Fractal phenomena. Dependence of hydrophobicity on surfaceappearance and structural features of SIR insulators

This paper reports the study of the dependence of hydrophobicity on the surface appearances and inherent chemical structure of the high-temperature vulcanized silicone rubber (HTVSR) insulator based on the fractal geometric principle. The fractal phenomena link the surface roughness with the hydrophobicity characteristics of silicone rubber (SIR) insulators. The rough surfaces of the insulators, no matter whether due to salt-fog aging or roughening by sandpaper, show the property of a fractal surface. According to the amount of hydrophobic material on the surface, surface roughness exerts different effects on the hydrophobicity of SIR insulators. Due to the behavior of fractal surfaces, the hydrophobicity of the appropriately roughened insulator can be enhanced, but depressed if dipped in hexane. Outdoor polymer insulating materials could be aged by environmental stresses and contaminants so that their surfaces become rough. The permanent change of surface roughness could be an indicator of aging state of the insulation material. The hydrophobicity of SIR insulators aged in salt fog in a fog chamber are investigated. During this aging, the inorganic filler and the hydrophobic groups in SIR are worn away so that the insulator surface becomes rough. The surface of the aged insulator becomes hydrophilic. After a subsequent rest, the hydrophobic polymer liquid in bulk diffuses and migrates to the insulator surface, which becomes more hydrophobic. Such a unique dependency of hydrophobicity on the surface appearance and chemical structure of SIR material is available for SIR insulator to maintain good contamination performances in outdoor 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.     

Flashover mechanism of silicone rubber insulators used for outdoorinsulation-I

Experimental results show that the contamination performance of silicone rubber composite insulators is better than porcelain insulators. This is attributed to the hydrophobicity of the silicone rubber. This suggests that the flashover mechanism of these insulators is different. This paper experimentally investigates contamination build-up, diffusion of low molecular weight (LMW) polymer chains, surface wetting, electrical field caused interaction between droplets, generation of conductive regions and filaments, field intensification caused spot discharge and the ultimate flashover of silicone rubber along wetted filaments. The studies resulted in a new flashover mechanism, which provides a better understanding of the superior performance of silicone rubber outdoor insulators. The results will be useful for the development of improved laboratory test procedures and to evaluate composite insulators' performance in contaminated conditions. The part of the paper describes an overview of the flashover mechanism and the diffusion of LMW polymer chains and wetting of the silicone rubber surface in detail     

Flashover of contaminated nonceramic outdoor insulators in a wetatmosphere

This paper deals with the flashover mechanism and performance of contaminated nonceramic (also known as composite and polymeric) outdoor insulators under wet conditions. Two types of wetting that occur in service, namely fog and rain, were considered. Different profiles of line insulators having weathersheds made from material families such as, high temperature vulcanizing (HTV) silicone rubber (SR) and ethylene propylene (EP) diene monomer rubber, were evaluated. The results demonstrate that the performance, and hence the relative ranking of nonceramic line insulators is influenced by the wetting mode and insulator geometry; that flashover of contaminated insulators can occur with rain at a much lower contamination severity than with fog; and that the mechanism of contamination flashover under rain is significantly different from that under fog condensation. New insulator profiles have been identified and tested to show that they can provide improved flashover performance, when compared to existing designs. This work proves that a comprehensive evaluation of outdoor insulator performance can be obtained by performing flashover tests that include more than one form of wetting, and hence illustrates the limitations of current standard contamination tests that employ only one form of insulator wetting     

Electrical performance of RTV silicone rubber coatings

The role played by two major constituents in a room temperature vulcanized (RTV) silicone rubber coating, namely, the inorganic alumina trihydrate (ATH) filler and the silicone polymer, on the coating's electrical performance was examined. The principal variable in the RTV silicone rubber coatings evaluated was the different weight fraction of the silicone polymer and ATH inorganic filler normally used for improving the tracking and erosion resistance. Other formulation details were essentially the same in the cured rubber. The coatings were spray coated on glazed porcelain rods and subjected to accelerated aging in a salt fog chamber. The changes produced by accelerated aging on the contamination withstand capability, leakage current suppression and erosion resistance were determined and correlated with the ratio of polymer to inorganic filler in the coating. It is shown that critical aspects of electrical performance necessary for satisfactory service operation such as contamination withstand capability before flashover and leakage current suppression are superior for the coating with a relatively high ratio of polymer to inorganic filler in comparison to the coatings with a relatively low ratio of polymer to inorganic filler. While all the coatings showed a high erosion resistance under mild discharge activity, the erosion resistance was found to be superior for the formulations with high inorganic filler under conditions of intensive surface discharge activity     

Electrical and environmental stress and the hydrophobic stabilityof SIR, EVA and their blends

This paper treats the effects of electrical and environmental stresses on the hydrophobicity of polymers intended for use in nonceramic insulators. We investigated the effect of UV radiation, corona discharges, dry-band arcing, acid rain and water absorption on hydrophobic transfers in, and the aging of, room temperature vulcanized (RTV)-silicone rubber (SIR), high temperature vulcanized (HTV)-silicone rubber (SIR), ethylene vinyl acetate (EVA) rubber and the blends made from HTV-SIR and EVA. Acid rain had little influence on hydrophobicity in any of the samples. EVA and the blends with low silicone ratios to EVA allowed much larger reduction in hydrophobicity after or while subjected to the above stresses (except for acid rain) than did the silicone rubbers and a blend with a high silicone ratio to EVA. Salt-fog tests were conducted to correlate the hydrophobic stabilities of materials with leakage current levels thereon. The hydrophobic stability of the silicone rubbers was attributed to low-molecular-weight silicone components that were able to migrate and finally cover the aged surface layers. Because EVA possessed less fluid components and less chemical stability under electrical and environmental stresses, its hydrophobicity was readily reduced and hardly recovered, which led to higher current leakage and early material aging. For each material, the critical hydrophobic level permitting leakage current to develop was determined by adding a non-ionic wetting agent to saline water     

Low-molecular weight silicone fluid in RTV silicone rubber coatings

Coatings of room-temperature vulcanized (RTV) silicone rubber (SIR) are used increasingly on outdoor HV porcelain and glass insulators and bushings when subjected to heavy pollution. An important property of the RTV-SIR coating is its ability to restore the hydrophobicity of the surface after a layer of contamination has built up on it, and thus continue to suppress the development of leakage current, dry band arcing and flashover. This is accomplished by the diffusion of low molecular weight (LMW) silicone fluid from the bulk to the surface of the coating and then to the top of the contamination layer. The presence of sufficient LMW silicone fluid in the bulk of the coating is a critical factor in both the recovery of hydrophobicity and its long term maintenance under wet and polluted conditions. This paper reports on several factors which affect the level of LMW silicone fluid present in RTV-SIR coatings. Different formulations of the RTV-SIR were used. The percentage weight of the LMW silicone fluid was determined in RTV-SIR coatings as a function of the size of the particles of the alumina trihydrate (ATH) filler which was varied from 1 to 75 μm, the concentration of the ATH filler from 50 to 150 pph of the base silicone rubber, the addition of 10%wt silicone fluid to the formulation of RTV-SIR, the carrier solvents of 1,1,1 trichloroethane and naphtha, exposure to continuous wind at a speed of 7.45±0.25 km/h for 22 months, exposure to stationary air for 14 months, immersion in low salinity water simulating rain (70 μS/cm) for 14 months and tests in energized low conductivity salt-fog. The mechanisms responsible for the observed phenomena are suggested and discussed     

Flashover mechanism of non-ceramic insulators

The contamination performance of non-ceramic (NC) insulators is better than porcelain insulators. The paper describes the pollution collection mechanism and concludes that silicone rubber insulators collect more pollution than porcelain insulators. Long term exposure of silicone rubber insulators produces a thin layer of pollution, which is a mixture of dust, salt and silicone oil. Fog or morning dew produces droplets on the flat surfaces and forms conductive regions. Spot discharge starts between the regions, which reduces hydrophobicity. Simultaneously, dry-band arcing starts on the shank of the insulator. The two arcs join together, which leads to flashover. The flashover voltage of polluted NC insulators is significantly higher than porcelain ones. Insulator performance is measured with laboratory tests. However, salt-fog and clean-fog tests can give different flashover values     

大气环境对合成绝缘子的憎水性影响分析

通过模拟不同的气候条件,试验分析了大气环境中影响合成绝缘子憎水性的各种因素,结果表明:分离水珠影响了硅橡胶表面的电场分布,局部小电弧导致了硅橡胶表面憎水性能的下降;合成绝缘子的憎水性随污秽层等值盐密的升高而降低,随污秽层附着时间的增长而增强,且酸性物质对合成绝缘子憎水性的影响较碱性物质影响小;低温特别是覆冰条件下,合成绝缘子的憎水性将减弱甚至丧失;随温度的升高,小分子聚合物挥发速度加快,硅橡胶的憎水性能越强,其老化速度加快。     

运行后复合绝缘子的理化分析

为了获得复合绝缘子在运行后老化的状况,选取了3个厂家生产的6支绝缘子试样,分别对这6支试样进行了憎水性测试、Fourier红外光谱分析、热分析和扫描电镜(SEM)检测。结果表明:硅橡胶表层的憎水性基团Si-CH3、-CH3各自与-Si-O-Si-基团的摩尔比明显低于中心层,而-OH未见明显规律;憎水性基团Si-CH3、-CH3摩尔比的相对变化能较好地表征憎水性,而-OH不宜作为复合绝缘子的憎水性表征指标。新复合绝缘子表层和中心层的组成基本相同。投运后复合绝缘子表层和中心层中无机化合物的质量规律性不明显;表层及中心层中高分子物质的质量随运行时间的延长而呈下降趋势,且中心层中高分子物质的质量高于表层;表层中低分子物质的质量高于中心层。投运时间对试样的扫描电镜结果具有一定的影响。新复合绝缘子试样喷水后的憎水性级别为HC1,运行后试样的憎水性级别处于HC3～HC4,同一复合绝缘子试样中不同伞裙、不同位置的憎水性也略有不同。研究结果对复合绝缘子老化评估具有参考价值。     

Flashover mechanism of silicone rubber insulators used for outdoorinsulation-II

Flashover mechanisms of silicone rubber used for outdoor insulation is presented and experimentally proven. A hydrophobic surface of silicone rubber wets and accumulates contaminants differently compared to conventional porcelain and glass insulators. This suggests that the flashover mechanism of silicone rubber insulators is different than porcelain and glass. This part of the paper experimentally investigates ohmic heating, electrical field caused interaction between water droplets, generation of conductive regions and filaments, field intensification caused spot discharges and the ultimate flashover of silicone rubber along wetted filaments. These studies have resulted in a new flashover mechanism which provides a better understanding of silicone rubber outdoor insulators and their excellent contamination performance. These results will be used for the development of a better laboratory test procedure to evaluate composite insulators' performance in contaminated conditions     

电力外绝缘用硅橡胶清洗修复剂制备及性能研究

硅橡胶广泛应用于电力行业的复合绝缘子和防污闪涂料,然而长时间工况运行后,容易发生老化粉化、污秽沉积,降低了表面憎水性。电网中目前有上亿支硅橡胶绝缘子服役多年,研究能够清除表面污秽,修复硅橡胶老化层的新型清洗剂迫在眉睫。本文探究不同非离子表面活性剂含量和修复剂含量对清洗效率和憎水性的影响。实验表明非离子表面活性剂含量在3%时,洗净率最高达到95%。而修复剂含量为11%时,憎水性最好,其憎水迁移性最佳可以达到HC1级。通过扫描电镜分析,发现清洗后硅橡胶表面微孔和裂缝消失,表面平整,表明该清洗剂具有去除老化层和污秽,修复老化绝缘子的效果。这为继续研发硅橡胶清洗修复剂,延长绝缘子服役期限提供一定的理论和实践基础。     

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     

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     

紫外线老化对复合绝缘子硅橡胶憎水性的影响

硅橡胶复合绝缘子的优良耐污闪性能源自于硅橡胶材料的憎水性以及憎水迁移性。本文研究了紫外老化对硅橡胶憎水性影响以及硅橡胶表面积污后的憎水迁移性。研究结果表明：经小分子处理的硅橡胶经过2 000 h紫外线辐射后仍能够保持良好的憎水性,静态接触角仍能达到102.3°,憎水性等级为HC2级;硅橡胶上的污层在较短时间内具有了憎水性,憎水性等级能达到HC2~HC3。并对硅橡胶紫外老化机理以及憎水迁移性机理进行了分析。     

特殊工业粉尘地区复合绝缘子自然积污特性研究

硅橡胶复合绝缘子具备机械强度高、防污闪性能优异、质量轻、运行维护方便等优点,在电网建设中应用广泛。在部分工业化工区,存在特殊工业粉尘污染,该类地区运行的复合绝缘子表面更容易积污,绝缘子表面在积污后憎水性显著降低,绝缘性能明显减弱。为研究特殊工业粉尘地区运行的复合绝缘子自然积污特性,文中对江苏某工业园区运行的复合绝缘子进行取样,使用污秽度与微观分析结合的方法对积污特性进行研究。结果表明,该地区运行的复合绝缘子普遍表面污秽度较重,污秽可溶物等值盐密(ESDD)数值较高,且污秽中存在的不溶灰分主要元素为碳(C)、氧(O)、硅(Si),含碳粉、二氧化硅(SiO 2)、氧化钙(CaO)等影响绝缘子表面导电性及吸水性的物质。文中所提方法能较好地揭示特殊工业粉尘地区的污秽特性。     

Effect of specimen configuration on deterioration of silicone rubber for polymer insulators in salt fog ageing test

The paper investigates the effects of specimen configuration on the deterioration of silicone rubber insulators in a salt fog ageing test. A cyclic salt fog ageing test was conducted on three types of high-temperature vulcanized silicone rubber (HTV SiR) specimens: two types of insulators having sheds, and plain rods without sheds. The test was based fundamentally on the CIGRE WG 15-04 specifications. After 50 test cycles, the three types of specimens showed different degrees of surface deterioration. Erosion on the straight shed insulator was more severe than that on the alternate shed insulator even though the two had the same leakage distance. No erosion was observed on the rod-type specimens, which had shorter leakage distances than the insulator-type specimens.     

不同伞形结构复合绝缘子的交流污闪有效爬电距离

为得出高海拔、低气压下污秽地区不同伞形结构复合绝缘子的爬电距离有效利用率,在大型多功能人工气候室内对高海拔、低气压条件下干弧距离比较接近的4组不同伞型结构的复合绝缘子及一张硅橡胶平板开展了污闪试验研究。根据真实复合绝缘子的污闪梯度与硅橡胶板的污闪梯度之比,可求得复合绝缘子的爬电距离有效利用率k。根据k及爬距与干弧距离之比(δCF系数),可计算得到电弧中间隙电弧的比例k1。分析了复合绝缘子污闪过程中k、k1的影响因素。结果表明,大小伞形δCF系数为3.55的复合绝缘子在海拔不超过2 500 m时爬电距离的利用率较高,当海拔升高到4 000 m时,大小伞形δCF系数为3.27的复合绝缘子具有较高的爬电距离有效利用率。爬电距离有效利用率与与盐密、气压均呈幂指数关系。     

高聚物复合绝缘子紫外老化性能研究

传统高温硫化（HTV）硅橡胶复合绝缘子以其优异的憎水性和憎水迁移性,广泛应用于国内外输电线路上。但随着特高压工程的开展,越来越多的输电线路需要跨越强紫外辐射、强风沙以及鸟啄频发地区,而在这类气候恶劣地区极易造成传统HTV硅橡胶绝缘子外绝缘性能的下降以及材料的损伤。针对这一情况,采用耐紫外线、耐气候性能强,机械性能稳定的高聚物复合绝缘子与传统HTV硅橡胶绝缘子在紫外老化性能方面进行对比,旨在研究高聚物复合绝缘子在西北高原强紫外辐射地区的老化特性。研究结果表明：在短波紫外光（UVC）持续照射下,环氧材料的拉伸强度和硬度均远高于硅橡胶,具有更好的韧性,已达到防鸟啄和耐踩踏的目的,但憎水性相比较于硅橡胶材料仍略有不及,但仍满足户外运行要求。     

硅橡胶复合绝缘子运行寿命的试验研究

为评估运行中硅橡胶复合绝缘子安全寿命,提高供电可靠性,从上海电网中实际运行的线路上,选取运行年限>10a的110kV硅橡胶复合绝缘子作为试验研究样品绝缘子,通过对其表面特性及工频和冲击电压的试验研究,并辅助硅橡胶材料的微观试验,分析和探讨硅橡胶复合绝缘子运行寿命特性、电气和机械性能的综合因子老化规律,评估运行中硅橡胶复合绝缘子安全运行状态。研究结果表明:在硅橡胶材料表面特性正常的情况下,其工频和冲击电压试验的干、湿闪电压较接近并与外绝缘结构有关;当硅橡胶材料表面特性老化时,其干、湿闪络电压出现差值,表现出与材料表面特性有关。