Nonlinear speed control for mitigating pump-turbine instability

The stability of a pump-turbine (PT) during transient variable speed (VS) operation is critically influenced by pressure pulsations inside the draft tube. This study adopts the analytical methods of entropy generation and rothalpy change. From the results, it can be found that the PT that adopted the linear VS strategy exhibits efficiency oscillations with oscillation amplitude of 43.48% at the beginning of the VS process. In accordance with the nonlinear VS strategy III, the duration of efficiency oscillation is reduced by 28.57%, and the oscillation amplitude is decreased by 4.01%. This strategy effectively suppresses the increase of the vortex volume in the draft tube, and also reduces the duration of high-amplitude low-frequency pressure pulsations. The region of high rothalpy change within the draft tube clearly indicates the location where the vortex is generated. The results indicate that optimizing the VS strategy of PT can improve its stability during the VS process.