Stator Winding Systems with Reduced Vibratory Forces for Large Turbine-Generators

It is shown that electromagnetically induced vibratory forces on the stator windings of large turbine-generators can be substantially reduced by using winding arrangements that permit an increased number of stator slots. Three methods for doing this without increasing the generator voltage are described: delta-connected windings, a new three-phase winding system using variable-pitch coils, and a " multi-three-phase" system using a six-phase generator, six buses, and a six-phase-three-phase power conversion in the generator stepup transformers. Generator voltages may be equal to, or slightly lower than, present levels. In some cases bus and transformer costs may be slightly higher, but this will be more than offset if the generator availability (over the life of the equipment) is increased by even a few days because of the reduced duty. Additional benefits in the generator design, in operating characteristics, and in the ability to build larger kVA ratings are described.