Дисертації з теми "Silicone rubber insulators"

Silicone rubber (SiR) outdoor insulators are increasingly being deployed in new AC and DC high voltage transmission systems thanks to their superior performance in wet-polluted conditions compared to traditional porcelain and glass insulators. However, in severely polluted environments, sustained discharge activities and dry band arcing due to surface contamination cause tracking, erosion, and the loss of hydrophobicity on the SiR insulator surface. This degradation can accelerate damage to the insulator surfaces, increasing the probability of a flashover and enduring insulator failure. This thesis presents an experimental study on the electric performance of polluted and aged outdoor SiR polymeric insulators using AC and DC voltages. The research involved an extensive review of the published literature and an investigation of the performance of SiR insulators subjected to different ambient conditions and identifies the modes where most failure and degradation occurred on SiR surfaces. Experimental investigations were carried out to compare the aging performance of two 11kV SiR insulator designs using a rotating wheel dip test under AC and positive DC excitations. A standard polymeric insulator design was used and compared with insulators that had a textured surface. Both insulator designs were fabricated in-house using a vacuum casting machine. Several electrical parameters were measured during the test to characterise the performance of each insulator. Dry band arcing activities were mainly observed on the trunk surface of the conventional profile. A decrease in hydrophobicity was measured on the tested surfaces, and tracking and erosion defects were also observed on both insulator designs. Leakage current measurements showed that drying and discharge activity was greater for a conventional insulator compared with the textured insulator, and more severe degradation appeared under positive DC tests than under AC. These studies showed that insulators with a textured design can improve the performance of SiR insulators against tracking and erosion under AC and positive DC excitations. The electric field and potential distributions along the leakage surface of the 11 kV SiR insulators under dry clean and wet polluted conditions were studied using finite element method COMSOL Multiphysics. The critical of high field regions on SiR surfaces were identified and the power dissipated in the pollution layer along insulator surface was calculated. This study showed useful information about surface heating, which could be used to predict of the formation of dry bands. An investigation of the pollution layer characterisation on conventional and textured pattern designs is described. Several tests were conducted to evaluate the behaviour of the insulator surfaces under different conditions. ESDD and NSDD parameters were measured for different materials, and evaluations for each design were also performed. Leakage conductance measurements on surface designs were determined, and the distribution trends of surface conductance were also characterised. Different rectangular SiR samples were assessed, and an improvement for reducing the pollutant deposition on textured surfaces was observed. In 4-shed insulators, the textured design showed comparable ESDD value with conventional profile. Textured designs also showed slower growth of the leakage current than the conventional design. Clean fog tests (based on a high voltage ramp test) were carried out to investigate the flashover performance of conventional and textured insulator designs. For different ranges of wetting and pollution severity conditions, the textured design showed an improvement in the flashover performance that could reach 16 % compared to the conventional surface. This indicates that the textured surface seems to be more effective under severe ambient conditions. It was also observed that the textured insulator design can improve the electrical performance of SiR insulators under AC and DC voltages.

Microbial colonization on the surface of silicone rubber high voltage outdoor insulators often results in the formation of highly hydrated biofilm that influence the surface properties, such as surface hydrophobicity. The loss of hydrophobicity might lead to dry band formation, and, in the worst cases, flashover and failure of the insulator. In this work, the biocidal effects of various antimicrobial compounds in silicone rubber materials were determined. These materials were evaluated according to an ISO standard for the antimicrobial activity against the growth of aggressive fungal strains, and microorganisms that have been found colonizing the surfaces of outdoor insulators in several areas in the world. Several compounds suppressed microbial growth on the surfaces of the materials without compromising the material properties of the silicone rubber. A commercial biocide and thymol were very effective against fungal growth, and sodium benzoate could suppress the fungal growth to some extent. Thymol could also inhibit algal growth. However, methods for preservation of the antimicrobial agents in the bulk of the material need to be further developed to prevent the loss of the compounds during manufacturing. Biofilm formation affected the surface hydrophobicity and complete removal of the biofilm was not achieved through cleaning. Surface analysis confirmed that traces of microorganisms were still present after cleaning. Further, surface modification of the silicone rubber was carried out to study how the texture and roughness of the surface affect biofilm formation. Silicone rubber surfaces with regular geometrical patterns were evaluated to determine the influence of the surface texture on the extent of microbial growth in comparison with plane silicone rubber surfaces. Silicone rubber nanocomposite surfaces, prepared using a spray-deposition method that applied hydrophilic and hydrophobic nanoparticles to obtain hierarchical structures, were studied to determine the effects of the surface roughness and improved hydrophobicity on the microbial attachment. Microenvironment chambers were used for the determination of microbial growth on different modified surfaces under conditions that mimic those of the insulators in their outdoor environments. Different parts of the insulators were represented by placing the samples vertically and inclined. The microbial growth on the surfaces of the textured samples was evenly distributed throughout the surfaces because of the uniform distribution of the water between the gaps of the regular structures on the surfaces. Microbial growth was not observed on the inclined and vertical nanocomposite surfaces due to the higher surface roughness and improved surface hydrophobicity, whereas non-coated samples were colonized by microorganisms.

QC 20151002

Silicone rubber (SiR) insulators are increasingly being used mainly because of their superior performance under wet polluted conditions compared to conventional porcelain and glass. However, in polluted environments with high moisture levels, electrical discharges will develop on the insulator surfaces. In the long term, electrical discharges cause degradation of SiR insulator in the form of tracking and erosion due to dry-band arcing that takes place when leakage current (LC) develops, and both are detrimental to the life of the insulation. This thesis presents an experimental study on the electric performance of aged high voltage outdoor silicone rubber insulators. In addition to the literature reviewed of the performance of silicone rubber insulators subjected to different climatic conditions, which is resulting in loss of their unique property named ‘hydrophobicity’, the research is concentrated in three areas: (i) Study of the effect of ultraviolet (UV) irradiation on the performance of silicone rubber insulation systems: 11kV non-textured and 4mm Textured Shank insulators (TS4) (TS4 is a new insulator design using intersection of 4mm square texture pattern on the shank regions of the insulator) were prepared in vacuum casting machine. Irradiated insulators were tested under different conditions. Dry-band formation and electrical discharges on their surfaces are dependent on the UV doses. Degradation of SiR insulators in a form of tracking and erosion is result in loss the surface hydrophobicity by UV irradiation, and decrease of the expected life of the insulators. Silicone rubber insulator electric performance was found to be affected by the experimental conditions. LC on relatively hydrophobic surfaces was found to exist and increase on overall insulators with increasing the exposure doses of UV irradiation, by increase the number of applied UV cycles. (ii) Development a new test set-up to determine the effectiveness of orientation angle on the performance of silicone rubber insulators subjected to different wet and pollution conditions: LC of silicone rubber insulators were increased with changes in the orientation angle from vertical to horizontal, where the TS4 showed its superior performance under fog conditions in the vertical orientation. (iii) A new test procedure, based on the high voltage ramp test method and standard wet test method, named ‘high voltage rain ramp test’, is proposed to evaluate the flashover performance of the TS4 silicone rubber insulators under simulated rain conditions at different orientations. Its flashover performance was compared with non-textured insulators. TS4 silicone rubber insulators are less suitable for wet weather conditions in any orientation, due to the following reasons: (a) in vertical orientation a very low flashover voltage was observed due to the development of high magnitude electric fields in the insulator shank region, and (b) for the inclined and horizontal orientations, non-textured insulators exhibit a better flashover performance than their textured equivalents. Hence, less improvement is achieved, due to use an expensive and intricate insulator design.

High-voltage (HV) outdoor composite insulators used in transmission lines are made of two polymers, comprising the core and housing, bonded together with metallic end-connections. The interface between these polymers is parallel to the electric field, which makes the insulators more prone to interfacial problems at these common points [1]. If interfacial ageing occurs, degradation and catastrophic breakdown can result [2]. Therefore, the design reliability of outdoor composite insulators depends on the high-strength bond between the core and the housing [3],[4]. Research findings by Kutil and Froshlic [5] indicate that delaminated areas, cavities and/or micro cracks in the medium are enough to initiate streamer discharges along the interface that are capable of degrading both insulating materials. The heat, UV radiation, and high-energy electrons produced from such discharge activity resulted in the growth of carbon paths along the interface, known as ‘tracking’, ultimately causing failure [6]. This investigation focuses on the development of tracking between silicone rubber and epoxy resin, with a view to replicating the tracking phenomena seen within composite insulators in service. A fine wire is placed between the dielectrics materials to enhance the local electric field magnitude and initiate discharge processes. The resulting partial discharge (PD) activity has been monitored. This Information has been used to understand the inception and propagation of the interfacial tracking. A strong relationship was found between maximum PD magnitude and track length. PD patterns and unique detailed images of the interfacial tracking development, allowed identification of the growth characteristics of interfacial channels and phases of tracking growth. Furthermore, a correlation in the mechanisms of interfacial degradation was found between the lab-fabricated samples and commercial composite rods. Finally, a growth model of interfacial ageing has been developed with the information from FEA models, PD patterns and the detailed images of tracking growth. The physical structure and chemical analysis of interfacial tracking is also disclosed to provide an insight into interfacial ageing mechanisms that occur in the composite insulators under electrical stress.

The degradation of high voltage electrical insulation is a prime factor that can significantly influence the reliability performance and the costs of maintaining high voltage electricity networks. Little information is known about the system of localized degradation from corona discharges on the relatively new silicone rubber sheathed composite insulators that are now being widely used in high voltage applications. This current work focuses on the fundamental principles of electrical corona discharge phenomena to provide further insights to where damaging surface discharges may localize and examines how these discharges may degrade the silicone rubber material. Although water drop corona has been identified by many authors as a major cause of deterioration of silicone rubber high voltage insulation until now no thorough studies have been made of this phenomenon. Results from systematic measurements taken using modern digital instrumentation to simultaneously record the discharge current pulses and visible images associated with corona discharges from between metal electrodes, metal electrodes and water drops, and between waters drops on the surface of silicone rubber insulation, using a range of 50 Hz voltages are inter compared. Visual images of wet electrodes show how water drops can play a part in encouraging flashover, and the first reproducible visual images of water drop corona at the triple junction of water air and silicone rubber insulation are presented. A study of the atomic emission spectra of the corona produced by the discharge from its onset up to and including spark-over, using a high resolution digital spectrometer with a fiber optic probe, provides further understanding of the roles of the active species of atoms and molecules produced by the discharge that may be responsible for not only for chemical changes of insulator surfaces, but may also contribute to the degradation of the metal fittings that support the high voltage insulators. Examples of real insulators and further work specific to the electrical power industry are discussed. A new design concept to prevent/reduce the damaging effects of water drop corona is also presented.

The degradation of high voltage electrical insulation is a prime factor that can significantly influence the reliability performance and the costs of maintaining high voltage electricity networks. Little information is known about the system of localized degradation from corona discharges on the relatively new silicone rubber sheathed composite insulators that are now being widely used in high voltage applications. This current work focuses on the fundamental principles of electrical corona discharge phenomena to provide further insights to where damaging surface discharges may localize and examines how these discharges may degrade the silicone rubber material. Although water drop corona has been identified by many authors as a major cause of deterioration of silicone rubber high voltage insulation until now no thorough studies have been made of this phenomenon. Results from systematic measurements taken using modern digital instrumentation to simultaneously record the discharge current pulses and visible images associated with corona discharges from between metal electrodes, metal electrodes and water drops, and between waters drops on the surface of silicone rubber insulation, using a range of 50 Hz voltages are inter compared. Visual images of wet electrodes show how water drops can play a part in encouraging flashover, and the first reproducible visual images of water drop corona at the triple junction of water air and silicone rubber insulation are presented. A study of the atomic emission spectra of the corona produced by the discharge from its onset up to and including spark-over, using a high resolution digital spectrometer with a fiber optic probe, provides further understanding of the roles of the active species of atoms and molecules produced by the discharge that may be responsible for not only for chemical changes of insulator surfaces, but may also contribute to the degradation of the metal fittings that support the high voltage insulators. Examples of real insulators and further work specific to the electrical power industry are discussed. A new design concept to prevent/reduce the damaging effects of water drop corona is also presented.

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Made available in DSpace on 2018-01-29T11:51:13Z (GMT). No. of bitstreams: 1 DANIELLA CIBELE BEZERRA - TESE PPGCMAT 2014.pdf: 4550868 bytes, checksum: d91b2b560040e4f5f101c7f129c13aab (MD5) Previous issue date: 2014-11-03
CNPq
Um dos principais problemas do isolamento elétrico externo é o acúmulo de contaminantes em sua superfície. A ocorrência deste evento faz com que a resistência da superfície diminua, aumentando a presença de correntes de fuga pela superfície. Entre as técnicas desenvolvidas para diminuir esse tipo de efeito, está o revestimento do isolador existente convencional (porcelana ou vidro, conforme o caso) com uma borracha de silicone que é vulcanizada à temperatura ambiente (BS). O objetivo deste trabalho foi desenvolver uma mistura de borracha de silicone/alumina trihidratada/nanosílica (BS/ATH:NS) para ser usada em revestimento de isoladores elétricos de vidro. Cargas de ATH e NS foram adicionadas à BS, produzindo uma mistura feita em diferentes proporções mássicas (79/21; 77/23 e 75/25) de BS/ATH:NS, utilizada nos revestimento dos isoladores elétricos de vidro. As cargas e os revestimentos foram caracterizados por difração de raios X (DRX), microscopia eletrônica de varredura (MEV), teste de inflamabilidade, ensaio na câmara de névoa salina, molhabilidade e avaliação da hidrofobicidade. As cargas apresentaram características morfológicas bem distintas, o que interferiu diretamente na morfologia dos revestimentos. A silanização das cargas favoreceu uma interação entre elas, assim como houve uma pequena melhora na interação das cargas com a BS. Observou-se também que no teste de inflamabilidade, os revestimentos que continham ATH, apresentaram resultados mais significativos, não havendo queima. Para os ensaios na câmara de névoa salina, o revestimento com BS/20:1 e os revestimentos com cargas silanizadas apresentaram os menores valores de corrente de fuga, o que foi comprovado com a avaliação da hidrofobicidade, na qual estes conseguiram recuperar a hidrofobicidade até HC=1 (nível de hidrofobicidade), de acordo com o guia STIR (Swedish Transmission Research Institute).
One of the main problems of external electrical isolation is due to accumulation of contaminants on its surface. The occurrence of this event causes the insulation resistance of the surface decreases, increasing the flow of leakage currents on the surface. Among the techniques developed to reduce this type of event is the coating of the conventional existing insulator (porcelain or glass, as appropriate) with a silicone rubber vulcanized at room temperature (BS). The aim of this study was to develop a mixture of silicone rubber/ alumina trihydrate/nanosílica (BS/ATH:NS) for use in coating glass electrical insulators. Loads of ATH and NS were added to BS, producing a mixture made in different mass ratios (79/21, 77/23 and 75/25) BS/ATH:NS used in the coating of glass electrical insulators. Loads and coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), flammability test, measurement salt spray chamber, test of wettability and hydrophobicity. Loads showed distinct morphological features, which directly affects the morphology of the coatings. The silanization loads favored interaction among them, as there was a small improvement in the interaction of the loads with the BS. It was also observed that the flammability test, the coatings containing ATH showed more significant results, without burning. For tests of the salt spray chamber, coat with BS/20:1 and with silanized fillers had the lowest leakage current, which was confirmed by measuring the hydrophobicity, where they managed to recover the hydrophobicity HC = 1 (level of hydrophobicity) according to STIR (Swedish Transmission Research Institute) guide.

Post insulators as the weakest component of transmission lines, are highly exposed to breakdown. Insulators breakdown leads to outage and economic loss for transmission companies. Faulted post insulator detection and location is very important to minimum duration of outage. Depending on type of post insulators types, they can be detected in case of breakdown. Silicon rubber post insulators as new generation of insulators, have better insulation characteristics compare to others. Away from all of its advantages, detection and location of broken ones are difficult for system operator due to no external damage. In this thesis design of a self-powered system to detect the broken silicon rubber post insulator has been pursued. An innovative optical effect triggered by self-powered system has been deployed to detect faulted post insulator.

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.

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.

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.

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.

碩士
國立臺灣科技大學
電機工程系
93
At present, silicone rubber insulators are likely reliable for outdoors power system applications in the world due to their better pollution withstand performance and aging withstand capability, especially the hydrophobicity of recovery. The goal of this study is to investigate the correlation between hydrophobicity and partial discharge of silicone rubber insulators. In the aging experiment, three samples of silicone rubber insulators were used. The aging process is to dip the samples in the salted water with different conductivity and then to stress them by A.C high-voltage. There are two stages of ten aging cycles each in this aging experiment. The time of dipping in salted water and the applied voltage magnitude are different for these two stages so as to observe the variation of hydrophobicity of silicone rubber insulators in terms of its discharge phenomenon during the aging process. The determination of hydrophobicity is based on IEC TS 62073－the spray method. The experimental results show that hydrophobicity aging process is highly related to the surface discharge and the time of dipping rather than the conductivity of salted water. Above all, similar to the related literatures, the thesis divides the ageing into three stages, the early ageing period, the transition period and the late ageing period. And, the relationship between hydrophobicity and partial discharge in each stage is then illustrated. Finally, the characteristics of corona discharges and dry-band arcing discharges are studied to verify its relation to the ageing of hydrophobicity. The results of the study shall be of value to the subsequent researches for on-line detecting of polymeric insulators in service.

abstract: This thesis concerns the flashover issue of the substation insulators operating in a polluted environment. The outdoor insulation equipment used in the power delivery infrastructure encounter different types of pollutants due to varied environmental conditions. Various methods have been developed by manufacturers and researchers to mitigate the flashover problem. The application of Room Temperature Vulcanized (RTV) silicone rubber is one such favorable method as it can be applied over the already installed units. Field experience has already showed that the RTV silicone rubber coated insulators have a lower flashover probability than the uncoated insulators. The scope of this research is to quantify the improvement in the flashover performance. Artificial contamination tests were carried on station post insulators for assessing their performance. A factorial experiment design was used to model the flashover performance. The formulation included the severity of contamination and leakage distance of the insulator samples. Regression analysis was used to develop a mathematical model from the data obtained from the experiments. The main conclusion drawn from the study is that the RTV coated insulators withstood much higher levels of contamination even when the coating had lost its hydrophobicity. This improvement in flashover performance was found to be in the range of 20-40%. A much better flashover performance was observed when the coating recovered its hydrophobicity. It was also seen that the adhesion of coating was excellent even after many tests which involved substantial discharge activity.
Dissertation/Thesis
M.S. Electrical Engineering 2013

abstract: This paper presents a theoretical model for evaluating flashover performance of insulators under contaminated conditions. The model introduces several new features when compared with existing models such as, the formation of dry bands, variations in insulator geometry and surface wettability. The electric field distribution obtained from software for 3-Dimensional models along with form factor are used to determine the dimensions of the dry bands and the onset of arcing. The model draws heavily from experimental measurements of flashover voltage and surface resistance under wet conditions of porcelain and composite insulators. The model illustrates the dominant role played by the insulator shape and housing material on the flashover performance.
Dissertation/Thesis
M.S. Electrical Engineering 2012

High voltage insulators used in overhead power transmission systems are of key im- portance for safe, reliable, and effcient operation of transmission line in transferring huge amount of electrical power. Conventionally, ceramic/ glass insulators were used in electri- cal power transmission, recently composite/ polymeric insulators are being used in over- head transmission lines. The polymeric insulators offer numerous promising advantages over conventional insulators such as light weight, higher impact strength, better pollution performance, resistance to vandalism due to its slim design etc. Moreover, the inherent property of polymeric insulators of being hydrophobic make it more advantageous over conventional insulators. However, the polymeric material being organic in nature, un- dergo degradation due to different stresses such as environmental stresses (thermal, UV, moisture, fog etc,), and electrical stresses. Further, the presence of electrical discharges such as corona, surface scintillations, occurrence of dry band arcing, operation in adverse conditions, etc., are recognized as the major accelerating factors for material degrada- tion. Presence of heavy ionic deposition over the surface leads to surface discharges and localized temperature rise leads to material erosion which results in electrical tracking formation and eventually leads to ash-over/ line outage of power transmission. In the present work, silicone rubber based polymeric insulators are experimentally in- vestigated for their material properties and long-term service performance. The available literature show that polymeric insulators are quite sensitive to their local environmental conditions and their long-term performance depends on field service environment. The material properties get affected in adverse environment and foul weather condition. The unavoidable service conditions such as moisture, fog, acidic rain, snow, pollution etc., alter their service life by accelerating their aging which directly affect their long-term performance. The evaluation of material properties such as surface electrical resistance and long-term performance of these composite insulators are still under consideration. The present thesis work focuses on the study of majorly two aspects of polymeric insulators which governs its performance in service life. One of them is the material prop- erty, which provides the surface electrical resistance; important role in avoiding the surface electrical tracking in the polymeric insulators under polluted/ contaminated conditions. The second aspect is the long-term performance analysis. In the first part of the thesis work, the experimental investigation are conducted using indigenously developed experi- mental arrangements. The investigation pertaining to surface resistance of silicone rubber insulating material investigations, Inclined plane tracking and erosion (IPT) method is utilized. The evaluation of the material property is conducted under different environ- mental condition simulating as per standard (normal condition) and acid rain condition under AC and DC voltage application. The acidity value and salt contents of artifi cial acid rain is simulated based on the physically observed acid rain information. In summary, the main contribution of present work is to understand the effect of different environmental condition on the degradation of silicone rubber insulators and its effect on material properties and long-term performance. Recurrence plot analysis performed on measured leakage current present the visual pattern that provide clear discrimination between fresh and degraded sample. It is proposed that this method could be used as an important tool to detect and diagnosis the loss of material property of polymeric insulators. The investigation presents more realistic contribution towards the surface degradation studies on Polymeric Insulators used for EHV and UHV Transmission.

High Voltage Ceramic and glass Insulators have been widely used by various transmission and distribution utilities for several decades across the globe. Recently composite or silicone rubber insulators have evolved and are now replacing ceramic/glass insulators due to their improved advantages; however, these Insulators suffer from degradation over a period of service. The first few chapters of the thesis deal with the study of silicon rubber/polymer insulators under various climatic conditions. Exhaustive experimental studies were conducted to understand the degradation of insulators under different climatic conditions which prevail in the Country. Studies on polymer insulators under sub-zero and under extremely high-temperature conditions were attempted experimentally to evaluate their performance. During experimentation, the leakage current was continuously monitored. Later, material analysis, which is a very important aspect and essential to correlate with the morphological changes of the insulator surface, was examined. The experimental investigations demonstrate that there is a need to conduct multi-stress experimentation under specific climatic conditions before the Insulators are installed in the field. The next portion of the thesis work deals with the failure mechanism of a Fibre Reinforced Plastic (FRP) Rod. Some portion of the work deals with mathematical analysis being extended to condition monitoring of dielectric surfaces and understanding the performance of FRP rods under high AC voltages. Further, experimental investigations are performed on FRP Rods to analyze the behaviour witnessed, as the field failures reported on Silicon rubber Insulators, interesting results are reported. Condition monitoring of dielectric surfaces is very important; hence it was felt necessary to analyze the field performance of transmission/ distribution composite Insulators. To understand further, a mathematical analysis based on Chaos has been evaluated for leakage current data and quantization of comparative degradation for a dielectric surface is presented. Later, Empirical Mode Decomposition is also used for understanding leakage current and implied degradation under minimal data conditions, and the results are analyzed and presented. Subsequently, the Surface electric field of insulators exposed to HVDC is studied considering the temporal boundary conditions which may arise due to the capacitive-resistive transients. The last portion of the thesis deals with a theoretical study of the bulk conductivity of polymer material. The Electric Field dependence of conductivity on the application of voltage and subsequent space charge distribution is attempted, and the results are analyzed and presented. In short, this thesis is a work where both experimental, simulation and theoretical studies pertaining to silicone rubber insulators are presented.