Abstract
The transformer thermal performance is a crucial issue that should be considered in the design stage. Recently, natural and synthetic ester oils have been proposed as environmentally friendly oils to substitute conventional mineral oil. This paper aims to evaluate the transformer thermal performance when using these environmentally friendly oils under different loading power factors. Three commercial types of environmentally friendly oils are considered, Midel 1204 and Midel 1215 as natural esters and Midel 7131 as a synthetic ester. Thermal-related properties of these esters are experimentally evaluated. These results include viscosity, thermal conductivity, specific heat capacity, and density. The experimental data is used to find a best representative formula for the thermo physical properties of these oils as a function of operating temperature by using least square regression method. A model 3D CFD COMSOL model is built including heat transfer model and laminar flow model to calculate the rated top oil temperature (TOT) and the rated winding hottest spot temperature (HST) for these environmentally friendly oils and for mineral oil. The rated output TOT and HST from COMSOL are used as inputs for a developed MATLAB model through which TOT and HST all day long can be obtained. Using these models, the TOT and HST for transformers filled with mineral oil and environmentally friendly oils are obtained. Also, the transformer aging and loss of life are assessed with these oils for the different scenarios of the loading power factor (PF). Finally, the annual energy losses of the transformer, their cost, and their impact on greenhouses gases emissions and corresponding environmental cost are evaluated as key issues for making proper decisions toward using environmentally friendly oils. It was found that using environmentally friendly oils reduces significantly the transformer aging compared to using mineral oil for different loading PFs.
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