Thesis (Ph. D.)--Anōtatē Scholē Ēlektrologōn Mēchanikōn E.M. Polytechneiou, 1981. / Summary in English. Includes bibliographical references (p. 139-142).

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The deterioration of electrical insulating oils results in the formation of complex oxidation products, many of which are polar in structure. The significance of the microwave-frequency dielectric-loss measurement, when applied to the evaluation of aged transformer oils, is investigated. A cylindrical cavity, operating in the TE₀₁ mode, is used to measure the loss tangent of aged transformer oils. Q-factor measurements are made by a dynamic method which is described. The problem of mode interference in the cavity is investigated in detail. It is found that the loss tangent of transformer oils, measured at X-band, increases as the oil deteriorates through oxidation. The increase is influenced by several factors but closely parallels the increase in acidity. Sludge particles do not in themselves cause a significant increase in the dielectric losses. An indirect correlation between the loss tangent and the sludge content of an oil may exist but has not been established. The change in the dielectric constant of an oil caused by the presence of dissolved water, or by the ageing process, is too small to be measured by the method used. A small but measureable increase in the loss tangent is produced by the presence of water in concentrations of approximately 75 parts per million. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

The work contained in this thesis is concerned mainly with conduction mechanisms and polarization processes in thin amorphous insulating films. A model has been proposed and its d.c. conduction characteristics computed. The numerical results show the possibility of obtaining either space charge, Schottky, or Poole-Frenkel characteristics depending on the model parameters. The transient electronic discharge current has been analysed and the results show that this electronic current is approximately independent of the preapplied voltage in contrast to the ionic discharge current which is linearly dependent on preapplied voltage. This result, together with the experimental results obtained on Ta/Ta₂O₅/Au diodes, suggests that the calculations of low frequency dielectric losses using step response measurements are complicated by space charge effects only when the preapplied field is relatively low (≤lMV/cm for Ta₂0₅ films). Ta/Ta₂O₅/Au diodes were prepared by solution anodization or plasma anodization. All prepared diodes exhibited a rectification behaviour. Over the frequency range 100 Hz -100 kHz capacitance and loss tangent were found to decrease slightly with increasing frequency while the equivalent series resistance was found to be approximately proportional to ω⁻¹‧º⁵. All prepared diodes, with gold counter electrodes less than 1000 Å thick, were found to withstand, under a slowly applied field, field strengths approaching the formation field value. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

A dissertation submitted in fulfilment of the requirements for the degree of Master of Science in Engineering to the Faculty of Engineering and the Built Environment, School of Electrical and Information Engineering, University of the Witwatersrand, Johannesburg, 2018 / Power cable terminations and joints experience high electrical stress due to the abrupt change in geometry of the cable; hence the insulation at these points is more prone to partial discharges and has been reported as the main root cause of power cable system failures. Electrical failure of the insulation is known to occur due to a phenomenon of Partial Discharges (PD). Modern practice (especially in higher voltage installations) entails installation of PD sensors at strategic locations during installation of electrical equipment such as cable joints and terminations. This enables continuous monitoring of PD events in the plant, and this is termed on-line PD diagnosis. However, with limited knowledge to interpret the meaning of certain PD changes during the service period, this practice remains limited. It is therefore the interest of the study to understand the time evolution behaviour of PD characteristics in order to discern the insulation condition or deteriorating stages. The present study is on XLPE power cables, focusing on PD in artificial defects in the cable termination insulation that in most cases arise from poor workmanship. The power cables were subjected to accelerated ageing to emulate their ageing under service conditions. PD measurements were then conducted at periodic time intervals and characterized PD in terms of PD Inception Voltage (PDIV), maximum apparent PD magnitude (Qmax), Pulse Repetition Rate (PRR) and Phase-Resolved-PartialDischarge-Pattern (PRPDP). The findings are that, Qmax, PRR and PDIV did not show any time-evolution trends unique to a defect, the general trends observed were that of a fairly constant PDIV with several fluctuations of a 5 kV band. Qmax showed a decreasing trend over ageing time. The PRR decreased overall, with a pick up increase near the end of the tests. Qmax and PRR were noted significantly fluctuative between 23% and 57% of the total ageing period, distinct characteristics were that, the tram line had the largest PRR which is expected since it is a flat cavity, and iii the PRPDP appeared more skewed than other defects. The semicon feather had a PRPDP that seemed like a combination of a void discharge and corona discharge. The ring cut PRPDP was similar to that of the tram line except that it was not skewed. Furthermore, a capacitance PD model was constructed in Matlab R Simulink R to emulate experimental observed PD behaviour and therefore confirm the theory explaining the observed time-dependency of PD phenomena. Simulated void discharge PRPDP which corresponded with experimentally measured PRPDP were obtained for the unaged, moderately aged and severely aged cavity defect. The corona-surface discharge effect observed in the semicon PRPDP was also successfully emulated. The study outcomes suggest that PD characteristics evolve over time, and that the behaviour of the observed trend is unique at different stages during ageing. The time evolution characteristics of PD are The PRPDP signatures did not change with time of ageing despite the variations in Qmax and PRR. This means that, defect signatures obtained prior ageing or in-service operation of the cables can still serve as a good reference of identifying the nature of the defect at different ageing stages except in the event of PD evanescence. From the simulations, it was derived that the PD region surface conductivity as well as the geometry of the defect are the main contributing factors to the unique signatures observed at different stages and per defect. / XL2019

Polymers are widely used as insulating materials in high voltage power apparatus because of their excellent electrical insulating properties and good thermomechanical behavior. However, under high electrical stress, polymeric materials can get deteriorated which can eventually lead to the failure of the insulation and thereby the power apparatus. Electrical treeing is one such phenomena whereby dendritic paths progressively grow from a region of high electrical stress and branch into conducting channels in a solid dielectric. The propagation of electrical trees is of particular interest for the power industry as it is one of the major causes of failure of high voltage insulation especially in high voltage cables, cast resin transformers as well as rotating machines. To improve the life time of the electrical insulation systems there is a need to improve the electrical treeing resistance of the insulating material for high voltage application. With the development of nanotechnology, polymer nanocomposites containing nano sized particles have drawn much attention as these materials are found to exhibit unique combinations of physical, mechanical and thermal properties that are advantageous as compared to the traditional polymers or their composites. Literature reveals that significant progress has been made with respect to the mechanical, optical, electronic and photonic properties of these functional materials. Some efforts have also been directed towards the study of dielectric/electrical insulation properties of these new types of materials. Considering the above facts, the present research work focuses on utilizing these new opportunities which have been opened up by the advent of nanocomposites to develop tree resistant insulating materials for high voltage power applications. Electrical treeing is a common failure mechanism in most of the polymeric insulation systems and hence electrical treeing studies have been carried out on two types of polymers (viz. polyethylene used in high voltage cable and epoxy used in rotating machines and resin cast transformers) along with three different types of nano-fillers, viz. Al2O3, SiO2 and MgO and with different filler loadings (0.1, 1, 3, 5 wt%). Furthermore, considering the fact that electrical treeing is a discharge phenomenon, the partial discharge characteristics during electrical tree growth in polymer nanocomposites was studied. As morphological changes in the polymer influence the electrical tree growth, the influence of nano-particle induced morphological changes on the electrical treeing has also been studied. Above all, an attempt has also been made to characterize and analyze the interaction dynamics at the interface regions in the polymer nanocomposite and the influence of these interface regions on the tree growth phenomena in polymer nanocomposites. A laboratory based nanocomposite processing method has been successfully designed and adopted to prepare the samples for treeing studies. Treeing experimental results show that there is a significant improvement in tree initiation time as well as tree inception voltage with nano-filler loading in polymer nanocomposites. It is observed that even with the addition of a small amount (0.1 and 1 % by weight) of nano-particles to epoxy results in the improvement of electrical treeing resistance as compared to the unfilled epoxy. In fact, different tree growth patterns were observed for the unfilled epoxy and epoxy nanocomposites. Surprisingly, even though there is not much improvement in tree inception time, a saturation tendency in tree growth with time was observed at higher filler loadings. To understand the influence of nano-particles on electrical treeing, the interaction dynamics in the epoxy nanocomposites were studied and it was shown that the nature of the bonding at the interface play an important role on the electrical tree growth in epoxy nanocomposites. The results of electrical treeing experiments in polyethylene nanocomposites obtained in this study also reveal some interesting findings. An improved performance of polyethylene against electrical treeing with the inclusion of nano-fillers is observed. It is observed that there is a significant improvement in the tree inception voltage even with low nano-filler loadings in polyethylene. Other interesting results such as change in tree growth pattern from branch to bush as well as slower tree growth with increase in filler loading were also observed. Another peculiar observation is that tree inception voltage increased with increase in filler loading upto a certain filler loadings (3 % by weight) and then decreased in its value at high filler loading. The morphology of polyethylene nanocomposites was studied and a good correlation between morphological changes and treeing results was observed. Effect of cross-linking on electrical treeing has also been studied and a better performance of cross-linking of nano-filled polyethylene samples as compared to the polyethylene samples without cross-linking was observed. The partial discharge (PD) activity during electrical tree growth was monitored and different PD characteristics for unfilled and nano-filled polyethylene samples were observed. Interestingly, a decrease in PD magnitude as well as the number of PD pulses with electrical tree growth in polyethylene nanocomposites was observed. It is known that PD activity depends on the tree channel conductivity, charge trapping and gas pressure inside the tree channel. The ingress of nano-particles into the tree channel influences the above known phenomena and affects the PD activity during electrical tree growth. The observed decrease in PD magnitude with increase in filler loading leads to the slow propagation of electrical trees in polyethylene nanocomposites. In summary, it can be concluded that polymer nanocomposites performed better against electrical treeing as compared to the unfilled and the conventional micron sized filled polymer composites. Even with low filler loading an improved electrical treeing resistance was observed in polymer nanocomposites. An optimum filler loading and a suitable filler to inhibit electrical treeing in the polymers studied are proposed. This work also establishes the fact that the characteristics of the interface region and the induced morphological changes have a strong influence on the electrical treeing behaviors of nanocomposites. These encouraging results showed that epoxy and polyethylene nanocomposites can be used as tree resistant insulating materials for high voltage applications. These results also contribute to widen the scope of applications of polymer nanocomposites in electrical power sector as well as development of multifunctional insulation systems.

Call number: LD2668 .T4 1966 F837 / Master of Science

No description available.

Buildings consume more than 30 percent of the primary energy worldwide with 65 percent of this attributed to heating ventilation and cooling. To help address this, stratified concrete panels (SCP) have been developed to provide insulation without compromising the thermal mass of concrete. SCP is created by vibrating a single concrete mix containing heavy and lightweight aggregates. Vibration causes the heavy aggregates drop to the bottom so that two distinct strata are formed; an internal structural/heavyweight layer providing thermal mass and an external lightweight layer for insulation. SCP incorporates waste products, for both financial and environmental gains, from which technical benefits also result. Stratified concrete panels have been made and tested during past research projects with results suggesting that SCP could be a competitive product in the residential construction industry, an area in which precast concrete systems have not been favoured in New Zealand. Consideration has been given to the specific rheological requirements of the concrete mix design and the hardened properties of the finished panels. This research considers the commercial viability of SCP using an industrial setting. For practicality of the setting, some materials were altered from past laboratory work to materials that are more easily sourced and better understood but with similar properties as those used previously. Several panels were cast at Stahlton precast yard in an effort to optimise the production process. Consistent results were not achieved and a range of stratification levels were produced. This showed that some capital investment is required to commercialise SCP to provide more energy for vibration such that sufficient stratification can be reliably attained. Two panels were then stood up in an exposed area with the exterior facing north to test for warping effects in a practical setting. No measurable warping occurred over this time which concurred with past work and long term readings that were taken of four year old panels. Structural, thermal and durability tests were carried out on panels with a range of stratification levels to assess the sensitivity of these properties to the level of stratification. From this it was found that the panels with better stratification had significantly better thermal properties than those with moderate to poor stratification. Generally the thermal targets for this project were not met with the total thermal resistance (R-values) not meeting current code requirements. In some cases structural properties were improved with better stratification as the structural layer was stronger through better consolidation. Delamination potential increased with stratification and with age. This requires further research to minimise this effect using fibres across the layer boundary. Porosity was increased in the structural layer in the poorly to moderately stratified panels as the structural layer was not consolidated enough due to lightweight aggregate contamination. As with any new innovation, market acceptance is largely governed by public perception. With appropriate marketing as a sustainable energy saving product, SCP has the potential to be competitive in the residential construction market with some capital investment.