Reduction of Loss and Local Overheating in the Tank of a Current Transformer

Transformer manufacturers often use low carbon steel for oil tanks to cut down production costs. The low carbon steel tanks experience not only high eddy currents loss, but also suffer from localized overheating in the tank particularly in the vicinity of insulated terminals. A numerical analysis of the eddy current loss generated in the tank walls of a current transformer (CT) due to adjacent high-current carrying primary conductor is presented. A 3-D nonlinear-transient finite-element method was used to determine the eddy current loss. Estimated losses at rated and overload conditions are then compared with the measured ones; they are in good agreement. In order to reduce the tank loss and local overheating, three nonmagnetic inserts are proposed and their effectiveness is analyzed. These inserts yield about 25 to 40% reduction in the total loss in the CT compared to that without any insert. Results also indicate that these inserts may be used to obtain a significant reduction in the local overheating, thereby contributing to improved life and reliability of the CTs.