Modelling the thermal-hydraulic behaviour of shell-type power transformers: on the influence of fluid properties in OD and ON cooling modes

The growing awareness of the environmental impact associated with industrial products has accelerated the development and adoption of eco-designed solutions. In the electric power sector – one of the major contributors to global CO₂ emissions – power transformers are key components, playing a critical role in energy transmission and distribution networks. As fundamental components of energy transmission and distribution networks, their design has a direct influence on the environmental footprint of the power grid.

In response to the rising global electricity demand, and associated greenhouse gas emissions, interest has intensified in the use of alternative materials, particularly in the use of insulating liquids as replacements for conventional mineral-oil dielectric fluids. Among these, biodegradable insulating liquids have emerged as promising candidates to replace traditional mineral-oil-based dielectric fluids, offering reduced environmental risks. However, the transition to such alternatives must be achieved without compromising the essential thermal, dielectric, and operational performance required for reliable transformer operation.

In addition to the environmental characteristics of the fluids, such as biodegradability, their thermal-hydraulic properties can influence the environmental impact of the transformer. An improved cooling efficiency can lead to a higher MVA/ton, resulting in reduced material usage during manufacturing and, in general, to a lower carbon footprint.

This study investigates the thermal-hydraulic performance of a Shell-Type power transformer winding using different insulating liquids. A purpose-built, highly instrumented experimental setup was developed to validate numerical models. The system allows dynamic control of cooling conditions and utilizes DC loading for precise control of heat losses. It was configured to operate under both ON (Oil Natural) and OD (Oil Directed) cooling modes.

In the present work, the thermal-hydraulic behavior of one of the pancake coils that comprise the experimental setup was evaluated, through Computational Fluid Dynamics (CFD). Two fluids were analyzed: a conventional mineral oil (NNT) and a bio-based hydrocarbon oil (NT3).

Their distinct thermophysical properties were integrated into the Shell-Type Winding model to evaluate thermal-hydraulic performance both under ON and OD conditions.

The results confirmed that fluid properties play a dominant role under ON cooling, leading to significant differences in flow rate and temperature distribution, while under OD cooling the pump governs the hydraulic behavior and limits these differences. For the same imposed average oil temperature, NB3 under ON cooling yields lower top-oil and hot-spot temperatures

(by approximately 9 K), a reduced inlet-to-outlet temperature increase, and a lower average winding-to-oil temperature gradient compared to NNT, indicating improved thermal performance. However, a higher hot-spot factor is observed for NB3 under ON cooling, showing that this parameter alone is insufficient to assess cooling efficiency and highlighting the need for optimized winding geometries to fully exploit the benefits of advanced insulating fluids.