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Transformer winding hot‐spot temperature (HST) is one of the important factors affecting transformer oil–paper insulation deterioration. This study presents a three‐dimensional coupled electromagnetic‐fluid‐thermal analysis method for HST calculation in a 10 kV oil‐immersed distribution transformer, the influence of the transformer internal metal structure parts on the HST of the winding is considered in the simulation. Combining electromagnetic‐field calculation with no‐load test and load test of the transformer provides a more accurate method to determine internal losses of the transformer. Taking those power losses as heat sources, the transformer fluid‐thermal field analysis is conducted with the finite volume method. The variation of physical parameters of transformer oil with temperature is considered in the simulation. On the basis of the equivalent thermal resistance theory, the equivalent thermal conductivities of transformer windings are obtained. The simulation results deduced from the proposed method agree well with the experimental ones, which are obtained with fibre optic temperature sensors during the transformer temperature rise test, the maximum temperature difference is <3°C. The results validated the validity and accuracy of the proposed transformer HST calculation method. [ABSTRACT FROM AUTHOR]

The frequency domain dielectric response method has been one of the most effective approaches for condition assessment of an electrical insulation system. The low‐frequency part is more effective for diagnosing the moisture content and aging status of oil‐paper insulation. To facilitate the study of the low‐frequency dielectric spectrum of oil‐paper insulation, a model applicable to the low‐frequency dielectric response of insulation oil is proposed in this study, which is based on the ionic motion of insulation oil. The creation of charge carriers under field test conditions mainly derives from the dissociation of ionic pairs. During testing, ionic motion of insulation oil is jointly affected by thermal diffusion and the electric field; moreover, dissociation and recombination of negative and positive ions also occur. Therefore, the above factors were all considered in the modelling process. The applicability of the model is verified against measured results. For further interpreting the proposed model, the characteristics of the ionic concentration distribution, the relation between ionic distribution and dielectric properties, and the application of the proposed model are analysed. [ABSTRACT FROM AUTHOR]

Copper sulphide deposition on insulating paper in oil‐impregnated power transformers affects the characteristics of oil‐paper insulation. Although several external factors, including electric field, contribute to the formation of deposition, the specific effect of electric field on copper sulphide deposition is still not well understood. This study presents an experimental research about the effect of an electric field on copper sulphide deposition. Paper pig‐tail wrapped copper winding were used, and 1.5 kV AC voltage at 130°C was applied on the winding for accelerated electric‐thermal ageing. The influence of partial discharge on deposition was investigated further. Results showed that the weight of copper sulphide depositions increased because of the presence of an electric field. Interestingly, this corrosion became serious under partial discharge effect. The electrochemical erosion and high temperature generated by partial discharge were considered as the main reasons for this phenomenon. The acid generated by the ageing of oil‐paper insulation further contributed to the acceleration of chemical reaction. The electric field aggravates Cu corrosion, which eventually leads to thermally induced dielectric breakdown of the insulating systems in transformers. [ABSTRACT FROM AUTHOR]

The ageing rate of cellulose insulation materials (paper/pressboard) in oil‐impregnated transformers is largely determined by moisture. In this paper, several experiments were conducted under controlled laboratory conditions to quantitatively diagnose the amount of moisture in cellulose insulation using frequency domain spectroscopy (FDS) technique, which has been proven to be an effective tool to predict the insulation condition in oil‐impregnated power equipment. The frequency dielectric response in oil‐impregnated pressboards, moisture content as well as test temperature, was monitored carefully, and a new dielectric characteristic parameter for the diagnosis of moisture content was proposed. In addition, the master curve technique was introduced to calibrate the temperature effect on FDS measurement results. These experiments revealed that a strong correlation exists between frequency dielectric parameter and moisture content in cellulose insulation. Such correlation can be utilised for better prediction of moisture content in paper–oil insulation. A modified assessment method for the diagnosis of moisture content in paper–oil insulation was investigated and its effectiveness was also demonstrated using a case. [ABSTRACT FROM AUTHOR]