Continuous Thermal Overloading Capabilities of Ester Oil Transformers in Oil Directed Cooling Conditions

The prediction of temperature distribution in ester oil (natural and synthetic ester oil) filled transformer due to different types of cooling modes are important for thermal designer in order to utilize the ester oil benefits of biodegradable, fire safety substations and continuous overloading capabilities with respect to mineral oil. It is still a technical challenge for ester oil to utilize in transformers since higher viscosity of ester oil will leads to reduce the heat transfer process mainly in convection. With the ester oil in a transformer, there will be reduction of convection fluid currents which leads to higher temperature in top temperature oil and winding temperature rise in ester oil filled transformers. In this paper, the effects of mineral oil and ester oil on top oil rise, winding rise and hot-spot temperature arising from their potentially different thermal properties are investigated using Thermal hydraulic network model (THNM) on 60/75/90MVA, 220/33kV transformer. The power losses are calculated from FEM based simulations and experimental data for thermal analysis using THNM model. The direct impact of oil flow rate and total pressure drop over the windings in oil directed (OD) cooling modes are analyzed under rated loading and 120% continuous overloading capabilities (108MVA) with respect to mineral oil.