Pressboard, a high density cellulose-based material used for insulation in high voltage power transformers, exhibits stress relaxation during compressive loading. Investigating the micro-mechanical mechanisms responsible for the relaxation can lead to modifications of the production process to control the behaviour of the material. This investigation can be done using Synchrotron X-ray micro Computed Tomography which provides sufficient temporal and spatial resolutions to capture the stress relaxation process. In the present thesis, a compression rig for in-situ mechanical loading during X-ray micro Computed Tomography was designed and constructed. Local tomography scans with sub-micrometre resolution were obtained at the TOMCAT beamline at the Swiss Light Source, Paul Scherrer Institut, Switzerland. Several fibre segmentation techniques are analysed, together with Optical Flow and Digital Volume Correlation (DVC), methods used for estimating displacement, strain and velocity vector fields. Suitability of the tested methods is evaluated, and it is found that segmentation of individual fibres in a cellulose material of such a high density is probably not possible using currently available segmentation techniques. The movements during relaxation are measurable at the used resolution, and can be estimated using Optical Flow. Further work into correction of image shift due to rig movement between scans, as well as image artefact reduction should allow for measurement and comparisons of displacement during relaxation as well as DVC-computed strain measurements during compression, recreating earlier results.

Pressboard, a high-density cellulose-based product, is used both as load bearing structures and dielectric insulation in oil-filled power transformers. During transformer operation, mechanical forces and vibrations are applied on the material. In particular, this investigation focuses on pressboard sheets placed between the turns of transformer windings, called the spacers, which during short circuit are subjected to high compressive forces of impulse nature. As a result of these forces, remaining deformations are created in the components. One step to reduce the negative consequences that come from the deformations is to understand how the fiber structure of the material changes by these forces, thus finding the week link. Understanding these changes could lead to future modification of the material to better withstand short circuits. To see the fiber structure and its changes in the material, pressboard has been investigated with a micro-CT at the Division of Applied Mechanics at Uppsala University. The scanned images have been reconstructed and analyzed in NRecon, CTAnalyser, and Matlab to investigate the density distribution changes and to identify the densification patterns. The study shows that pressboard initially has an inhomogeneous density distribution through the thickness and after mechanical indentations, the densification tends to begin in the more porous parts of the material. The project also included a prediction of the densification pattern, performed by finite element analysis (FEA) using a simplified material model. The results show that a material model with varying Young’s modulus through the thickness, based on a stiffness and porosity relationship, can produce similar densification patterns as in the experiments.

Ester liquids including natural ester and synthetic ester are considered as potential substitutes for mineral oil, due to their good biodegradability and high fire points. Although these liquids have been widely used in distribution and traction transformers, research efforts are required for the purposes of design and manufacture of high voltage and large power transformers which are filled by esters. Indeed, it would be risky to apply esters in large power transformers without thorough understandings of their behaviours in large gaps and/or when combined with pressboard insulation. Therefore, investigations of creepage discharges along the surface of pressboard in esters are vitally important and their behaviours should be compared with those of mineral oils. This thesis is aimed to investigate the creepage discharges along pressboard in esters and mineral oil under ac divergent electric field. Apparent charges, current signals and images of streamer channels were obtained synchronously to identify whether and how the introduction of pressboard surface would influence the inception and propagation of discharges as compared to tests in open gap. When over-stressed by higher voltages, the surface tracking along the pressboard-ester interface, triggered by sustaining creepage discharges, was studied and the evolutions of accompanying creepage discharge patterns were investigated. In these experiments, both esters and mineral oil impregnated pressboards were comparatively studied. The test results indicated that at the inception stage, the presence of pressboard or any other solid types in different liquids under test do not influence the PD inception voltages; in the propagation stage, solid surface tends to promote the development of discharges, especially those occurring in negative half cycles, and shifts more discharges towards the zero-crossing phase angles. This discharge promotion effect is much more evident in esters than in mineral oil, probably because of higher discharge intensity in esters and higher viscosity of esters. The space charge effect and the residual low density channel effect are proved as the mechanisms best explaining the influences of solids on creepage discharges. Under higher voltages, it was found that the impregnated pressboard is susceptible to discharge erosion characterized by “white and carbonized tree-shaped marks”, due to intense discharges occurring on or near the pressboard surface. The “white mark” appears at a lower voltage and propagates more easily on ester impregnated pressboard. The gaseous “white mark” channels will attract the subsequent discharges to follow the same discharge routes; the accumulative energy dissipation in these channels will then result in the carbonization of the channels. Once formed, the surface tree-shaped mark can continue to grow even under reduced voltage levels until it bridges the gap and causes the final flashover.

Ester liquids including both natural ester and synthetic ester are being considered as potential alternatives to mineral oil, due to their better environmental performance and for some liquids their higher fire point. Although these liquids have been widely used in distribution and traction transformers, it is still a significant step to adopt ester liquids in high-voltage power transformers because the high cost and severe consequence of a factory test failure and the high level of safety and reliability required in service for these units, tend to lead to a cautious approach to any step change in technology. Lightning impulse strength as basic insulation level is of importance for insulation design of power transformers and lightning impulse test is commonly required in the factory routine tests for high-voltage power transformers, so this thesis is aimed to investigate the electrical performances including pre-breakdown and breakdown of natural ester and synthetic ester under impulse voltage. Two types of field geometry were considered in the study, one is sphere-sphere configuration which represents the quasi-uniform fields inside a transformer and another is strongly non-uniform point-plane configuration which represents the situation of a defect or a source of discharge. In quasi-uniform field study, standard breakdown tests were carried out under negative lightning and switching impulse voltages. Influence of various testing methods on the measured lightning breakdown voltage was studied and the 1% lightning withstand voltage was obtained based on Weibull distribution fitting on the cumulative probability plot built up using the approximately 1000 impulse shots. As for strongly non-uniform field study, streamer propagation and breakdown event in ester liquids either with or without pressboard interface were investigated at various gap distances under both positive and negative lightning impulse voltages. A relationship between the results under lightning impulse and previously published results under step voltage was built up to predict the lightning breakdown voltage of ester liquids at very large gaps. The results indicated that impulse strengths of ester liquids for both breakdown and withstand in a quasi-uniform field, are comparable to those of mineral oil. In a strongly non-uniform field, streamers in ester liquids propagate faster and further, than in mineral oil at the same voltage level. Thus breakdown voltages of ester liquids are generally lower than those of mineral oil, which could be as low as 40% at a large gap distance of approximately 1000 mm. Introduction of parallel pressboard interface has no influence on the streamer propagation and thus does not weaken the breakdown voltage, but it tends to reduce the acceleration voltage particularly for mineral oil under positive polarity. Last but not least, a unique phenomenon of secondary reverse streamer (SRS) was observed in ester liquids, which occurs subsequently and well after the extinction of the primary streamer (PS) propagation within a single shot of impulse voltage and has the reverse polarity to the PS. The formation mechanism of SRS is explained mainly due to the reverse electric field induced by the residual space charges left by the PS.

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