Mechanism analysis and suppression strategy of reverse flow phenomenon in variable speed pumped storage units under pump mode

The pump mode of variable-speed pumped storage units (VSPSUs) is significantly influenced by speed regulation. Inadequate speed regulation during transients may induce reverse flow phenomenon (RFP), inducing flow disorder, intensifying local pressure fluctuations and water hammer effects, thereby compromising system stability. However, existing studies lack detailed analysis of RFP mechanisms. Therefore, this paper analyzes the mechanisms of RFP and proposes a suppression strategy. First, RFP is preliminarily analyzed using the pump-turbine's full characteristic curve. A VSPSU model is developed to identify RFP-triggering conditions: low-opening operation or reduced input power. Following this, a detailed examination is performed on how the input power change amplitude, input power change rate, and guide vane closure rate influence the phenomenon. The mismatch between the rates of change in speed and opening leads to a significant disparity between the mechanical and electromagnetic torques of the unit, resulting in increased amplitude of speed fluctuations and, ultimately, RFP. Finally, enhancements are made to the optimal speed generator based on the insights gained from the mechanistic analysis, and a suppression strategy is proposed. This strategy enhances the match between the rates of change in speed and opening, effectively suppressing RFP.