Optimization of LV Winding Lead Routing in Three Phase Power Transformers to Minimize Induced Losses in Tank

The most economical design of a power transformer, taking capitalization costs into account, should offer a compact design with minimum stray losses. The design clearances between the tank on low voltage (LV) side and active part is dependent on induced magnetic flux density which generates eddy currents and resulting hot spot temperatures. Design of tank with clearances below the required values will lead to increase in tank eddy losses, while large tank clearances make the equipment uneconomical. The stray losses in structural components of large rating transformers account for 20-30% of the total load losses. The common practice of minimizing these losses is by arrangement of magnetic laminated shunts or copper screens on tank wall, which minimizes the incident winding leakage flux from reaching the tank. However, in higher rating transformers the additional flux generated by leads pose a serious problem, as the tangential flux generated by the high current carrying leads cannot be effectively shunted or screened by above arrangements. The distribution of eddy currents on the tank surfaces is quite complex, especially for higher rating three phase transformers where high current carrying LV leads are routed from winding bottom end to the turret covering the whole of top tank. Investigations were carried out on 3D electromagnetic model of three phase transformer to evaluate the eddy current distribution and resultant losses in tank wall for different combinations of lead routing. Based on the detailed studies, an optimal routing of LV leads is proposed, which will minimize the induced losses and hot spot temperatures in large three phase power transformers.