Porcelain and glass insulators have been used for over a hundred years and although these materials have proven themselves resistant to environmental ageing, their pollution performance has often been relatively poor due to hydrophilic surfaces. In 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 linear 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 end 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 past INMR article by Dr. Weiguo Que of Axcelis Technologies and Professor Stephen Sebo of Ohio State University, discussed the specifics of just such a model.
Development of Insulator Computation Models
A typical 34.5 kV composite insulator has 12 weather sheds and a length of about 0.8 meters. By comparison, a typical 765 kV composite insulator has over 100 weather sheds and is nearly 5 meters long. Therefore, to obtain accurate results, considerably more elements have to be used for the electric field analysis of such a 765 kV insulator than is case for a unit designed for 34.5 kV. When using the boundary element method to calculate the electric field and voltage distribution along such insulators (EFVD), the greater the number of elements used, the more time is needed for computation. Therefore, in order to reduce calculation time when analyzing relatively long composite insulators, some simplifications of the insulator model are necessary.
A composite insulator, depending on design, can have up to four main components: the fiberglass reinforced (FRP) rod; the polymeric sheath on the rod; the polymeric weather sheds; and two metallic end fittings. To determine which component can be simplified with the least influence on the accuracy of the calculated results of EFVD, a 34.5 kV composite power line insulator was studied. The detailed geometry and dimensions of this 34.5 kV insulator are shown in Fig. 1.
The insulator, equipped with metallic fittings at both line and ground ends, is made of silicone rubber with a relative permittivity of 4.3 and a rod with a relative permittivity of 7.2. There are 12 weather sheds on the housing. The insulator is surrounded by air with a relative permittivity of 1.0. The top metal end fitting is taken as the ground electrode and the bottom electrode is connected to a steady voltage source of 1000 V for the purpose of calculations. The insulator is positioned vertically (but shown horizontally in Fig. 1 for convenience). Four simplified computation models are used for a step-by-step comparison process. In addition, a three dimensional “full” insulator model is set up as a reference for study of the EFVD. These five calculation models are shown in Fig. 2: (a) two electrodes only, (b) two electrodes and the fiberglass rod, (c) two electrodes, rod and sheath on the rod without weather sheds, (d) two electrodes, rod, sheath, two weather sheds at the each end of the insulator, and (e) the “full” 34.5 kV insulator.
As an example to show the element configuration, the “full” insulator model has 12,553 four-sided elements applied to the surface of boundaries and the interfaces of different media. The element configuration on the surface of the insulator is partially shown in Fig 3.