Prediction of Static Pressure Drop, Velocity and Flow Rate of Higher Viscous Nature of Ester Oil in Power Transformers under Oil Directed Cooling Conditions

The study on oil flow rate, pressure drop (pressure losses) behaviour of ester oil group (both natural and synthetic ester oil) due to its higher viscous nature is important for thermal designer during thermal design stage, since oil flow distribution within the winding has a direct impact on thermal performance of the transformer. During oil directed (OD) cooling modes, the static pressure drop over the continuous disc type of winding determines the ester oil split among low voltage windings (LV) and high voltage windings (HV) which is connected hydraulically in parallel. In addition, the oil flow distribution in disc type of transformer windings in conjunction with power loss (no-load and load loss) of transformer will decide the location and magnitude of the hotspot temperature in transformer. Hence, there is still a technical challenge with ester oil for transformer manufacturers and utilities to use in large rating power transformers mainly in OD cooling modes. In this paper, the effects of mineral oil and ester oil on frictional pressure, gravitational pressure, oil flow rate, velocity in cooling duct, temperature distributions in top and bottom insulation, main winding zones, core, radiators and coolers are investigated under OD cooling conditions. The typical transformer rating of 60/75/90MVA, 220/33kV is considered for thermal analysis. Since, natural ester oil and synthetic ester oil are potentially different in thermal properties, the thermal analysis results are compared with mineral oil. The thermal hydraulic network model (THNM model) are used for fast calculations of thermal parameters in typical large power transformer. The power losses distributions are calculated from FEM based simulations as an input for thermal analysis.