hundred years. Although these materials have proven themselves resistant to environmental ageing, their pollution performance has typically been relatively poor due to their hydrophilic surfaces. During recent decades, polymeric insulators have become more widely used because of the advantages they offer in terms of excellent hydrophobic surface properties under wet conditions.

However, the bonds of polymeric materials are relatively weak in comparison to those of inorganic ceramic materials. Therefore, they are more susceptible to chemical changes under the various stressors encountered in service. These include electric stresses due to the operating voltage, corona and arc as well as environmental stresses such as contamination, UV and heat cycling. Under these stresses, the hydrophobicity of the shed surfaces on these insulators can be temporarily or even permanently lost resulting in worsened pollution performance.

Generally-speaking, the electric field distribution along a polymeric long rod is not so linar as that of a porcelain insulator string because there are no intermediate metallic parts. High electric field strength can cause corona on these insulators resulting in corona cutting, deterioration and ageing of the polymeric material. Therefore, controlling electric field strength along non-ceramic insulators is an important aspect of their design and also of the design of their grading devices.

When installed on a power line, the tower geometry and nearby line and hardware and conductors will also affect electric field distribution around an insulator.

Depending on voltage level, the magnitude of the electric field strength on the insulator's surface may exceed recommended corona-related values. Grading rings are then used to modify the electric field distribution and reduce its

maximum value. Given all these considerations, a three dimensional model should ideally be set up in order to evaluate electric field strength and voltage distribution near as well as along a non-ceramic insulator.

This article, contributed by Dr. Welguo Que ofAxceis Technologies and Professor Stephen Sebo of Ohio State University, discusses the specifics of just such a model. Their study employs the commercially-available, 3-D electric field analysis software - COULOMB - based on the boundary element method and developed by Integrated Engineering Software.