Tank Losses in a Generator Step-Up Transformer

Abstract

The background for the project is the 320 MVA generator step-up transformer for Jostedal Power Plant. The transformer experiences excess heating in a limited part of the steel tank and lid from eddy current loss caused by the low voltage busbar configuration. This is unfavorable for the gasket and the transformer oil. In the project a test rig has been constructed which features a mild steel plate, being subjected to a magnetic field. The temperature in the plate is recorded and compared versus a numerical model created in Comsol Multiphysics and the use of the New Thermometric Method. The non-linear magnetic permeability in mild steel has also been investigated. The heat transfer analysis from the numerical model showed an acceptable temperature deviation compared with the actual temperature in the steel plate. The knowledge from this has been used to create numerical model of a selected part of the transformer utilizing symmetry conditions. The Heat Transfer analysis of this coincides well with thermography images of the overheating area. Poor electromagnetic contact between tank and lid may be an important part of the excess heating in the flange.It is recommended to mount braided copper links over the flange to see if this will reduce some of the excess temperature. If this is not adequate, the evaluation of the different measures that could be done to reduce stray loss concluded that mounting an aluminum shield would be the preferable choice. Mounting such a shield would repel the flux from entering the tank steel, and thereby reduce the temperature. However, the shield itself may experience heating, but not to an extent that the steel experience today.