Temperature distribution in ventilated dry-type transformer windings

Temperature is an important factor when designing a transformer since thermal stress deteriorates electrical insulation and results in the failure of the transformer. Most of the manufacturers obtain winding temperatures from empirical thermal models based on energy balance and/or by heat run tests once the transformer is manufactured. However, the empirical models and heat run tests only provide average winding temperature rise and estimated hottest-spot temperature. The location of the hottest-spot temperature cannot be determined. These methods require the additional cost of reassembling if the transformer fails the test.;In this study, new thermal finite element models for both foil-type and disc-type windings in ventilated dry-type power transformers were proposed. All cooling surfaces were identified, and heat transfer coefficients for each surface were presented. The proposed thermal models were applied to a ventilated dry-type transformer rated at 2000 kVA whose winding combination was foil-disc type under different load conditions in two-dimensional (2-D) space using the finite element method. The thermal models were solved by coupling them with an electromagnetic finite element model to deal with non-uniformly generated power losses due to induced currents. The calculated surface temperatures were compared with the experimental values collected by thermocouples and an infrared thermometer while short-circuit testing of the selected unit. The numerical results were in good agreement with the experimental results and the proposed model showed fair accuracy. The effect of the heat from the core on winding temperature distribution was investigated numerically. The impact of ambient air temperature on the thermal models was also studied numerically.;Obtaining temperature distribution by the heat diffusion equation with appropriate boundary conditions can solve these problems. Despite the considerable advantages, the mathematical thermal models for dry-type transformers have received little attention in the literature while many research works have been reported for liquid-filled transformers. Moreover, a few problems have been found in previous thermal models for dry-type transformers. No solutions were presented for local overheating due to localized induced currents. Temperature distribution was obtained only under steady-state conditions. Finally, no heat transfer coefficients were proposed for disc-type winding in spite of their wide usage.