Submitted by Lucienne Costa (lucienneferreira@ufcg.edu.br) on 2018-04-17T14:10:59Z No. of bitstreams: 1 KAL-EL BASÍLIO BRITO – DISSERTAÇÃO (PPGEE) 2017.pdf: 3481089 bytes, checksum: 8584b99da594f1f67c9b3daa41e27564 (MD5) / Made available in DSpace on 2018-04-17T14:10:59Z (GMT). No. of bitstreams: 1 KAL-EL BASÍLIO BRITO – DISSERTAÇÃO (PPGEE) 2017.pdf: 3481089 bytes, checksum: 8584b99da594f1f67c9b3daa41e27564 (MD5) Previous issue date: 2017-03-17 / Capes / Em inspeção, com câmara sensível à radiação ultravioleta, são fornecidas apenas informações acerca do local onde estão concentradas as descargas corona e uma estimativa de intensidade dessas decargas em equipamento de alta tensão. Nesse intuito, uma metodologia de extração de atributos das fáculas de imagens ultravioletas de isoladores poliméricos de 230 kV foi desenvolvida. Para isso, foram usados vídeos captados pela câmera de detecção ultravioleta como material. Essas informações, quando adequadamente dispostas, podem servir de entrada a um sistema de auxílio à tomada de decisão que poderá classificar, finalmente, os isoladores em padrões predefinidos. Na metodologia do trabalho, desenvolveram-se várias rotinas de processamento digital de imagens para, inicialmente, adequar as imagens e, posteriormente, quantificar os atributos de interesse extraídos. As rotinas de adequação de imagens buscam segmentar o shape do isolador do plano de fundo, segmentar em uma imagem as fáculas, que representam descargas causadas pelo efeito corona, integralizar as fáculas de vários quadros em apenas uma imagem, eliminar o que vier a ser considerado ruído e segmentar o shape do isolador em partes de interesse. As rotinas de quantificação de atributos são destinadas a calcular a área, o perímetro, o fator de forma e a persistência das fáculas, além de sua distância às partes de interesse do isolador polimérico de 230 kV. Como resultado do desenvolvimento das rotinas, é apresentado um algoritmo para extração de atributos de fáculas. Finalmente, é feita uma análise de sensibilidade dos parâmetros usados no algoritmo no intuito de delimitar suas condições de uso. Dois estudos da aplicação do algoritmo foram realizados, em que, para cada um deles foram processados 64 combinações de número de quadros e de limite de ruído. Foram apresentadas as superfícies dos atributos de área, perímetro, fator de forma e persistência das fáculas em função dos dois parâmetros citados. A fim de determinar qual par ordenado (número de quadros, limite de ruído) seria adequado, adotou-se como métrica buscar o menor valor de gradiente das superfícies dos atributos. Concluiu-se que a metodologia de extração de atributos de fáculas de imagens ultravioletas de isoladores, para aprimoramento da classificação de isoladores foi desenvolvida com sucesso e a contribuição do trabalho se apresenta na metodologia para extração de atributos e na determinação do número de quadros e limite de ruído adequados para vídeos ultravioletas. / On inspection with ultraviolet sensitive camera, it is only supplied information on where the corona discharges are concentrated and an estimative of intensity of these discharges in high voltage equipment. For this purpose, it was developed a methodology of faculae attribute extraction of ultraviolet images from 230 kV polymeric insulators. In order to do this, videos captured by the ultraviolet detection camera were used as work material. The methodology of the work consists of the development of several digital image processing routines, initially, to adapt images and, later, to quantify the attributes of interest extracted. The image adaptation routines seek to segment the shape of the insulator from the background, to segment in an image the faculae that represent discharges caused by corona effect, to integrate faculae from multiple frames in only one image, to eliminate what is to be considered noise and to segment the shape of the insulator into parts of interest. The attribute quantification routines are destined for faculae area, perimeter, form factor and persistence, in addition to their distances of the 230 kV polymeric insulator parts of interest. As a result of the development of these routines an algorithm for attribute extraction is presented. Finally, a sensitivity analysis of the parameters used in the algorithm is made, with the intention to delimit its use conditions. Two case studies of the application of the algorithm were performed, for each of them 64 combinations of number of frames and noise limit were processed. Presenting the surfaces of area, perimeter, shape factor and persistence of the faculae attributes in function of the two cited parameters. In order to determine which number of frames and noise limit would be suitable, it was adopted as metric to look for the lowest gradient value of the attributes surfaces. In conclusion the methodology for extracting attributes from the UV image faculae of insulators to improve the classification of insulators was successfully developed and the contribution of the work is presented in extracting attributes and determining suitable number of frames and noise limits for UV videos.

Submitted by Lucienne Costa (lucienneferreira@ufcg.edu.br) on 2018-04-17T16:49:57Z No. of bitstreams: 1 RENATA GARCIA DUTRA DE OLIVEIRA – DISSERTAÇÃO (PPGEE) 2017.pdf: 2837305 bytes, checksum: 244c93af337ec0f89cf4eb7ae057d905 (MD5) / Made available in DSpace on 2018-04-17T16:49:57Z (GMT). No. of bitstreams: 1 RENATA GARCIA DUTRA DE OLIVEIRA – DISSERTAÇÃO (PPGEE) 2017.pdf: 2837305 bytes, checksum: 244c93af337ec0f89cf4eb7ae057d905 (MD5) Previous issue date: 2017-03-31 / Capes / A análise da variação de temperatura ao longo do isolador, por meio da termografia, pode consistir em um dos métodos mais eficazes de avaliação de isoladores em serviço, pois considera o efeito combinado da poluição, do estresse elétrico, defeitos internos e das condições ambientais. No entanto, analisar apenas de modo visual imagens termográficas é difícil e, muitas vezes, apenas os engenheiros especializados são capazes de fazer julgamentos corretos. Neste trabalho foi desenvolvida uma técnica para análise de imagens termográficas baseada em expansão não-linear de sinais, equalização de histogramas e análise de componentes principais com o objetivo de auxiliar no diagnóstico do nível de degradação de isoladores poliméricos. O processamento digital das imagens termográficas permitiu a obtenção dos valores de temperatura que indicam o comportamento térmico dos isoladores sob inspeção. Ensaios em laboratório foram realizados com isoladores poliméricos de 230 kV, com diferentes níveis de degradação, com a finalidade de captar imagens utilizando uma câmera de detecção de radiação infravermelha. Os resultados evidenciam a eficiência da metodologia desenvolvida, atuando de forma semiautomática no processamento digital de imagens termográficas, para obtenção do perfil térmico dos isoladores inspecionados. Com base nos produtos do processamento digital das imagens termográficas, tem-se uma melhor avaliação dos níveis de degradação em que se encontram os isoladores, além de auxiliar de forma eficiente a tomada de decisão para a classificação dos seus estados operacionais. / The analysis of the temperature variation along the insulator through thermography can be one of the most effective methods of evaluating insulators in service, for it considers the combined effect of pollution, electrical stress, internal defects and environmental conditions. However, analyses based only on visual inspection of images are not enough. In fact, only specialized engineers are able to make correct judgments. In this work a computational routine was developed, based on the digital image processing. The routine utilizes the signal nonlinear expansion technique, histogram equalization and principal component analysis. The digital processing of the thermographic images is used to obtain the temperature values that indicate the thermal behavior of the insulators under inspection. Laboratory tests were performed with 230 kV polymeric insulators with different levels of degradation in order to capture images using an infrared radiation detection camera. The results demonstrate the efficiency of the digital thermographic imaging technique to obtain the thermal profile of the insulators inspected. Based on the products of the digital processing of the thermographic images, we have a better evaluation of the degradation levels of the insulators, besides efficiently support to the classification of the operational states of the insulators.

Submitted by Lucienne Costa (lucienneferreira@ufcg.edu.br) on 2018-04-25T18:49:44Z No. of bitstreams: 1 GIRLENE LIMA RIBEIRO – DISSERTAÇÃO (PPGEE) 2017.pdf: 3769966 bytes, checksum: 1e2c04beeac23084837591c1bfec0869 (MD5) / Made available in DSpace on 2018-04-25T18:49:44Z (GMT). No. of bitstreams: 1 GIRLENE LIMA RIBEIRO – DISSERTAÇÃO (PPGEE) 2017.pdf: 3769966 bytes, checksum: 1e2c04beeac23084837591c1bfec0869 (MD5) Previous issue date: 2017-03-31 / CNPq / Nesta pesquisa é desenvolvida uma metodologia para aprimoramento da classificação de isoladores poliméricos por medições termográficas e radiação UV utilizando o Processamento Digital de Imagens (PDI) e Redes Neurais Artificiais (RNAs). A metodologia é baseada na análise da ocorrência de descargas corona e nas variações de temperatura ao longo do isolador a fim de classificá-los quanto seu estado de degradação. Cada isolador utilizado foi submetido à tensão de 133 kV fase-terra durante um período de 30 minutos, com o objetivo de ocasionar aquecimento e evidenciar descargas corona nos isoladores. As medições foram realizadas utilizando um detector de corona para medição de UV e os dados de temperatura foram adquiridos utilizando-se um termovisor. As imagens adquiridas pelos instrumentos de monitoramento, durante os ensaios, foram submetidas a um processamento digital de imagem, para extrair informações de densidade de pixels, persistência das descargas e distâncias relativas das áreas de descargas ao isolador. A partir de informações obtidas de imagens de infravermelho (temperatura) foi aplicada a estatística descritiva e o teste discriminante de Fisher, para apresentar ao sistema de classificação, parâmetros objetivos e com alto nível de separabilidade. O sistema de classificação utilizou RNA para determinar o estado de degradação dos isoladores. A classificação foi realizada de forma individual e combinada, com vetores formados pelos atributos UV e infravermelho. O sistema desenvolvido permitiu o auxílio à tomada de decisões quanto à necessidade de intervenção ou não aos isoladores. A classificação dos isoladores, de forma individual, obteve acurácia média para temperatura de 80,00% e UV 74,05%. A classificação dos isoladores, de forma combinada (UV e infravermelho), obteve acurácia média de 92,58%, evidenciando o aprimoramento na classificação. / This research presents a methodology for the improvement of the classification of polymeric insulators by using thermographic measurements and UV radiation in combination with Digital Image Processing (DIP) and Artificial Neural Networks (ANNs). The methodology is based on the analysis of the occurrence of corona discharges and temperature variations along the insulator in order to classify their stage of degradation. Each insulator was subjected to the 133 kV phase-to-ground voltage over a period of 30 minutes, in order to cause heating and corona discharges in the insulators. The experiments were performed using a corona detector for UV measurement and the temperature data were acquired using a thermal imager. The images acquired by the monitoring instruments during the tests were subjected to digital image processing to extract information of pixel density, persistence of discharges and relative distances from the discharge areas to the insulator. From information obtained through infrared (temperature) images descriptive statistics and Fisher's discriminant test were applied to present objective parameters with high level of separability to the classification system. The classification system used ANN to determine the insulators degradation state. The classification was performed in individual and in combination ways, with vectors formed by UV and infrared attributes. The developed system helped on the decision making, concerning to the necessity of intervention or not to the insulators. The classification of the insulators, in an individual way, obtained accuracy for temperature of 80.00% and UV 74.05%. The classification of the isolators, combined (UV and infrared), obtained an average accuracy of 92.58%, evidencing the improvement in the classification.

Outdoor electrical insulator is one of the important components of a power system which directly influences the system reliability. Traditionally ceramic insulators have been used for close to a century in both transmission and distribution lines. In the last few decades, polymer based outdoor insulators are being increasingly used in the above application. Polymeric insulators offer attractive advantages such as light weight, resistance to vandalism and they also outperform conventional ceramic insulators under contaminated wet conditions at least in the initial stages of their usage. However, there are certain disadvantages with polymeric insulators which have made the utilities hesitant to replace readily the ceramic insulators with polymeric insulators. One of the major concerns with the polymeric insulators is the aging w.r.t time due to the presence of multiple environmental stresses (fog, humidity, temperature, rain as well as contamination due to industrial, sea and agricultural pollution) along with electrical stress. The manifestations of the aging of insulators include tracking or/and erosion of the weathersheds. Polymers in pure form (unfilled) can not perform satisfactorily all the required functions (electrical, mechanical, thermal etc.) of an insulator used in such high voltage transmission lines. Polymers have inherently poor thermal stability. Thermal stability directly influences the tracking and erosion resistance of the weathershed. Without adequate tracking and erosion resistance, polymeric insulators can not perform satisfactorily under contaminated wet conditions. Hence the common practice to improve the tracking and erosion resistance (and other properties such as mechanical, thermal) is by filling the base polymer with large loadings (> 30 wt %) of micron sized fillers. This makes the processing of the polymer composite difficult as the viscosity of the material rises substantially at such large loadings. Due to the large filler loadings beyond a certain limit, the flexibility of the end product also suffers. Though tracking and erosion resistance of the polymer has been improved substantially at these large filler loadings, the recent failures in the field suggest the need for an alternate material with higher tracking and erosion resistance than what is achieved at these large loadings of micron sized fillers. Of late nanocomposites are emerging as promising alternatives which can offer the above mentioned functionalities at low filler loadings itself without sacrificing the flexibility in the end product as well as ease of processing. There are even indications suggesting that the tracking and erosion resistance performance is better than what is obtained using micronsized fillers. As the development of nanocomposite dielectrics/insulation is still at its infancy, it is required to investigate their specific properties needed for outdoor applications and to understand the various mechanisms responsible for the interesting behaviour of the nanocomposites. Also, it is known that dc pollution performance of ceramic insulators is much inferior to the performance under ac stress. With the introduction of higher ac/dc transmission voltages in many countries including India, it is required to design insulators with better performing materials so as to get a reliable performance under polluted wet conditions. Due to the hydrophobic nature of the polymers, it is believed that polymers especially silicone rubber insulators can perform better as compared to the ceramic insulators under polluted conditions under ac and dc. As the dc tracking and erosion (T&E) resistance of polymer is poor compared to the ac tracking and erosion resistance, it is required to investigate the T&E resistance characteristics of the nanocomposites under dc stress. In addition, due to the enhanced electric fields at the line end of the insulators in extra and ultra high voltage transmission lines, there is always a possibility of corona generation on the hardware at the metal-sheath junction and at the water droplet tips on the weathersheds of the polymeric insulators especially under foul weather conditions. It is reported that the long-term exposure to such corona has the potential to degrade the polymeric material. The effects include reduction of the hydrophobicity, surface oxidation of the weathersheds and development of microcracks on the surface of the polymeric material. These cracks (corona cutting) can worsen the wet pollution performance of the insulator. If the cracks grow deeper, then FRP rod would get exposed to the atmospheric conditions leading to brittle fracture of the FRP rod and finally resulting in the line drop. Hence, the corona aging resistance of nanocomposites has also been studied especially at low filler concentrations to see its performance under the above mentioned adverse conditions. Therefore, the research work presented here deals with three aspects of the aging (1) Study the ac and dc tracking and erosion resistance performance of silicone rubber nanocomposites with low concentrations of fillers and their suitability for outdoor applications (2) Study the corona aging performance of silicone rubber nanocomposites with low concentrations of fillers and (3) To develop a model to explain the unusual behaviour of nanocomposites observed in the above studies. The thesis also reports results of the accelerated multistress weathering studies conducted on normal polymeric outdoor insulators under prolonged dry conditions. The major challenge in case of the polymer nanocomposite processing is getting uniform distribution of the fillers. A protocol has been standardised for the processing which comprises high shear mechanical mixing followed by sonication to get good dispersion of the fillers. Room Temperature Vulcanised (RTV) silicone rubber was successfully processed with different micron and nanosized fillers and with different weight (wt.) percentages in the present work. For carrying out the T & E resistance, corona aging and multistress aging studies, facilities (such as Inclined Plane T & E Resistance Test Apparatus in line with IEC/ASTM standards and aging chambers) have been designed and developed in house as a part of the thesis work. The ac tracking and erosion resistance performance of the unfilled, microcomposite (filled with alumina trihydrate filler of 5, 10, 15, 20 and 30 % by wt) and nanocomposite (filled with alumina, silica and magnesium hydroxide fillers of 2.5 and 4 % by wt) have been compared in inclined plane (IP) tracking and erosion resistance test facility specifically developed for the work. It was very interesting to observe that nanocomposites at 4 % performed on par with the microcomposites at 30 % filler loadings. Leakage current was also measured during the IP test and it was found that the form factor (ratio of r.m.s to average leakage current) was in good agreement with the variation in the erosion resistance of the silicone rubber composites and hence it can be used as a diagnostic tool for assessing the aging state of the polymeric materials. It was also observed that the performance under positive dc stress was much inferior to the performance under ac stress. The dissipation of power under dc stress was estimated by measuring the leakage current through the sample and is found to be about four times (towards the end of the test) higher as compared to the power dissipation under ac stress. Intense electrolytic corrosion has been observed (under positive dc) on the grounded electrode and on the sample and chemical studies of the same have been carried out. The poor performance under dc is due to the absence of the voltage zero crossing, more accumulation of the contaminant (scaling) and electrolytic corrosion. It was also observed that to get the same tracking and erosion resistance under dc as in the case of ac during IP test, dc stress levels have to be reduced to about 60 % of the ac stress. This information would be helpful to the design engineer of the outdoor insulators for the HVDC transmission lines. To understand the different mechanisms responsible in improving the tracking and erosion resistance of the micro and nanocomposites, thermal, SEM and FTIR studies have been carried out. Thermal stability of the samples was measured using thermogravimetric analysis (TGA) and differential thermo gravimetric (DTG) studies. It was observed that thermal stability of nanocomposites even at low filler loadings (4 wt %) was comparable with the microcomposites at higher filler loadings (30 wt %). SEM studies indicate that the barrier resistance (against discharges) offered by the fillers in the nanocomposites even at low filler loadings (4 %) could be comparable with the microcomposites at higher filler loadings (30 %). The interaction between the fillers and the host matrix has been studied using various techniques. SEM studies done on the eroded regions of the composites revealed that a honey comb type formation had taken place on the nanocomposites during the IP test which was believed to be due to the interaction of the filler and the polymer. This honey comb structure formation at the eroded site in the nanocomposites greatly helps to protect the sample from further damage due to the discharges. The interaction at the interface between the polymer and fillers could also lead to further improvement in the thermal stability of the nanocomposite. A model was proposed which considers barrier resistance and a single-layer interaction around the fillers to explain the improvements offered by the nanocomposites. Corona aging studies have been carried out on unfilled silicone rubber, micro and nanocomposites for 25 h and 50 h of aging using a needle-plane electrode arrangement. Different parameters such as hydrophobicity, surface roughness, microcracks width on the aged surface, FTIR and SEM studies were carried out to study the corona aging resistance of the new and aged samples. The studies indicate that silicone rubber samples containing nanofillers at 3 wt % are able to impart significantly enough corona resistance compared to the unfilled and microcomposite samples. It is known that the discharge resistance offered by the fillers and the interaction/bonding between the fillers and polymers directly influences the corona aging resistance. Hence, the model proposed (discussed above) is valid for understanding the corona aging performance of the nanocomposites which is better than the unfilled and ATH filled silicone rubber. In addition to the tracking and erosion resistance and corona aging studies, multistress aging of commercially available polymeric insulators containing micron sized fillers has been carried out. The aging behaviour of the polymeric insulators under tropical and subtropical conditions (in the absence of discharges under wet conditions) has not been explored. Further, the long-term influence of the UV radiation on silicone rubber in the presence of temperature and electric stress is also not explored. Hence, to understand the aging phenomena (weathering characteristics) under multistress (electric, thermal and UV), distribution class composite polymeric insulators were aged for 30,000 h in a multistress aging chamber developed specifically for the studies. Insulators were continuously subjected to the accelerated electric and thermal stresses as well as UV radiation. Different studies like leakage current, SEM, hydrophobicity, surface roughness and low molecular weight (LMW) molecules content in the samples before and after the aging have been investigated. It is interesting to observe that even in the absence of electrical discharges on the surface of the material, significant monotonous reduction in LMW molecules has been observed w.r.t weathering time. Appreciable increase in the surface roughness (at least 200 % as that of the new material) as well as increased oxygen levels on the surface has also been observed. The results indicate that surface hydrophobicity is dynamic in nature and may not reflect the slow and permanent changes taking place in the bulk of the material. The results obtained for the nanocomposites enable us to design a better material with improved tracking, erosion and corona resistance without sacrificing the flexibility in the end product as well as ease of processing. The silicone rubber nanocomposites also open up the possibility for economically designing a smart material possibly with a higher reliability for outdoor insulator application.

Outdoor insulators are one of the most important parts of a power system. The reliability of a power system depends also on the reliability of the insulators. The main functions of an insulator used for outdoor applications are to give the necessary insulation, provide the necessary mechanical support to the transmission line conductor and also to resist the various environmental stresses like pollution, ultra violet rays etc. Traditionally porcelain and glass insulators have been used for outdoor insulator applications. They are good insulators under normal conditions and the cap and pin arrangement allows them to take up the mechanical load of the line. But owing to their large weight and brittle nature they are susceptible to vandalism and also they have increased cost of installation and commissioning. But the main problem of porcelain and glass insulators is its performance under polluted environmental condition. Under wet and polluted conditions, the porcelain insulators allow the formation of a conducting layer on the surface which results in setting up of leakage current, dry band arcing and power loss. This problem is further augmented under dc voltages where the stress is unidirectional and the contaminant deposition is higher as compared to ac. Polymeric insulators are a good alternative for porcelain and ceramic insulators for use especially under dc voltages because of their good pollution performance. The property of surface hydrophobicity resists the setting up of leakage currents and hence polymeric insulators help in reducing power loss. They are also light in weight and vandalism resistant and hence easier to install. But being polymeric, they form conductive tracks and erode when exposed to high temperatures which occur at the surface during dry band arcs and when exposed to corona discharges. The surface hydrophobicity is also temporarily lost when exposed to different electrical stresses. Silicone rubber is the most popular among the various choices of polymers for outdoor insulator applications. They have good surface hydrophobicity and tracking performance. But polymers in their pure form cannot be used as insulators because of their poor mechanical strength. Adding inorganic fillers into the polymer matrix not only improves its mechanical properties but also its erosion resistance. Micron sized Alumina Trihydrate (ATH) is used traditionally to improve the tracking and erosion resistance of polymeric insulators. A very high loading (up to 60%) is used. Adding such a high filler loading to the base polymer hampers its flexibility and the material processing. With the advent of nanotechnology, nano fillers have come into vogue. Studies conducted on nano filled polymers showed exciting results. A small amount of nano fillers in the polymer matrix showed significant improvement in the mechanical strength without hampering its flexibility. The electrical properties like tracking and erosion also improved with filler loading. Hence the use of nano filled silicone rubber is a good alternative for use as a high voltage insulator especially under dc voltages. Reports suggest that adding nano fillers into the silicone rubber matrix improves the tracking and erosion resistance and the corona degradation as compared to the unfilled samples under ac voltages. The literature on the dc performance of silicone rubber nano composites is scarce. So the present study aims to evaluate the performance of silicone rubber nano composites for tracking and erosion resistance and corona degradation under dc voltages. The tracking and erosion resistance under dc voltages was measured using the Inclined Plane Tracking and Erosion Resistance set up as per ASTM D2303 which was modified for dc voltage studies. The performance of nano Alumina and nano Silica fillers were evaluated under negative dc and the performance was compared with micron sized Alumina Trihydrate filled samples. The effect of filler loading was also studied. It was seen that the performance of the silicone rubber improved with filler loading. A small loading percentage of nano fillers were enough to give performance similar to silicone rubber filled with micron sized ATH filler. The silicone rubber performed better under negative dc as compared to ac and positive dc. The positive dc tests showed a migration of ions from the electrodes onto the sample surface. The increased surface conductivity resulted in very heavy erosion in the case of positive dc tested samples. The corona aging studies were also conducted on silicone rubber nano composites. Nano silica was used as filler in this case. Different filler loadings were employed to understand the effect of filler loading. The corona was generated using a needle plane electrode and samples were exposed to both positive and negative dc corona. The samples were exposed to corona for different time intervals – 25 and 50 hours to study the effect of exposure time. The hydrophobicity, crack width and surface roughness were measured after the tests. Adding nano fillers into the polymer matrix improved the corona performance. With filler loading, the performance improved. The samples exposed to positive dc corona performed better than those under negative dc corona. The loss of hydrophobicity, surface cracks and the surface roughness was less in the case of positive dc corona tested samples. With exposure time, the performance of silicone rubber became poorer for positive dc corona tested samples. For the negative dc corona tested samples, the performance seemed to improve with exposure time. The tracking and erosion resistance and the corona aging studies conducted showed that the performance of silicone rubber is improved by adding nano fillers into the polymer matrix. A small amount of nano filler loading was enough to perform similar to a heavily loaded micron filled sample. Hence nano fillers can be used as a good functional material to improve the performance of silicone rubber insulators especially under wet and polluted conditions.

Recently high temperature vulcanized (HTV) silicone rubber (SIR) / polymeric/composite insulators are gaining wider acceptance as overhead transmission line insulators for extra high voltage (EHV) and ultra-high voltage (UHV) systems due to some promising features like hydrophobicity recovery, light weight, ease of handling and installation, better pollution ashover performance, admirable resistance against vandalism etc. Since polymeric insula-tors are of recent origin, their long-term eld performance is yet to be understood. Owing to their organic nature, and exposure to environmental stresses like pollution, temperature, UV radiation, humidity, fog, rain etc., the insulator performance degrades over a period. The sheds/petticoats of the insulators become wettable leading to frequent ashover in humid and contaminated environment. Hence, long term reliability of the composite insulators is of foremost concern to the power utilities. The available literature on the long term eld performance of these insulators for di erent climatic conditions and under multiple environ-mental stresses for both the HTV SIR and Liquid Silicone Rubber (LSR) is scant. Also there is no reference standard for evaluation of these insulators for pollution/contamination test methods in the laboratory. However currently, CIGRE Work Group is working towards the standardization of the test methods for arti cial pollution tests for polymeric insulators. The thesis addresses some of the issues in detail. In the first part of the thesis, a new and simple pre-treatment methodology to achieve uniform contamination layer on inherently hydrophobic HTV SIR Insulator samples is presented for laboratory pollution performance evaluation. The surface water level di usion in the dipping period is found to make the insulator surface temporarily hydrophilic. Then the uniform contamination layer is applied by dipping the sample immediately in the pollution slurry. Exhaustive experiments were conducted on full scale SIR insulators as well as SIR slabs to investigate the hydrophilicity appearance on the SIR surface. A specially fabricated arrangement for assessment of Wettability Class (WC) is made as per IEC stds. The results of WC measurement and wet ashover studies support the temporary reduction in hydrophobicity of SIR due to dipping phase in the proposed pre-treatment methodology. The next part of the thesis presents the results for the effeect of long term thermal aging experimentation conducted on HTV SIR with difffeerent degrees of pollution (medium, heavy), the effeect of arid desert climate on polymeric insulators is studied. The experimental set-up consists of controlled HVAC source, temperature controlled furnace with a provision for high voltage (HV) and Leakage Current (LC) monitoring, a Digital Storage Oscilloscope (DSO), compact DAQ-9201 of National Instruments operated in LabVIEW platform etc. Two types of HTV SIR Insulators are considered for the study. Flat slabs as well as full-scale insulator samples of creepage length 725 mm are stressed simultaneously to simulate the in-service condition. The experimentation is conducted for about 575 hours with application of 21.0 kVrms at 60oC. The results of the hydrophobicity recovery for thermally aged contaminated polymeric insulators are reported. Besides, monitoring electrical and mechanical proper-ties, changes in material properties of SIR are also analyzed using Physiochemical analysis techniques like Fourier transform infrared (FTIR) spectroscopy, X-Ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Thermo-Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC). Some of the key findings of the study are increased surface oxidation, surface roughness and mechanical stress due to thermal aging of polymeric insulators. Experimental investigations show that the characteristics of power frequency component of leakage current can be linked with thermal aging of SIR. Further, a unique climatic aging experimental facility is established to evaluate the long-term reliability of SIR under environmental stresses like UV, Humidity, temperature and applied electric stress. The investigations are conducted on two different types of HTV SIR and LSR at samples as well as full-scale insulator samples. The experimentation is conducted for 500 hours with 10.0 kVrms at 50oC, with 85% humidity and 1 W/m2 UV ir-radiation which is in accordance with the aging cycle specified in IEC standard. The results of the comparative studies conducted for the electrical, mechanical and material properties indicate leakage current pulses, brittleness, Salt deposition for multistress aged samples. In summary, an attempt has been made to contribute a pollution methodology with sim-ple pre-treatment technique for inherently hydrophobic HTV SIR surface to achieve better uniformity of contamination layer. Also, electro-thermal and multiple stresses investigations were conducted for long term performance on polymeric insulators.