Rotor-stator interaction and vortex rope in the high-head variable speed reversible pump-turbine operating at minimum head

The pumped storage hydropower (PSHP) has large head variations during its cyclic operation, and using variable speed units of reversible pump turbines (RPT) can extend its operating range and grid stability. In minimum head conditions, the performance of RPT in turbine mode is more important because it may experience fatigue and vibrations due to hydraulic instabilities caused by rotor-stator interaction (RSI) in vaneless space and vortex rope in the draft tube. Therefore, it is important to investigate these phenomena in the RPTs operating at minimum head. At minimum head and flow rate, the variable speed RPT controller adjusts the rotational speed to the value that yields the highest efficiency, and the rotational speed becomes "fixed" at that operating condition. The present work investigates RSI and vortex rope and its associated pressure fluctuations (PFs) in RPT at minimum available head operating conditions. The analysis was performed at the best efficiency point (BEP) and minimum head with three flow rates having fixed rotational speed (corresponding to the highest efficiency). It was observed that the pressure fluctuates in the vaneless space because of the RSI as a result of the presence of a low-pressure stagnation region at the inlet of the pressure side of the runner blade. The vortex rope developed in the draft tube because of the presence of a stagnation region at its center as a result of the high-velocity gradient. It can be concluded that the main source of PF in the RPT at minimum head operating condition is RSI.