Reduction of Stray Losses in Flange–Bolt Regions of Large Power Transformer Tanks

In large power transformers, the presence of stray currents in the structural elements near the high current bushings can be considerable, and this leads to hot spots. This work presents a practical analysis of overheating in the bolts that join the tank and the cover, which are near the high current bushings of the transformer. Overheating results are analyzed and discussed for the case of a 420-MVA transformer. The hot spots in the flange-bolt regions are discovered by thermal maps that are obtained during power transformer operation as a part of a preventive maintenance program. In this paper, we use copper links to ensure the connection of both the cover and tank body, significantly reducing the overheating of the flange-bolt region. The copper link solution has been validated by measurements. We have used calibrated measurement instruments in all the experiments. Moreover, a 3-D finite-element analysis of the geometry of interest has been used to verify the copper link solution.