Coupled Electromagnetic-Thermal Analysis and Evaluation of Ester Oil under Power Frequency Conditions

Abstract

With rapid increase in the demand of electrical energy, it becomes indispensable to transmit large quantity of power from major generating stations to load centers with minimum losses. This has necessitated the development of higher rating power transformers. The losses in power transformer need to be estimated accurately as even 3-5kW reduction in losses give a competitive advantage. Stray load loss contributes significantly 20%- 30% of the total load losses. This loss primarily depends upon the leakage flux. Hence it is quite evident to study the electromagnetic behavior of each component of transformer to accurately estimate stray loss. Finite element method (FEM) is a highly efficient numerical method to analyze and compute the electromagnetic field and power losses in conducting parts. Stray losses occur in windings and structural parts of transformer. The other important aspect is to enhance the continuous overloading level of transformer. In oil filled transformer, mineral oil plays a role of coolant and insulator as well. Owing to the good environmental performance in terms of higher flash and fire point as compared to mineral oil, esters liquids are considered as an alternative for higher breakdown strength and more safety and reliability in operation. The study in this project is aimed to estimate the stray losses for optimal operation of power transformer. Further it is aimed to investigate the ac withstand level of natural ester oil, synthetic ester oil and the effect of polar contamination in transformer oil by considering pure mineral oil as a benchmark. Rising voltage method is used to measure AC breakdown voltage in all cases.Sphere-Needle electrode configuration is considered to create non-uniform field in order to evaluate the winding insulation under actual operating conditions. Weibull distribution is used to model the non-uniform field with 1% breakdown probability. Finally, FEM technique is used to estimate the maximum stress and the degree of uniformity in the consideredliquidunder observation. It is concluded that the stray losses in the power transformer can be reduced by splitting winding base and use of tank shields.On the other hand the breakdown strength of the transformer insulating liquid having different grades of mineral oil is affected by adding the polar contamination and aromatics content.