Thesis (Ph. D.)--University of Hong Kong, 2008. / Includes bibliographical references. Also available in print.

Inaug.-Diss. (Ph. D.)--Friedrich-Wilhelms-Universität zu Berlin, 1894. / Includes bibliographical references.

Inaug.-Diss. (Ph. D.)--Friedrich-Wilhelms-Universität zu Berlin, 1894. / Includes bibliographical references.

Thesis (Ph. D.)--Mississippi State University. Department of Electrical and Computer Engineering. / Title from title screen. Includes bibliographical references.

Thesis (DR. ENG.)--John Hopkins University, 1938. / Cover title. Mimeographed. Bibliography: leaves 71-72.

Thesis (Professional Degree)--University of Missouri, School of Mines and Metallurgy, 1935. / The entire thesis text is included in file. Typescript. Title from title screen of thesis/dissertation PDF file (viewed July 15, 2010) Includes index (p. 23-24).

Thesis (DR. ENG.)--John Hopkins university, 1938. / Cover-title. Mimeographed. "References": leaves 71-72.

Η μόνωση του εξοπλισμού και των εγκαταστάσεων υψηλής τάσης είναι αναγκαία για τη διατήρηση της διαφοράς δυναμικού μεταξύ των υπό υψηλή τάση αγώγιμων μερών. Οι μονωτήρες είναι οι διατάξεις οι οποίες χρησιμοποιούνται στα ηλεκτρικά δίκτυα και στηρίζουν, διαχωρίζουν και περιέχουν αγωγούς υψηλής τάσης. Όλοι οι μονωτήρες έχουν διπλή λειτουργικότητα, μηχανική και ηλεκτρική, κάτι που δημιουργεί αντικρουόμενες απαιτήσεις στο σχεδιασμό τους. Επιπλέον, αποτελούν βασικά δομικά στοιχεία ενός συστήματος μεταφοράς και διανομής ηλεκτρικής ενέργειας. / -

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.

Pollution is the single largest cause of transmission/distribution line outages, next to lightning, which result in expensive power outages. A major significance of the problem is that it can repeatedly occur even at normal working voltages. As a result, it has become the most detrimental factor affecting the safe operation of extra and ultra high voltage (EHV/UHV) transmission lines and substations. In reality, the phenomenon of pollution-induced flashover is a very complex, and vexatious problem that continues to challenge high voltage engineers even today. In spite of knowing this phenomenon for the past several decades, a solution has remained still elusive. Although there exist some remedial measures, there are associated limitations, which will become evident on long runs. The guaranteed solution seems to be the washing and cleaning of insulators for which utilities spend significant amount of money. Therefore, there is a need to develop a suitable mitigation technique which is cost effective and yield better performance in the field. Motivated by this, the present study is undertaken and it essentially aims to seek simple alternative solutions for the problem for strings with ceramic insulator discs. All the ten types (normal and anti fog) ceramic insulators, which are commonly employed in our country, are considered in the study. Amongst the several controllable and uncontrollable physical quantities leading to the pollution flashover phenomena, the maximum surface field identified as on of the major influencing factor. In fact, the field concentration near the pin can lead to early formation of dry band and scintillation/partial arcs. Considering this, it is intended to seek possible minimization of the maximum surface field occurring at the pin region. This is expected to yield enhanced pollution/contamination flashover strength and in addition, show an improvement in normal operation. The intended study requires a detailed knowledge on field distribution. However, the required data is found to be rather scarce. In view of this a detailed study on field distribution is taken up for all the ten types of disc insulators used in our country. For the problem under investigation, the governing equation has been identified for both clean and polluted conditions along with pertinent boundary conditions. Considering the open geometry nature of the problem along with presence of multiple dielectrics, Surface Charge Simulation (SCSM) methodology was found to be most suitable and hence adopted for the work. In particular, the Galerkin method with piecewise linear interpolation function is employed in the formulation. The method employed and the codes developed are verified with suitable examples. First, a detailed quantification of the field distribution under clean conditions is made for all the ten types of discs in single disc and string configuration. Subsequently, the task of reducing the maximum surface field gradient, which occurs at the pin, is attempted. Several consideration lead to an artificial extension of pin as one of best feasible choice. However, any attempt to extend the pin would lead to some reduction in total creepage length, possible enhancement of bulk stress in air and enhancement of stress in triple junction. After a careful study, involving several experimental trials, a novel field control element (FCE) is developed both for normal and anti-fog types of insulator discs. From the electric field simulation study, it is shown that the use of field control element for uniform pollution deposition prevailing under laboratory test conditions yields a significant reduction of maximum surface field for discs by about 47 to 54%. Similarly a reduction of about 37% to 55% in case of 3-disc string (for 33 kV class), 30% to 52% in case of 9-disc string (for 132 kV class), 27% to 52% in case of 14-disc string (220 kV class), 27% to 54% in case of 23/20 disc string (for 400 kV class) and 41% to 48% in case of 35/29 disc strings (for 765kV class) is achieved respectively for different strings. It is anticipated that this will lead to retardation in inception of scintillations/partial arcs, which in turn can reduce the risk of pollution induced flashover. Subsequently, it was aimed to experimentally evaluate the impact of field control element on the performance of disc/string under normal and that during polluted conditions. For this a national level unique artificial pollution test facility as per the international standards has been established for conducting pollution studies on disc insulator/strings up to 132kV system voltage (The rating of test source: 150kV/2A,100kV/3A,50kV/6A of 300kVA). Experimental investigations for the normal operation involving dry and wet power frequency flashover strengths, lightning impulse strength, radio interference level (RIV), visible discharge inception (Corona) level and voltage distribution along the string are carried out. The study showed that with the insertion of field control element, performance under normal condition is maintained and in fact noticeably improved in certain cases. Subsequently pollution flashover strength is evaluated using solid layer and cold fog methods. The pollution flashover strength exhibited an overall improvement 15 to 20% for all types of discs considered in the study. Similarly an improvement of 16 to 19% and 12-14% is observed for 3-disc string and 6-disc string respectively. The reasons for relatively lower gain in the pollution flashover strength as compared to reduction in maximum surface field are investigated. In particular, the reduction in improvement with number of discs in a string is dealt with. The non-uniform wet-ting against highly non-uniform drying of insulator surface and discs in a string, are identified as the cause for deviation. For an experimental verification of the same, the pollution layer resistance of individual discs in a 3-disc string is measured prior to and immediately after flashover. It is shown that the resistances of the different units become grossly different even though initial values were substantially the same. The values measured immediately after flashover show that the resistance of the top unit develops voltage enough to result in its flashover and subsequently, the flashover of the whole string. In summary, the main contribution of the present work is the development of novel field control element (FCE) for both normal and anti-fog type of ceramic disc insulators, which will enhance their pollution flashover strength. The actual cost of these elements is estimated to be about 1-2% of the cost of the disc, while the gain in strength is shown to be more than 12 - 20%.