The hydraulic characteristics of pump turbines in off-design conditions, especially the S-shaped characteristics, are crucial for the safety and stability of the unit. To explore the S-characteristics of pump turbines through a transient method, an experimental investigation was conducted based on a pumped-storage model system at Wuhan University. By shutting down the circulating pump, a special transient process was triggered, forcing the pump turbine to operate in turbine mode, turbine brake mode, and reverse rotational pump mode. As the rotational speed of the pump turbine was maintained almost constant in the oscillation process with a maximum deviation of 0.6%, this transient operation was named as constant-speed oscillation (CSO). The parameters for global performance and pressure pulsations in the vaneless gap were measured and analyzed. In addition, the one-dimensional rigid column theory was used to establish a mathematical model for simulation. The results from simulation were quantitatively compared with the experimental results. Finally, the reason for the CSO was theoretically explained based on stability analysis through the established mathematical model. It was observed that the positive slope of ned–Qed characteristic curves at no-flow resulted in this oscillation. In contrast, the simulation was performed under the same conditions with a modified ned–Qed characteristic curve, which had a negative slope at no-flow. However, the results showed that, with the modified characteristic curve, the pump turbine would stabilize at no-flow.

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