Evaluation of eddy current losses in the cover plates of distribution transformers

The elimination of hot spots and the reduction of eddy current losses in bushing mounting plates is an important consideration in transformer design. The currently used bushing mounting plates are either mild steel plates, or mild steel plates with non-magnetic stainless steel inserts or stainless steel plates. The authors calculate the eddy current losses in bushing mounting plates using four different methods; (i) an analytical formulation; (ii) a three-dimensional finite element method (FEM); (iii) from measured values of the initial temperature rise; and (iv) from measured values of the steady-state temperature rise. There is a close agreement between the loss values obtained using these four methods. The analysis has resulted in a detailed understanding of the loss pattern and temperature rise phenomenon in bushing mounting plates. The authors also analyse tank plates of small pad-mounted distribution transformers. It is shown that judicious use of non-magnetic stainless steel can result in considerable energy savings for pad-mounted transformers. FEM simulations are performed to find out cost-effective materials for the tank plates of the pad-mounted transformers. A T-shaped stainless steel plate is found to significantly reduce the load loss. The results of the simulations have been verified on a 225 kVA pad-mounted transformer.