Analysis of Aged Oil on the Cooling of Power Transformers from Computational Fluid Dynamics and Experimental Measurements

In this paper, experimental and numerical investigations were conducted to study the aging impact on the cooling capacity of mineral oils in power transformers. The experimental investigations had three objectives. Firstly the study of the impact of oil aging on its physicochemical properties using two diagnostic techniques which are: DDP (Dissolved Decay Products) according to ASTM (American Society for Testing and Materials)-D6802 standard and Interfacial Tension (IFT) according to ASTM-D971 standard. Secondly,the study of the impact of oil aging on its viscosity according to ASTM D445. Finally the development of empirical equations depicting oil viscosity changes, to be implemented in the numerical model. To achieve this, accelerated thermal aging tests of mineral oil samples were conducted in laboratory conditions according to ASTM D-1934standard. Data from experimental investigations were used to conduct the numerical investigations. A 2D axisymmetric numerical model was developed with COMSOL Multiphysics 4.3a to study the cooling capability of oil aged at different levels. The results indicate that the oil physicochemical properties are affected by aging. Furthermore the results show that the oil viscosity increase with aging and give a good correlation between viscosity and aging indicators. Finally it was found that sludge from oil oxidation byproduct contributes to the formation of hotspots and this leads to a significant increase of hotspot temperature in the power transformers.