An Improved Dynamic Thermal Circuit Model for Transformers and its Application to Evaluate Capacity Increase

The thermal circuit model is widely used in the estimation of transformer temperature, especially the hot-spot temperature which is extremely challenging to be acquired. In this paper, a dynamic thermal circuit model for calculating the transformer internal temperatures is established considering actual operating conditions and environmental factors synthetically. In our model, load losses are calculated according to the operation currents and the tap positions, while the effects of sunshine and wind are reflected in thermal resistances. The parameter-estimation methods are also analyzed to solve the problem of information incompleteness in practice. To implement and verify the dynamic thermal circuit, the real-time loss and cooling thermal resistance of a transformer are calculated first. Then, the hot-spot temperature based on the average oil temperature is calculated for a period of 24 hr. The calculated results were compared with the temperature-rise test result and practically measured data. The comparison shows that the estimation accuracy of the proposed model is satisfactory. The model is then used to evaluate the feasibility of increasing operating capacity and the maximum safe load factor for a long-term operation.