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This paper reviews the aging mechanisms and stresses that affect toughened glass-suspension-type insulators. Procedures and criteria for determining the overall condition of inservice toughened glass-suspension insulators are described so that a decision can be made on their suitability for continued service or to replace the insulators.

This paper reviews and discusses the rated strength of insulators as defined in ANSI, CSA, and IEC product standards. The definition of rated strength, which for a particular insulator, requires reference to the tests and acceptance requirements given in the appropriate standard for that insulator is discussed. An insulator may not exhibit noticeable changes at loads sufficient for the initiation of irreversible damage, referred to as its damage limit. The significance of this is discussed in the paper as insulator application loads which must be below the damage limit of the insulator and be made in accordance with the relevant standard, including consideration of the allowable variation in strength implied by strict conformance to the standard. Damage limits for ceramic and composite insulators based on the minimum allowable strength according to the current standards are given.

This paper reviews and discusses the mechanical loads that overhead lines experience as defined by applicable codes and the strength requirements for insulators that support the lines as described in ANSI, CSA, and IEC standards. Consideration of the probable occurrence of extreme weather conditions requires the insulator strength to be such that irreversible damage to the insulator does not occur at the maximum load considered. The inherent variation of the mechanical strength of the insulator dielectric material is discussed as a factor for its ultimate strength, and as a basis for limiting line loads below the damage limit for the insulator. Time-load tests suggest that reasonable damage limits for ceramic and composite insulator dielectric materials can be established. As discussed in a companion paper, the assignment of damage limits for specific insulators depends not only on the inherent mechanical strength of the dielectric, but also on the standard upon which the mechanical strength of the insulator is rated. Common line design requirements, coupled with insulator manufacturers' recommendations that the maximum load not exceed the routine proof test load each insulator is subjected to has, in general, given good field experience.

Insulator contamination remains an important technical problem because contamination-caused insulator flashovers may jeopardize the security of high voltage power systems. This paper updates an insulator selection guide with respect to contamination first published 25 years ago. As in that guide, this update relates primarily to contamination conditions in North America. Results of research that have led to a better understanding of contamination-caused flashovers, particularly for some types of nonceramic insulators are summarized. Assessment techniques for site severity are discussed. Even though they have improved significantly, no leading method has emerged. Artificial contamination test methods have been standardized. Mitigation techniques are discussed for extreme contamination-caused dangers, including important new knowledge regarding ice and snow flashover voltage reduction. caused.

Special measures may be needed to select line insulators for transmission and distribution lines in locations exposed to freezing conditions. This second part of the paper describes the strength of typical line insulation in these conditions and then deals with selection and mitigation methods based on the in-service stresses described in Part I.

Special measures may be needed to select line insulators for transmission and distribution lines in locations exposed to freezing conditions. This first part of the paper describes the environmental and electrical stresses encountered in service that influence the risk of flashover on line insulators. Part II of the paper describes the strength of typical line insulation in these conditions and deals with selection and mitigation methods.

In locations exposed to freezing conditions over the station service life, additional measures for selecting adequate station insulators at distribution and transmission voltage levels should be taken. This second part of the paper describes the selection process and mitigation options, based on the environmental parameters, as discussed in Part I. These include insulator size, shape, surface material, surface quality, electric field improvement, and orientation.

In selecting adequate insulators for substations at distribution and transmission voltage levels, special measures may be needed in locations exposed to freezing conditions. This first part of the paper describes the environmental and insulator parameters that influence the risk of flashover on station insulators exposed to freezing conditions.

The paper provides a review of the electrocution and collision hazards of overhead distribution lines of large birds, particularly raptors, which can be reduced through modifications to existing design standards. The modifications to overhead distribution lines discussed in the paper include pole-line reconfiguration, the use of cover-up materials, perch deterrents, and line markers. The paper also discusses the effectiveness of these measures and research that is under way to develop tools to better understand where the problems are occurring and to determine if existing mitigating measures are working.

Bird related outages are more common and widespread than generally believed. The objective of this paper is to review the existing bird related problems and to recommend practices and procedures that are successful in reducing bird related outages on transmission lines, in substations and on overhead distribution lines. Experience has shown that use of perch guards, metal or plastic spikes, insulating caps, alternate perch sites and washing have significantly reduced bird related streamer and contamination outages.

Nonceramic insulators (NCIs) with crimped metal end fittings were studied to understand the nature of the interfacial stresses at the rod-metal fitting interface. The study involved both experimental and computational aspects. Monotonic tension tests were performed on rod assemblies (bare rod with end fittings crimped) as well as full-scale composite insulators. Different rod formulations (E-glass, E-CR glass fibers, resins made of epoxy, polyester, and vinyl ester) were evaluated. The experimental results were used to calculate the stresses at the interface. It is shown that the onset of debonding/detachment of the rod from the end fitting can occur at load magnitude close to the proof-test load. Radial cracks in the rod can also develop in the crimped portion of the rod. A theoretical model for pullout was developed to validate the experimental observations. A companion paper identified the role played by imperfections in the GRP rod matrix and electrical stress on brittle fracture. It is concluded that imperfections caused by crimping, together with mechanical stress concentrations are a greater threat to brittle fracture than posed by imperfections in the GRP rod matrix and electric stress concentrations near the line end fitting, only if the housing does not protect the rod from the elements. This highlights the importance of good quality control in manufacturing, and complete protection of the rod by the polymer housing.

A description of the composite insulator brittle fracture phenomenon suitable to allow correct diagnosis of a failure is provided. The paper reports on the number of brittle fractures that have occurred around the world and contains specific empirical data surrounding composite insulator brittle fractures in the United States. It also includes several case studies of in service failures to inform the reader of the diversity of initiating causes. The current state of knowledge on the brittle fracture mechanism is presented. Based on what was learned during the study, possible actions that can be taken by manufacturers and users to reduce frequency of occurrence are suggested.

A procedure for estimating the flashover probability of aged nonceramic insulators in-service is presented. The study was carried out using two types of 138 kV EPDM line post insulators received from the Florida Power and Light system (USA). Presented are the laboratory characterization of the naturally contaminated insulators, the development of a linear regression model, and the estimation of the statistical parameters that define the relation between probability of failure and applied voltage for a given contamination level. The results obtained in the laboratory were in agreement with the current and past field experience. Conservative probabilities of flashover were estimated under the assumption of a cumulative normal distribution. The estimators of the parameters that define the normal distribution were assessed with the original up and down method. The results suggest that for one type of EPDM insulator, contamination flashover performance was distinctly inferior to the other.

This paper reports the results of material tracking and erosion tests, and flashover tests of full-scale insulators made from epoxy compounds. Six different formulations, all based on the cycloaliphatic epoxy family, were evaluated as flat slabs and complete insulators rated for 25 kV (line to line) system voltage. In addition, line insulators in the range 11-66 kV, that had been operating satisfactorily for several years in light to heavy contaminated regions in different countries, were evaluated to obtain information on their future expected performance. Material tracking and erosion tests were performed using the standard ASTM D-2303 inclined plane test, and a variation of the same test that was developed in-house. Flashover tests were performed using the IEC Standard 507 "clean-fog" method. Results demonstrate that all material formulations evaluated had adequate tracking and erosion resistance for outdoor use. In addition, the insulators evaluated did not flashover at nominal service voltage (and higher) under a wide range of simulated contamination conditions. The overall good performance of the insulators has been related to material formulation, design and manufacturing of the insulators.

This, two-part paper, describes the results of a research project aimed at understanding the magnitude of aging of polymeric cable terminations used in distribution. The termination types evaluated are currently used; hence, they have satisfied IEEE Standard 48. This standard does not address the issue of aging. Terminations made from 3 different polymer families and porcelain were evaluated. Presented in Part I, are the results of field aging from 5 outdoor sites in the USA over a 3 year period; and in Part II, results from accelerated aging fog chamber laboratory tests. The results from field aging show that only minor changes were produced. Some differences were observed in the electrical performance of formulations within the same polymer family, and among different polymer families. However, it was concluded that the performance of the terminations evaluated would not be compromised by aging produced changes in the weathershed housing.

For pt.I see ibid., vol.13, no.2, p.316-22 (1998). This paper presents the results of laboratory aging tests on polymeric cable terminations. The goals of the laboratory aging were two-fold: (1) develop an accelerated aging test that can simulate the changes produced in the field, and (2) provide an assessment on the magnitude of aging induced changes on the electrical performance of terminations. Monitoring leakage current and material changes during laboratory tests were useful for assessing the variations possible in aging due to experimental parameters used. It has been shown that a laboratory aging test that is based on evaluating the termination's leakage current suppression capability and the ability to withstand a moderate level of discharge activity is suitable not only for assessing the long term performance, but also for ranking the electrical performance in accordance with field exposure.

This paper presents an overview of the practices employed by utilities and industries to combat the contamination problem with ceramic insulators. It emphasizes two popular methods, namely, coating insulators with greases and RTV (room temperature vulcanizing) silicone rubber coatings. The experience to date, application methods, user concerns, current theories on how and why the coatings perform, life expectancy, and future direction of the technology, are outlined. The paper also describes research activity in the committee on RTV coatings, which is the state of the art in protective coatings. >

AC clean fog tests were performed on nonceramic materials used for outdoor high voltage insulators, namely, room and high temperature vulcanizing (RTV and HTV) silicone rubber, and ethylene propylene rubber (EPR), with porcelain used as the reference. The steam input rate was varied upwards from the value standardized for porcelain insulators. Results indicate that higher steam input rates produce a significant reduction in the flashover voltage of silicone rubber family materials, although it is always higher than that obtained for EPR and porcelain. For EPR, the reduction is less and is similar to that established for porcelain. The mechanisms involved have been examined. The trend in the results is found to be consistent for different formulations and insulator geometries of the generic polymer (e.g. silicone rubber, EPR) evaluated. A new, simple method for consistently applying uniform contamination on silicone rubber (both RTV and HTV) is described, without the use of extensive physical or chemical treatments, or prior conditioning by dry band arcing. >

The behavior of 138 kV nonceramic line post insulators is investigated by means of clean fog tests conducted before and after aging in a specially designed accelerated aging chamber. The laboratory aging cycles are justified on the basis of actual weather in the coastal regions of Florida. Analytical measurements quantifying the degree of artificial aging are discussed, and artificial aging is compared with service experience. Observations of audible noise and radio influence voltage during the clean fog tests are reported. >

A comparison is presented of the electrical performance of room-temperature vulcanized (RTV) silicone rubber coatings for insulators under artificial contamination produced in a salt-fog chamber with DC voltage. Comparison of the RTV coatings with silicone grease coating has also been made. Correlation of the electrical parameters, such as leakage current and flashover voltage, with material degradation due to aging produced by dry band arcing is established. It is demonstrated that there are distinct differences among the RTV coatings evaluated, and between RTV coatings and silicon grease. >

The electrical performance of polymer-housed zinc oxide distribution arresters has been investigated in a fog chamber. Two types of housing materials, copolymers of ethylene and propylene (commonly known as EPM rubber) and terpolymers of ethylene propylene and diene (commonly known as EPDM rubber), both containing alumina trihydrate (ATH) filler, have been examined. It has been demonstrated that, under relatively low (250 mu S/cm) and moderate (1000-2000 mu S/cm) fog conductivity, degradation in the form of tracking and erosion of the housing occurs. On some housings, degradation is dominant in the region of the mold line and is due to nonuniformity in filter dispersion. The electric field has been computed and a few electrode modifications to improve the surface electric field distribution are suggested. >

For pt.1 see IEEE Trans. Power Deliv., vol.6, no.4, p.1366-73 (1991). This study is a follow-up of a previous report describing a systematic study of the contamination performance of silicone rubber cable terminations in a fog chamber. The goal of the study is to aid the development of an artificial contamination laboratory test for evaluating the electrical performance of all types of polymeric terminations in the field. This is achieved by understanding the role played by the various experimental factors on the electrical performance. The effect of varying the test method off-time on the regeneration of the silicon rubber's hydrophobic or water repellant surface is studied. The effect of load current on dry band arcing, which occurs on a moist and contaminated surface of an energized termination and can result in material degradation or flashover, has also been evaluated. It has been shown for the first time that the surface temperature rise due to the full load current causes a significant reduction in dry band arcing activity and therefore has a beneficial effect on the contamination performance of polymeric terminations. >

The authors present the results of fog chamber experiments done to examine the tracking and erosion performance of polymer insulator profiles. The effect of weathershed material, construction, orientation, and AC and DC voltage are examined. Correlation between cylindrical rod specimens of materials and insulator profiles is shown. The protected leakage path provided by the weathershed is found to play a major role in the tracking and erosion performances of polymer insulators. The resistance to tracking and erosion of insulator profiles with DC is shown to be reduced in comparison to AC. Cylindrical rods of material yielded the same ranking of material performance as insulator profiles but in a shorter time. >

The results of a study on silicone rubber and ethylene propylene diene monomer (EPDM) polymers are presented. The study was done to understand the mechanisms involved in the loss and subsequent recovery of surface hydrophobicity due to dry-band arcing and to investigate various experimental techniques which could be used for characterizing surface hydrophobicity. The materials are subjected to dry-band arcing in a log chamber. Several techniques such as measurement of contact angle, determining surface composition using electron spectroscopy for chemical analysis (ESCA), and measuring crossover voltage (COV) using a scanning electron microscope are examined. The experimental results and theoretical calculations demonstrate that the mobile species in the polymer are responsible for the surface hydrophobicity. In terms of repeatability and simplicity of measurement, the COV determination appears to be the most suitable technique for hydrophobicity studies. >