Satisfactory operation of composite insulators is intimately related to the surface electric field (stress) distribution. This paper presents results of calculation of the electric field distribution for composite insulators up to 1200 kV, using a 3D software package based on the Boundary Element Method. The impact of corona and grading rings, single and bundled conductors, insulator orientation (dead-end and suspension), single and double units, and surface condition (dry and wet) on the electric field distribution has been analyzed. For UHV systems (higher than 750 kV ac) the use of dual insulators with individual corona rings at the line and ground end and a common grading ring at the line end is beneficial. The existence of optimal dimensions of corona and grading arrangement has been illustrated on a 1000 kV insulator string. It was found that the dead end insulators experience higher electric stress when compared to their suspension counterparts and this difference gets more prominent as the system voltage increases.

Electric field computation of composite line insulators up to 1200 kV AC

This paper deals with the use of corona ring at the HV end fitting for improving the electric field and potential distributions and then for minimizing the corona discharges on 230 kV AC transmission line composite insulator. A single corona ring is installed at the energized end side. Three-dimensional finite element method (FEM) software is employed to compute the electric field. As the performance of high voltage insulator strings closely depends on designs and locations of corona ring, the effects of the corona ring radius, the ring tube radius and the ring vertical position are examined. The minimization of the electric field necessitates the optimization of corona ring. For this purpose, new nonlinear mathematical objective function linking the electric field strength to the corona ring structure parameters is established. The optimization problem is achieved by minimizing the objective function using a modified particles swarm optimization (PSO) algorithm with a dynamic population size. The algorithm adjusts the size of population for each iteration. Based on the average value and the best solution of the objective function, we propose a new mathematical model to update the population size. This algorithm enables the population size reduction leading to computing time decrease. According to the results, FEM-PSO hybridization technique could be very helpful in optimization of corona ring design.

Optimal design of corona ring on HV composite insulator using PSO approach with dynamic population size

?Polymer? is used in this article, but often ?nonceramic? or ?composite? is used in place of ?polymer.? So, polymer insulators are synonymous with nonceramic or composite insulators. This article focuses on the development of suspension-type polymer insulators, used on overhead distribution and transmission lines, and on the present-day issues that limit their greater use.

50 years in the development of polymer suspension-type insulators

Water droplets on hydrophobic surfaces of polymeric insulators oscillate under applied high voltage electric field stress. Deformation of the droplets increases local electric field stress in the triple zones, causing lower partial discharge inception voltage and consequently affecting the ageing performance of the insulator. For this contribution dominant modes of single water droplet oscillation on hydrophobic surfaces were investigated in a wide range of droplet volume and frequency of applied electric field using a high speed camera. 3D frame analysis showed for all volumes an increasing trend of mode number with frequency in the range of 20 to 100 Hz. Experimentally found resonance frequencies for water droplets on a hydrophobic surface approximately correlate with published frequencies of free water droplets. Conductivity showed no effect on the pattern of water droplet oscillation. Partial discharge inception voltages on two different hydrophobic surfaces (silicone rubber and epoxy resin) with conductive and non-conductive (distilled) water droplets were measured. These measurements showed that inception voltage makes a step whenever the oscillation mode of water droplet changes. Conductivity of the water droplets was found to have no effect on the inception voltage on silicone rubber surfaces having a high degree of hydrophobicity, whereas conductive droplets on epoxy resin with lower hydrophobicity lead to lower inception voltages.

Experimental investigations on water droplet oscillation and partial discharge inception voltage on polymeric insulating surfaces under the influence of AC electric field stress

A major consideration for utilities applying composite insulators on overhead lines and substations is the aging of the polymer materials used for insulator housing. Historically, a primary driver for composite insulator development was to achieve a good contamination of flashover and aging performance. However, the application of composite insulators is not only limited to highly contaminated areas since large numbers are now also installed in locations with low contamination severities. The study of the long-term aging and life expectancy of composite insulators in clean environments is, therefore, of fundamental importance. Recent work at the Electric Power Research Institute (EPRI) and STRI has identified water-induced discharge activity as an important aging mechanism in these environments. This paper summarizes the findings of the extensive research carried out by these two organizations to determine a practical limit for the permissible E-field on insulator surfaces for design purposes. The initial work by the EPRI to determine e-field thresholds for water-induced corona, published in 1999, is expanded on the basis of small- and full-scale tests by both organizations to refine threshold levels and determine the influence of insulator parameters, such as hydrophobicity and shed profile. The influence of water-induced corona on the aging performance of insulators is also investigated on the basis of in-service inspections, accelerated aging tests, and natural exposure tests. The implications of this work on the need and dimensioning procedures for grading rings are also discussed.

Electric-Field Limits for the Design of Grading Rings for Composite Line Insulators

This paper provides an overview of the electric field (E-field) distribution on transmission line composite insulators applied in alternating current applications. Factors that affect the E-field distribution are discussed as well as the influence of the E-field distribution on the short and long term performance. Modeling and measurement methods are reported and examples of calculated E-field magnitudes determined are presented together with corona ring application information. This paper was developed by the IEEE Task force on electric fields and composite insulators.

B. Vancia

Papers

Electric Fields on AC Composite Transmission Line Insulators