Smart load management of distribution-class toroidal transformers using a dynamic thermal model

Thermal behaviour is a prime factor in the accurate performance assessment of power transformers as well as in the prediction of their life expectancy. This study presents a computer modelling tool based on an electro-thermal equivalent circuit of transformers that is able to predict the hot-spot temperature and average surface temperatures for all internal layers of distribution-class toroidal transformers. Temperature is the limiting factor that prevents running transformers for hours or days in overload conditions. The modelling tool presented in this study is capable to identify the safe maximum overload current and duration that a transformer can handle without introducing damage or loss of life. The model is helpful to predict the short-term (few hours) and long-term (few days) overload capabilities of transformers. The electro-thermal model can also be used as a tool to optimise the design and evaluate the performance of transformers. This study is specifically focused on the implementation of the proposed method on dry-type distribution-grade toroidal transformers. The model is built using circuit components (lumped R and C) obtained from the thermal-electrical analogy. The model is validated with numerous finite-element method simulations and laboratory tests with transformers of various power ratings.