Thermal Performance Study of Power Transformer Filled with Biodegradable Oil Using the Effective Approach for Electromagnetic and Computational Fluid Dynamics Models Multiple Coupling

Large power transformers used to transmit electrical energy from the power plants to the grid are the units where significant amounts of heat are generated due to ohmic and core losses. Consequently, efficient heat dissipation must be ensured. Nowadays, most of the large power transformers are cooled in air-natural, air-forced, or oil-directed air-forced regimes with dielectric oils used for the active elements cooling to prevent their overheating. Unfortunately, commonly used mineral oils are characterised by low biodegradability, environmental contamination, and hazards in case of leakage and low fire point. The sustainability of mineral oil production, utilisation, and disposal is questioned nowadays due to limited natural resources and environment degradation. Therefore, biodegradable synthetic and natural ester oils are recognised as a promising alternative for the application of electrical devices cooling. This study presents a numerical investigation of 8.5 MVA disk-type power transformer cooling using biodegradable ester oil and conventional mineral oil as a reference. Operating conditions were defined for a wide range of ambient temperatures from -5.8 degrees C to 32.0 degrees C, which are characteristic for different seasons in Polish and Argentinian climate zones. For a detailed modelling of the entire power transformer, a computational strategy was introduced to develop several numerical models of selected parts of the transformer and to couple them using properly defined boundary conditions. Consequently, a detailed analysis of the power loss, oil flow, and the temperature level in different parts of the unit can be performed with a satisfying computational effort. In particular, the velocity field within the entire transformer tank and inside the windings oil ducts is evaluated and the local temperature values including the hotspot temperature can be determined. According to the results, the average windings temperatures are higher from 2 K to 9 K when the ester oil is used for the cooling of the analysed device. Moreover, the hotspot temperatures increased by up to 9 K and 18 K in low voltage and high voltage windings, respectively. The analysis of the fluid flow suggests that the oil ducts require modification if the mineral oils would be replaced by esters in the analysed construction.