Evaluation of the Effects on Electrical Losses and Thermal Ageing of Transformer Heat Recovery

To achieve net-zero carbon targets, alternatives to fossil fuels for space and water heating are required. Direct heat recovery from grid transformers becomes an option when heat demand is close to those transformers and the transformers can be run hot enough to provide heat at an acceptable temperature. However, increasing the operational temperature of a transformer tends to result in increased electrical losses and reduced lifetime. In this paper, three transformer thermal models were developed to evaluate transformer oil and winding temperatures and hence to derive losses and insulation thermal aging rate. First, a standard WTI (winding temperature indicator) controlled ONAN (Oil Natural and Air Natural)/ODAF (Oil Directed and Air Forced) cooling system transformer model was developed based on an IEC thermal model and is used as the reference. Second a quasi-static WTI model, ignoring the time constants, was developed and validated against the reference model. Third, a quasi-static model simulating a heat recovery system using a constant oil flow and constant top oil temperature was developed. It was found that the lifetime reduction for heat recovery using a $70{ }^{\circ} \mathrm{C}$ top oil temperature is negligible in comparison to the expected lifetime of the transformer investigated in this study. There is a modest increase in winding losses at the top oil temperature of $70{ }^{\circ} \mathrm{C}$, but this increase is approximately halved if the top oil temperature setting is reduced to $60^{\circ} \mathrm{C}$ and becomes negligible when the setting is reduced further to $50^{\circ} \mathrm{C}$.