Abstract: Pumped-storage power stations are promisingly developed in China, but they suffer unit power oscillation that, caused by cavity vortices in draft tube, occurs frequently in generating mode, severely threatening the safety of their operation and the power grid. To study the influence of such cavity vortices on the stability of a hydropower generating system, we construct a transfer function for the regulating system with a single unit and a global matrix for the system with two units sharing the same tunnel, from the perspective of one-dimensional frequency domain with consideration of the cavity vortices. Then, calculations and analysis of system stability are carried out for these two cases. The results indicate the influence of cavity vortices plays a decisive role in system stability in comparison with that of the governor. When power oscillation is induced by the cavity vortices, no system-stable region will exist; when the system is affected by the vortices but kept from power oscillation, its stable region is basically the same as that without the vortices. Nevertheless, the cavity vortices are usually characterized by small values of parameters that may change responding to the variations in unit operating conditions, and thus will result in power oscillation. This work highlights the role of draft tube cavity vortices to avoid system instability in the operation of hydropower stations.

Key words: hydropower generating system, cavity vortices in draft tube, stability, transfer function method, global matrix method