HV composite polymeric insulators are being accepted increasingly for use in outdoor installations by the traditionally cautious electric power utilities worldwide. They currently represent -60 to 70% of newly installed HV insulators in Nortb America. The tremendous growth in the applications of non-ceramic composite insulators is due to their advantages over the traditional ceramic and glass insulators. These include light weight, higher mechanical strength to weight ratio, resistance to vandalism, better performance in the presence of heavy pollution in wet conditions, and comparable or better withstand voltage than porcelain or glass insulators. However, because polymeric insulators are relatively new, the expected lifetime and their long-term reliability are not known and therefore are of concern to users. Additionally they might suffer from erosion and tracking in the presence of severe contamination and sustained moisture. This leads to the development of dry band arcing that under certain circumstances could lead to failure of polymer insulators. In this paper a review is presented of the recent performance experience of HV composite polymeric insulators in outdoor service, testing methods, aging, the ranking of the materials, the role of fillers, the role of low molecular weight components present in the insulators, the mechanisms responsible for the loss and recovery of hydrophobicity, one of the most important properties of polymers, the mechanisms of failure, detection of faults, type and quantity of natural contaminants, effects of exposure to rain, hydrocarbons, stationary air and wind, various methods to optimize the electrical performance and a relatively new method for evaluating the performance status of polymeric insulators in the field.

Outdoor HV composite polymeric insulators

This is a laboratory study on the aging of SIR and hydrophobicity recovery. UV radiation, corona discharges, acid rain, dry-band arcing are employed as sources of the aging. Chemical and morphological analysis is used to detect the surface chemical and structural changes derived from these stresses. The nature of highly mobile low molecular weight (LMW) chains achieving a quick recovery of the hydrophobicity is carefully examined. Their generation and extinction caused by these stresses are investigated. From this study, it is shown that oxidation that induces crosslinking, branching, interchanging and a formation of silanol groups are the most dominant chemical reactions during the aging of SIR. Furthermore, silanol groups that are byprodncts of oxidation restrict the diffusion of mobile LMw chains, which decreases the recovery speed of hydrophobicity and accelerates the aging. A typical aging scenario of SIR from the installation to the end of their life is drawn.

Electrical and environmental aging of silicone rubber used in outdoor insulation

Tracking and erosion of high temperature vulcanizing (HTV)-silicone rubber (SIR) and the suppression mechanism of alumina trihydrate (ATH) filler were investigated in the present study. The tracking and erosion resistance of HTV-SIR filled with 0 to 60%wt ATH was evaluated by employing an IEC 587 inclined plane (IP) tracking and erosion test, during which leakage current pulses on HTV-SIR were counted. Surface temperature distributions and the occurrences of thermal spots >400/spl deg/C also were observed by means of an infrared thermovision study. We employed thermogravimetry (TG)-differential thermal analysis (DTA)-mass spectrometer (MS) to observe the thermal degradation of unfilled and filled HTV-SIR in both air and argon. The test results indicate that 40%wt is a critical ATH level and whether tracking and erosion is allowed in the IP tracking and erosion test. Highly filling ATH (>40%wt) reduces the number of low unit silicone oligomer precursors which promote dry-band arcing as well as the presence of residual carbon which leads to carbonization. Chemical modifications of water vapor liberated from heated ATH to methyl groups, which occurs at the thermal decomposition temperature of silicone rubber, were found to result in the above process. The protection mechanisms of ATH for the tracking and erosion of HTV-SIR are formulated herein.

Tracking and erosion of HTV silicone rubber and suppression mechanism of ATH

Composite insulators are considered mature products and a broad range of insulator types is nowadays available, including line insulators, hollow-core insulators and apparatus insulators, for applications ranging from distribution voltages to the highest transmission voltages. This development has created a need to elaborate diagnostic methods and tools for determining the quality of composite insulators. A review on recognized insulator faults and methods for their detection is provided.

Diagnostic Methods for Outdoor Polymeric Insulators

This paper deals with the analysis of leakage current and phase angle characteristics of porcelain and silicone rubber insulator in order to develop a better diagnostic tool to identify the pollution severity of outdoor insulators In this work, laboratory based pollution performance tests are carried out on porcelain and silicone rubber insulator under ac voltage at different pollution levels and relative humidity conditions with sodium chloride as a contaminant Multi resolution signal decomposition (MRSD) using discrete wavelet transform (DWT) is employed to understand the time-frequency characteristics of the leakage current signal Fast Fourier transform (FFT) spectral analysis is adopted to calculate the phase angle values of the applied voltage and leakage current signals Reported results on porcelain and silicone rubber insulators show that the pollution severity of outdoor insulators could be identified from the DWT STD-MRA (standard deviation multi resolution analysis) distortion ratio pattern analysis of leakage current signals

Investigations on leakage current and phase angle characteristics of porcelain and polymeric insulator under contaminated conditions

A new laboratory test for evaluating the tracking and erosion performance of HV outdoor polymeric insulating materials is described. The materials evaluated include various formulations of HTV (high temperature vulcanized) silicone rubber and polyolefin polymers. The test is based on combining some features of the ASTM D2132 DF (dust and fog) test and the ASTM D2303 IP (inclined plane) test. The new test employs IP test geometry, IP test equipment, and IP specimen plaques. The plaque is coated with a mixture of clay and salt similar to the contaminant of the DF test and identical to the contaminant used in the IEC clean fog test to rate ceramic insulators for use in contaminated environments. A liquid contaminant with a conductivity similar to that of the DF test is applied to the test specimen in the same way as in the IP test. Data collected from the field on the maximum concentration of insoluble and soluble ionic materials on surfaces of contaminated insulators which have been for 21 years in contaminated regions, provide a basis for choosing the minimum concentration of the solid contaminant to apply to the specimens. This choice also serves to define, at least tentatively, the geographical area where the results of this test have significance. It is expected that this test could be used also for screening materials and obtaining a relative ranking of the tracking and erosion resistance of various materials. Measurements of the leakage current via a computerized data acquisition system, and the discharge activity with a high speed camera were performed, and have resulted in a better understanding of the onset of material degradation.

A laboratory test for tracking and erosion resistance of HV outdoor insulation

This paper describes the identification of a common mold (i.e., fungus) growing on nonceramic insulators (NCIs) in service, and analyzes its impact on the electrical performance. The insulators evaluated were line posts removed from a 138-kV transmission line serving the interior (away from the coast) regions in Florida. The insulators had housings made from different types of silicone rubber (SR) and ethylene propylene rubber (EPR) polymer families. Electrical tests performed include surface resistance measurements and determination of contamination withstand capability (CWC) using the clean-fog procedure. Changes in the CWC were correlated to material changes by using micro-Fourier transform infrared (FTIR) analysis and surface resistance measurements. It was concluded that the mold growth is dependent on material formulation and outdoor environment. On SR insulators that exhibited mold growth, there was a reduction in the CWC when compared to the same insulator without the mold; however, even the reduced level of CWC was superior to that obtained on similarly rated EPR and porcelain insulators without any visible mold growth. Lastly, simple methods to remove the mold from the insulators are listed.

Mold growth on nonceramic insulators and its impact on electrical performance

The results of an investigation concerning a mechanism of brittle fracture in glass reinforced plastic (GRP) rods used in non-ceramic insulators (NCI) are presented. Commercial grade GRP rods and GRP rods from actual insulators were exposed to ultra-pure water (UPW) and acids while being subjected to mechanical stresses. The experimental results revealed that water has the potential of inducing stress corrosion cracking on the fibers and hence brittle fracture in the rods. It is observed that the fracture proceeded faster when the rods were exposed to UPW than when exposed to acids. Furthermore, a brittle fracture in an epoxy cross-arm, which was installed in a region where the formation of acids in the atmosphere can be neglected, is analyzed. Based on these evidences, it is postulated that the failure of in-service NCI in the brittle mode can occur under the influence of water and mechanical stresses, and that the failure is more likely to happen with water than with acids.

Mechanism of brittle fracture in nonceramic insulators

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.

Mechanical aspects of crimped glass reinforced plastic (GRP) rods

This paper describes the identification of a common mold (i.e., fungus) growing on nonceramic insulators (NCI) in service, and analyzes its impact on the electrical performance. The insulators evaluated were line posts removed from a 138 kV transmission line serving the interior (away from the coast) regions in Florida. The insulators had housings made from different types of silicone rubber (SR) and ethylene propylene rubber (EPR) polymer families. Electrical tests performed include surface resistance measurements and determination of contamination withstand capability (CWC) using the clean-fog procedure. Changes in the CWC were correlated to material changes by using micro-Fourier transform infra-red (FTIR) analysis and surface resistance measurements. It was concluded that the mold growth is dependent on material formulation and outdoor environment. On SR insulators that exhibited mold growth, there was a reduction in the CWC when compared to the same insulator without the mold; however, even the reduced level of CWC was superior to that obtained on similarly rated EPR and porcelain insulators without any visible mold growth. Lastly, simple methods to remove the mold from the insulators are listed.

J. Montesinos

Papers

A laboratory test for tracking and erosion resistance of HV outdoor insulation

Mold growth on nonceramic insulators and its impact on electrical performance

Mechanism of brittle fracture in nonceramic insulators

Mechanical aspects of crimped glass reinforced plastic (GRP) rods

Mold growth on nonceramic insulators and its impact on electrical performance