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This study experimentally investigated and theoretically analysed the evolution of gas bubbles in a wet oil-paper insulation system. Water vapour bubbles significantly decrease the dielectric withstand strength of insulating oil. The influence of moisture content on paper, quality of oil, quality of paper, ageing state and temperature increase on this 'Bubble Effect' was systematically investigated. New thermally upgraded paper combined with new insulation oil shows the best resistibility against bubbling because of its microstructure. Paper ageing increases the bubble inception temperature, whereas oil ageing decreases this parameter. Bubbles appear only for a steep temperature rise of more than 3 K/min; for lower values the moisture diffuses into the oil only. An empirical equation that calculates the inception temperature with respect to paper and oil quality and ageing is derived. The evolution of gaseous bubbles constitutes a risk for transformer operation if a steep temperature gradient occurs, if the moisture in paper is above 2% and if bubbles move into an area with high dielectric stress. [ABSTRACT FROM AUTHOR]

The ability of a detailed RLC model of a winding to calculate the transfer function (TF) and to demonstrate the effects of various winding damages on TF has been proven on small laboratory windings by previous studies. However, this ability is not adequately known for real high-voltage windings. Hence, a 400 kV disc winding consisting of 86 discs is used in this work to study, whether the detailed model is able to represent the behaviour of large-power windings in a TF form. In addition, disc space variation (DSV), known as a frequently occurring mechanical damage in transformers, is analysed experimentally and mathematically. The accuracy of the model to identify the DSV is verified with the help of the measured TFs. [ABSTRACT FROM AUTHOR]

In the context of thermal performance and thermal design, it is of significance to predict the magnitude and the location of the 'hot-spot' temperature inside a power transformer. In an attempt to accurately predict this hot-spot in an oil-immersed transformer, various numerical modelling approaches have been developed for calculating the cooling oil flow distribution, which are generally categorised as either 'network models' or the methods that incorporate forms of computational fluid dynamics (CFD). In network modelling, the complex pattern of oil ducts and passes in a winding is approximated with a matrix of simple hydraulic channels, where analytical expressions are then applied to describe oil flow and heat transfer phenomena. On the other hand, CFD models often adopt discretisations of much higher fineness, which can be expected to offer a higher order of accuracy but also comes with a large increase in the required computational resources. In order to compare both modelling approaches, the network model implementation TEFLOW and a commercial CFD package, ANSYS-CFX, were applied on a typical 'zigzag' oil channel arrangement of a disc-type winding to predict oil flow distribution and disc temperatures; experiments on hydraulic models have also been performed to validate the models. The principle work of this study is then comparing the results and concluding recommendations to industrial practices. [ABSTRACT FROM AUTHOR]