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One of the cellulose insulation in high voltage power transformers is paper, and it is widely used in transformers due to low cost and excellent physical/electrical properties. However, the performance of paper could be affected by ageing. Ageing of paper is a complex phenomenon and can be influenced by many factors. This PhD thesis aimed to examine the ageing of paper in transformers based on Tensile Index (TI), dielectric strength and Low Molecular Weight Acid (LMA) measurements. The effect of ageing on the dielectric strength of paper was examined, through studying the influence of paper and oil ageing by-products such as LMA and HMA on the AC breakdown voltage. Meanwhile, the end of life for transmission transformer population in the UK was examined by studying the profile of LMA and TI of paper in scrapped transformers, with phase, winding, and layer locations taken into consideration. The relationship between TI, LMA and transformer age was modelled using previously published formulae in literature. The partitioning of LMA and HMA between mineral oil and paper was also investigated. Since TI is not directly measureable when a transformer is in-service, a TI prediction model was developed based on LMA measured in oil. The results indicated that there is no reduction effect of ageing (moderate towards severe) on the dielectric strength of paper, whereas the dielectric strength of paper is mainly influenced by the oil condition. LMA significantly reduces the breakdown voltage and accelerates the ageing of paper in mineral oil. The end of life based on TI is best represented by the 20 % retention of TI. Based on the case studies, the estimated end of life of transmission transformer population in the UK is around 43 years. The vertical distribution profile of LMA in paper and TI of paper along a winding is mainly influenced by vertical temperature profile. As paper ages, the amount of LMA in paper tends to increase as TI reduces. The percentage of LMA in service aged oil is about one fourth of Total Acid Number (TAN). The partition of LMA in mineral oil and paper favours its stay in paper and is heavily influenced by the weight ratio of paper and oil. The performance of the natural ester under high temperature ageing in open or sealed condition was evaluated based on TAN and viscosity measurements. The ratio of LMA and HMA in aged natural ester was examined. The effect of LMA and HMA on the AC breakdown voltage of natural ester and ageing of paper was also investigated. It was suggested that natural ester is suitable for application in hermetically sealed transformers. High temperature oxidation has significant effect on the physical and chemical properties of natural ester. Due to the polar nature of natural ester, the stay of LMA in natural ester is favoured over in paper which could be the reason why the breakdown voltage of natural ester and the ageing of paper are not significantly affected by LMA as in mineral oil.

Epoxy resin is widely used in high voltage apparatus as insulation due to its excellent mechanical, electrical and chemical properties. Fillers are often added to epoxy resin to enhance its mechanical, thermal and chemical properties. With the new development in nanotechnology, it has been widely anticipated that the combination of nanoparticles with traditional resin systems may create nanocomposite materials with enhanced electrical, thermal and mechanical properties. The project aims to improve the overall electrical performance by adding nanoparticles into epoxy resin. In the present thesis a detailed study on dielectric permittivity, AC breakdown strength and space charge behaviour of epoxy resin/nanocomposites with nano-fillers of SiO2 and Al2O3 has been carried out. The epoxy resin/nanocomposite thin film samples were prepared and tests were carried out to measure their dielectric permittivity and tan delta value in frequency range of 1Hz- 1MHz. The space charge behaviours were also observed by using the pulse electroacoustic (PEA) technique. The influence of filler type, filler size and filler concentration on nanocomposites ac breakdown strength were also examined. In addition, traditional epoxy resin microcomposites were also prepared and tested and the results were compared with those obtained from epoxy resin/nanocomposites. The present results indicate that the presence of nano-sized fillers enhances the insulation properties of the epoxy resin and the dielectric properties are strongly influenced by the interfacial region between epoxy and nano particles. It is the key factor that affects the electrical performance of epoxy nanocomposites. The multi core model has been applied to explain the effects of such interfacial region on the electrical performance epoxy nanocomposites. A new phenomenon of space charge accumulation at higher nano size filler loading concentration has been observed at a filler loading concentration above 3wt%. This phenomenon is a result of the formation of electrical double layer surrounding the nano particles. A comparison study between epoxy nanocomposites loaded with both surface treated and non-surface treated nano particles has also been carried out. The results indicate that nano particle dispersion rate is an important factor in determine the electrical performance epoxy nanocomposites. Surface functionalisation on nano size fillers by using silane as a coupling agent could help avoiding the formation of large agglomerations resulting in better insulating performance. In addition, it has also been found that the presence of water inside epoxy nanocomposites also leads to the reduction in dielectric properties due to the formation of water layers surrounding the nano particles. Those water layers could act as a conductive path to help charge carriers travelling through the bulk of the materials.

Although the cost of investment in power lines insulators is 3-5% of the total cost of the installation, the impact of their performance on reliability, failure costs, maintenance routines, etc in power systems is tens of times higher. Composite insulators were introduced 50 years ago and have been used around the world with consistently good experience. Low weight, easy handling, good performance under high pollution, low maintenance costs, and resistance to vandalism are some of their advantages. Nevertheless, acid rain, salty dust deposition, corona discharges, ozone, UV radiation, and humidity among other factors, deteriorate the quality of the polymeric housing reducing their hydrophobicity. The synergistic action of ageing factors is extremely complex and the whole degradation process may change when any one variable is slightly modified. Many studies have been carried out to increase understanding of the physicochemical processes which control the electrical and mechanical stability of polymers during in-service ageing with the objective of predicting remaining life-times. Vital areas of knowledge about polymer insulators are still incomplete and lacking; three of them are: (1) early stages of degradation in service under different environmental conditions, (2) monitoring and diagnosis techniques suitable for distribution installations and (3) steps to establish an insulators management plan based on condition and risk of failure. In this research these three topics are covered. A full review of literature about management of electrical distribution assets is included, followed by a specific plan developed for monitoring, diagnosis and ranking of insulators mainly supported by visual inspections. Diagnosis of medium voltages EPDM insulators recovered from service aged under different conditions is done using both traditional techniques and, uniquely, dielectric impedance. The relationship between surface roughness and static contact angle is also used to characterize insulators' surfaces. Early stages of degradation are studied focusing the experimental work to evaluate the electrohydrodynamic processes which occur on new samples under different conditions, giving special attention to leakage current pulse analysis, electric field enhancement, and resistance/capacitive behaviour including phase of leakage current. Results from each specific topic offer additional understanding of polymer insulators degradation providing insight to monitoring, diagnosis and management. Additionally, results open new topics in which new investigations are proposed.