Effect of long-term corona on non-ceramic outdoor insulator housingmaterials

The influence of long-term corona on the degradation of non-ceramic composite insulator (NCI) housing materials was studied. Polymer samples were obtained from full-scale NCI, employing three different housing materials, namely silicone rubber, ethylene propylene diene monomer rubber and a blend of EPDM with silicone rubber. The effect of relative humidity and externally applied mechanical stress as acceleration factors was assessed. Several modes of degradation were observed, namely, cracking, roughening, and discoloration. In general the severity of degradation increased with the relative humidity level and application of mechanical stress. Periodic monitoring of corona discharge magnitude and pulse repetition rate allowed for a simplistic computation of the energy needed for initiation of surface changes. A comparison of these calculated values with expected conditions in the field was made, It is postulated that even under severe operating conditions over a period of many years, degradation of the housing materials evaluated due to water droplet corona is highly unlikely     

Extending the applicability of insulator flashover models by regression analysis

A combination of experiments and regression analysis has been utilized to extend the useful range of models for estimating flashover performance of insulators under contaminated conditions. Experiments were performed on distribution class insulators employing porcelain, silicone rubber and ethylene propylene diene monomer (EPDM) rubber as housing materials. It has been shown that it is possible to include important factors such as hydrophobicity, aging and contamination accumulation in the assessment of insulator performance. The results from this work are useful for selecting the appropriate insulator design (dimensions and material) for different system voltages     

Leakage current and flashover of field-aged polymeric insulators

In this paper results from leakage current (LC) measurements under steam fog and voltage withstand tests under rain conditions obtained on various field aged polymeric insulators are reported. Included in this work are several silicone rubber (SIR) and ethylene propylene diene monomer (EPDM) rubber insulators of ac and dc design that have been energized at suitably high ac and dc voltages under coastal conditions for a period of more than nine years. Their performances are compared to two sets of identical insulators, namely one that was outdoors exposed without being energized and one that was stored indoors. The LC of each insulator was studied three times, namely as received from the field, after it was gently washed to remove soluble pollution, and finally after a voltage withstand test. In between the measurements, the insulators were left to recover for a period of a week. The results show that the LC of the SIR insulators were generally lower than those of the EPDM. In both cases, the LC of the insulators that were stored indoors were significantly lower than those of the outdoor exposed insulators. The voltage withstand tests revealed an almost linear relation between the flashover voltage (FOV) and the arcing distance of the insulators. In general, for similar SIR and EPDM insulators the FOV of the SIR ones were higher than those of EPDM insulators     

Quantification of corona discharges on nonceramic insulators

This paper attempts to establish a correlation of the visual images of corona obtained from a camera with discharge magnitude measured with conventional partial discharge equipment. A linear relationship is shown between the transformed image parameters and the discharge magnitude, thereby providing a means for quantifying corona observations made during routine maintenance inspections, of insulators from ground. Different insulator designs using silicone rubber (SIR) and ethylene propylene diene monomer (EPDM) housings were examined. The effect of fog has been examined by performing experiments inside a fog chamber. It is shown that this information can be used along with the corona degradation characteristics of housing materials to identify discharge patterns that can pose problems to the integrity of the insulator.     

Characterization of a severely degraded silicone elastomer HV insulator-an aid to development of lifetime assessment techniques

A high voltage composite insulator showed severe degradation and failure of the silicone elastomer sheds following service in a substation environment. Sensitive analytical techniques were used to investigate the extent of degradation of the silicone elastomer (SiE). The polymer content of some surface regions decreased as degradation increased. This was quantified through observation by scanning electron microscopy (SEM), weight loss upon heating using thermogravimetric analysis (TGA), determination of surface "silica" levels in damaged areas by X-ray photoelectron spectroscopy (XPS) and Raman mapping through a cross-section of the elastomer producing a degradation depth profile. It is believed that due to the environmental conditions at the site of installation, the electric field may have reached corona onset threshold on parts of the elastomer surface. Because of the moist conditions the corona was maintained, effectively preventing the elastomer from regaining hydrophobicity and leading in time to severe degradation. This investigation has also identified techniques applicable to the assessment of condition of other, less damaged SiE insulators which may in turn lead to the prediction of the remaining useful lifetime.     

Quantification of degradation in nonceramic housing materials by laser ablation

A laser irradiation technique has been explored to quantify visible as well as invisible degradation of housing materials used for nonceramic insulators. Nonceramic insulators that were removed from service and new samples of the housing that were subjected to accelerated testing in the laboratory have been evaluated. The method uses a constant power laser source for delivering the same energy to the test specimen. The proposed method is fairly quick and could be looked upon as an additional method for characterizing aging of nonceramic insulators. It has been shown that this method provides a better characterization of aging than is obtained from Fourier Transform Infrared Spectroscopy. The housing materials evaluated include silicone rubber, ethylene propylene diene monomer rubber and cycloaliphatic epoxy resin.     

Charging Characteristics of EPDM and Silicone Rubbers Deduced from Surface Potential Measurements

Surface potential dynamics on corona charged surfaces of thick samples of EPDM and silicone rubbers have been studied and analyzed by examining the so called tdV/dt characteristics. This methodology allows for directly obtaining information about charge trap distributions in materials from measurements of potential decay without a-priori knowledge about trap parameters. More importantly, the combination of the above mentioned methodology in conjunction with measurements of other material parameters may provide information desired when analyzing the surface charge behavior on composite insulators for HVDC applications.     

Effect of Hydrophobicity on Electric Field Distribution and Discharges Along Various Wetted Hydrophobic Surfaces

According to hydrophobicity and hydrophobicity transfer studies on silicone rubber surfaces, although the silicone rubber can transfer its hydrophobicity to various contaminants deposited on its surface, the hydrophobic states of water droplets on clean silicone rubber surfaces and on various contaminated silicone rubber surfaces were found to be different based on evaluation by contact angle measurement. Furthermore, surface discharge and aging of materials decreases the hydrophobicity of the surface. Focused on the difference of hydrophobicity and the effect of water droplets on the hydrophobic contaminated surfaces, some contrastive computations and tests of water droplets on various surfaces were carried out in this study. The results showed that the water droplets existing on the surface of silicone rubber materials might change the electric field and voltage distribution along the surface, and even reduced the initial voltage of the corona discharge, which was verified with a comparison test on a high voltage side of polymer insulator monitored by a photomultiplier tube (PMT) system. Since the leakage current provides much useful information to diagnose the state of outdoor insulation, this study investigates the time and frequency distributions of surface discharges on ceramic insulators, new room temperature vulcanized (RTV) silicone rubber coated insulators and aging RTV insulators with various hydrophobic states with the short time Fourier transform and Gabor transform methods. Then a correlation was found between the distribution of the spectrum of surface discharges and hydrophobicity via the joint time and frequency analysis of discharges on various hydrophobic surfaces.     

Electrical performance of non-ceramic insulators in artificialcontamination tests. Role of resting time

The paper presents the results of an investigation on the electrical performance of artificially contaminated non-ceramic insulators as a function of resting time. Resting time is defined as the time interval between the application of contamination and start of testing. New (unaged), full scale, 69 kV non-ceramic insulators using different types of silicone rubber and ethylene propylene diene monomer (EPDM) rubber as weathersheds were studied. Their electrical performance was assessed using the clean fog technique. The applied voltage was kept constant throughout the test, while contamination severity and resting time were varied as parameters. The transfer of hydrophobicity through the contamination layer was demonstrated by flashover tests. This was also visually depicted by analyzing samples in a scanning electron microscope. The results show that resting time exerts little influence on the electrical performance of EPDM insulators. However, for silicone rubber insulators, resting time is shown to drastically improve its electrical performance. Experimental evidence indicates that the transfer or recovery process of hydrophobicity in silicone rubber insulators occurs as a progressive superposition of silicone oil layers with time. Thus, the net effect of resting such insulators before tests can be thought of as a gradual reduction of the effective contamination layer thickness. Such a reduction alters the way in which the contaminant layer interacts with external stressing agents, which could impact the insulator's electrical performance in service. In this work, an attempt has been made to identify and quantify the electrically significant changes introduced by a reduction in the effective contamination thickness. It is in the light of this reduced effective contamination layer that we explain how a seemingly wettable silicone rubber insulator is still able to hold the applied voltage without flashover     

Monitoring the condition of insulator shed materials in overheaddistribution networks

Methods of sampling and analyzing the surface material of sheds of composite insulators are reviewed. It is shown that scanning electron microscopy gives useful information from slivers cut from the insulator surfaces, and emission Fourier transform infrared (FTIR) spectroscopy enables derivation of indicators of material oxidation to be developed from surface swabbing with solvents. FTIR absorption spectroscopy and diffuse reflectance FTIR spectroscopy give indicators of surface chalking for ethylene propylene diene monomer (EPDM) insulators. X-ray photoelectron spectroscopy is used to determine surface layer elemental composition and concentration of oxidized bonds. The techniques are applied to field aged 275 kV EPDM and silicone rubber insulators and tentative correlations developed between surface condition and leakage current. Cluster analysis of results from sampling of a large population of EPDM insulators shows that the surface analysis methods facilitate grouping by location and manufacturer     

Contamination performances of silicone rubber insulator subjectedto acid rain

Acid rain causes one of the more severe environmental contaminants, and there is an increasing interest in the degradation of outdoor polymer insulating materials in severe pollution conditions such as acid rain. The performance of high temperature vulcanizing (HTV) silicon rubber insulators in accelerated aging of artificial acid rain is investigated by the salt fog method. Experimental results and theoretical analysis show that acid rain exerts a large erosion effect on the silicone rubber insulator. The inorganic filler alumina trihydrate (ATH) in the silicone rubber easily dissolves in acid rain, and the increasing discharge heat from dry band arcing due to acid rain can further cause the loss of ATH. The hydrophobic groups of hydrocarbon branch (ie. methyl in silicone rubber), are also degraded in acid rain. Therefore, the surface of aged insulators becomes rough, they become less hydrophobic, and the surface discharge current increases. However, in a subsequent dry environment, the low molecular weight (LMW) polymer silicone liquid or oil in the bulk of the insulator diffuses and migrates to the surface to provide a hydrophobic layer so that the degraded surface of silicone rubber insulator is compensated, and the contamination performance is restored to some extent. Such recovery of contamination indicates why silicone rubber insulators can be used for long times under severe environmental stress     

复合绝缘子沿面电晕放电特性的试验研究

为了研究复合绝缘子沿面电晕放电状况,以110kV复合绝缘子为试验对象,利用一种基于光电倍增管的紫外电晕检测装置来测量绝缘子电晕产生的紫外光,探讨复合绝缘子沿面电晕放电发展和分布特征。通过逐步升高施加电压的试验方法,检测得到复合绝缘子在不同表面条件下(干燥、清洁、潮湿、染污)沿面电晕放电发展和分布状况的脉冲量化特征数据。测量结果表明:当电压升高时,表面干燥条件下的复合绝缘子电晕放电沿串长发展和分布,而表面潮湿条件下的复合绝缘子电晕放电随机性较大。电晕紫外光脉冲个数以及脉冲平均峰值可以用作检测电晕放电的特征参数,以判定复合绝缘子电晕放电状态。     

Properties of Interfaces between Silicone Rubber and Epoxy

An experimental set-up was designed for testing the impact of adhesion defects at interfaces between silicone rubber and epoxy in composite insulators. Test samples were aged in this set-up by discharges appearing under a presence of tangential electric field at the interfaces exposed to humid conditions. This combination of stresses simulated realistic conditions, in which ageing at the interface could possibly accelerate further deterioration of the adhesion between silicone rubber and epoxy. The test samples were prepared by joining both the materials with and without application of primer. Without the primer, bonding between epoxy and silicone rubber was reduced. The interfacial strength after the ageing was investigated by means of a modified peel test, whereas the changes of the dielectric properties were assessed by dielectric response measurements. Furthermore, the resulting degradation was analysed with infrared spectroscopy and optical microscopy. The analyses showed that the combined stresses affected to some extend both the epoxy and the silicone rubber. However, the interfacial mechanical strength was not reduced significantly, which is an indicator for possibility to tolerate existence of small interfacial defects in real composite insulators.     

Sudden flashover of nonceramic insulators in artificialcontamination tests

This paper presents experimental and theoretical results to explain the phenomenon of sudden flashover on artificially contaminated nonceramic insulators. A sudden flashover in this paper is defined as a flashover occurring on a partially contaminated insulator, and which is preceded by little or no surface discharge activity. Sudden flashovers could occur in the field on those insulators where the wet surface resistance varies over a wide range along the insulator length. The mechanism of sudden flashover is significantly different from conventional contamination flashover that occurs in standard contamination tests. The insulators evaluated had weathersheds made from silicone and ethylene propylene diene monomer (EPDM) rubbers. The impact of the present study from the point of view of weathershed material, insulator design, development of a comprehensive laboratory test and field monitoring for evaluating the performance of nonceramic insulators in service are discussed     

In-service aging and degradation of 345 kV EPDM transmission line insulators in a coastal environment

Aging and degradation of 345 kV ethylene propylene diene monomer (EPDM) transmission line insulators removed from service is presented. These suspension type insulators were installed in a New Hampshire coastal area in 1995 and were removed in 2000 after unexplained outages in that structure. The purpose of this paper is to better understand the aging and degradation mechanisms of EPDM insulators in service. The insulators showed severe chalking and discoloration and partial loss of hydrophobicity on the side facing the sun. The surface structural changes were studied in detail using advanced surface analysis techniques, such as attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). For the first time, the significant differences in surface properties between the chalked/discolored (white) and the other surfaces (dark) were studied quantitatively. The Fourier transform infrared (FTIR) absorption spectra showed a significant decomposition of the CH groups of the white surface, elucidating the effect of photo-oxidation on the EPDM polymer. The SEM micrographs showed the cracking of the surfaces. The XPS spectra showed the formation of various polar carboxyl groups and the presence of high surface energy compounds, such as silica, and silicates. This study provided valuable basic information on the changes in the surface properties of EPDM insulators during service in a coastal environment.     

Electrical Insulation Characteristics of Silicone and EPDM Polymeric Blends. I

The typical parts of a polymeric insulator are core, metal end fittings and polymeric housing material. The housing is intended to protect the fibre glass rod from the environment and electrical surface discharges. Since the housing materials are made of organic polymeric material, its insulation characteristics need to be studied. Amongst the many different polymers available, this work focuses on silicone rubber and ethylene propylene diene monomer (EPDM). Blends of EPDM and silicone rubber are prepared in a two roll mixing mill. Dicumyl peroxide is used as vulcanizing agent. The blends consisting of various proportions of component polymers are prepared, compression moulded into sheets, and post cured. The blends are tested for their insulation characteristics as per IEC and ASTM standards. Volume and surface resistivity, dielectric strength, dielectric constant, tan delta, tracking resistance, arc resistance, comparative tracking index, tensile strength, and percentage elongation at break of the blends are studied and discussed. The test results show that the increasing proportion of silicone enhances the electrical insulation properties whereas increasing weight percentage of EPDM improves the mechanical strength of the blends.     

On the breakdown strength of silicone rubber

Polymeric materials are an alternative to porcelain and glass regarding outdoor HV insulators. Silicone rubber appears to be one of the main candidates to replace porcelain and glass. Although much work has already been done regarding the behavior of complete polymeric insulators in the laboratory and in the field, relatively little is known with respect to the breakdown strength of silicone rubber. In this paper the breakdown strength of silicone rubber with respect to gap spacing and the electrical behavior of built-in interfaces inside the silicone rubber are investigated     

Aging of polymeric and composite insulating materials. Aspects of interfacial performance in aging

Aging of polymeric and composite materials is reviewed mainly from the standpoint of their interfacial performance. Insulating materials can be divided into simple polymers and composites. Polymers for power cables, transformers, insulators and rotating machines consist of thermoplastics such as PE, PET and PPS, elastomers such as silicone, EPR and EPDM, and thermosets such as epoxy. Composites for GIS, rotating machines and insulators comprise epoxy/glass, epoxy/silica/alumina, and epoxy/mica systems. Aging processes are complicated in general, and take place under simultaneous multiple stresses such as electrical, thermal, mechanical and environmental stresses. Some of the phenomena covered in this paper are associated with the degradation by tracking and erosion and the loss of hydrophobicity in case of surface properties, and with PD, electrical treeing, water treeing and combined phenomena. The quality of cable insulation such as XLPE has been improved extensively from the standpoint of design electric strength. Interfacial problems will emerge for modification of cable joints. It is expected that polymers for outdoor use and filled epoxy resin systems should be improved from their environmental stability and from their design stress enhancement, respectively.     

高海拔地区硅橡胶合成绝缘子电晕老化特征分析

模拟高海拔低气压环境,采用多针-板电极放电模型产生电晕电场,分别对硅橡胶合成绝缘子试样进行老化处理,通过静态接触角法测量表面憎水性的变化,分析低气压环境对合成绝缘子电晕老化的影响,得出合成绝缘子的电晕老化速度随着气压的降低而加快等3个试验分析结果。     

Utilization of fog chambers for non-ceramic outdoor insulatorevaluation

Non-ceramic (polymer, composite) insulators consist of a fiberglass core rod, weathershed housing, and metal end fittings. The housings generally are manufactured from polymeric materials. In service, the weathershed materials may be subject to degradation or aging due to harsh environmental conditions. One of the most commonly used laboratory tests to evaluate the performance of the weathershed materials is to employ fog chambers. Design and operational features of various fog chambers are reviewed in this paper, including size and main layout of fog chambers and their HV sources, data acquisition systems, systems for simultaneous visual observations and electrical measurements, and test cycles and techniques