Thermal Analysis of a Dry-Cast Medium-Voltage Medium-Frequency Transformer

The thermal analysis of a 600 kVA dry-cast medium-voltage medium-frequency transformer is presented. The transformer is based on litz-wire windings and a nanocrystalline core, is designed to operate at 2.5 to 8 kHz, and is cooled with forced air convection. The experimental part of the analysis comprises a DC test, an AC load test, and an AC no-load test, all for different losses and cooling rates. An additional numerical analysis was carried out using computational fluid dynamics with conjugate heat transfer. First, different turbulence models were benchmarked using a simplified 2D configuration of channel flow. The transition shear-stress-transport model was found to be suited best and was used in the subsequent 3D analysis of the transformer. In the paper, practical insight is provided into the various aspects of the thermal analysis, highlighting the roles of the different types of tests and the 3D simulations. For cast windings, the surface temperatures are found to be sensitive to the location of the temperature sensors, making the positioning of these sensors important. Heat transfer via the leads is found to play a minor role to overall cooling and needs no correction. For the core leg, radiative heat transfer from the LV winding plays a non-negligible role. Computational fluid dynamics is found to be capable of realistically reproducing the temperature trends and influence of the cooling strength when using an appropriate turbulence model and mesh.