Experimental validation of a new methodology to reduce hot spots on the screws of power transformer tanks

In power transformers, the presence of stray currents in the structural elements near to the high current bushings can be considerable and this usually leads to hot spots. This work presents the analysis of the overheating of the screws that join the tank and the cover; these screws are near to the low voltage bushings of the transformer. Overheating results are analyzed and discussed for the case of a 420 MVA, 20/230 kV, OA/FOA transformer. The hot spots in the screws are discovered by thermal maps (thermography) that are obtained during the power transformer operation as part of the preventive maintenance program. This paper proposes the use of copper sill to ensure the connection of both the cover and the tank body because this solution significantly reduces the overheating of the screws. The proposed solution, which has been validated by measurements, significantly reduces the hot spots on the screws of power transformer tank.