Optimization of Transposition in Stator Bars of Hydro-Generators

Large-scale generators tend to have high power density. In hydro-generators, stator windings commonly use Roebel bars in which strands are transposed in slot region to suppress circulating currents between strands. The Roebel bar allows increasing machine efficiency, and consequently energy savings. But in end regions of stator windings, the unbalanced leakage fluxes still produce circulating currents. To alleviate the circulating currents in strands, one more transposition can be made in end regions. Such reduced circulating current is achieved at the cost of the consumption of more copper. Therefore, a trade-off between circulating current losses and copper losses must be made. To this end, the transposition method is optimized based on the theoretical analysis of the losses caused by circulating currents.