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. 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 constant-speed oscillation 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.
