Determination of local losses and temperatures in power transformer tank

Abstract

Paper presents research of local losses and temperature rise in transformer steel tank. First experimental method based on initial rate of rise of temperature is presented. This is a direct method for determining distribution of losses in transformer structural steel parts. Technique relies on the fact that after a body has settled at a steady state temperature and the internal heat source is suddenly removed or applied, the initial rate of temperature change at any point is proportional to heat input (loss density) at that point. To test applicability of sensors and instrument for the local loss measurement method, measurement system was tested on conductors (strips) and magnetic steel rings. Second part of experimental work consisted of investigations on model for tank local overheating. The model consisted of excitation windings that were sources of magnetic field. Existence of three separate windings gave the possibility to change value and position of magnetic field source inside the tank. Local losses in the tank were evaluated by proposed method of initial rate of rise of temperature. Heat-run tests were made on the model and local temperatures on the tank were measured. Measured local losses and local temperatures were used for determining local heat transfer coefficients on tank – oil interface. It was concluded that heat transfer coefficients can be presented as function of heat flux from tank to oil. Finally, temperatures in transformer tank were calculated by finite element method. Losses calculated by electromagnetic calculation represented heat sources in thermal numerical model. Heat transfer equations were solved in solid domain (tank) while cooling conditions were defined by heat transfer coefficients checked experimentally. Calculated temperatures were compared to measured temperatures and gave good agreement.