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Circulating Currents in Stator Coils of Large Turbine Generators and Loss Reduction

Stator coils transposed by 540deg are widely used as one of the solutions to reduce the circulating current loss in the coil strands of turbine generators. However, the difference of the leakage fluxes in the two coil end parts, which is possibly caused by the coil end length deviation, the winding pitch, and the presence of the connection rings, may induce unbalanced voltage in the stator coil strands, which increase circulating current loss even in the transposition coils. This paper describes the mechanism and the analysis results of the circulating current increase in the coil strands due to the coil end flux difference. Then, some new schemes to reduce the circulating current loss are proposed and discussed.