LAUSR

Abstract The S-shaped characteristic of a pump-turbine poses a latent risk to problems such as failure to start the unit, excessive water hammer pressure after load rejection, severe pressure pulsation, and intense vibration. However, S-shaped characteristics of a pump-turbine are usually investigated by static model tests, while the scope of this method is relatively limited. In a previous study exploring the S-shaped characteristic curve of pump-turbines, the constant-speed oscillation (CSO) was experimentally discovered. This transient process can be utilized to acquire the dynamic trajectory of the unit at a fixed rotational speed in the turbine mode, turbine brake mode, and reverse pump mode. On this basis, this study investigated the hydraulic oscillations between two pump-turbines after the net flow of the system was reduced to zero when the two units were operated in parallel. It was found that the two pump-turbines had an equal oscillation cycle, but their transient trajectories were significantly different. One of the runners oscillated only in the reverse pump mode, whereas the other runner oscillated in the turbine, turbine brake, and reverse pump modes. This process was termed the CSO interaction (CSOI). Subsequently, a one-dimensional rigid column simulation and a sensitivity analysis were performed to analyze the CSOI further, thereby verifying that the S-shaped characteristic curve was the root cause of the oscillation period and amplitude variation of the unit in the CSOI. The runner with the more prominent S-shaped characteristic curve tends to operate in the reverse pump mode during oscillation. The novelty of the work is that a new hydraulic interaction between pump-turbines, CSOI, was discovered. CSOI provides a metric for comparing the intensities of the S-shaped characteristic curves of two pump-turbines experimentally or theoretically.