Mitigating Low Pressure in Draft Tubes of Pumped-Storage Units by Installing Tailwater Connection Pipes Under Successive Load Rejection Conditions: Mechanism and Effectiveness Analysis

The large-scale integration of intermittent renewable energy has increased operating-condition transitions in pumped storage hydropower plants, thereby raising the probability of successive load rejection (SLR) and associated hydraulic transient risks. To mitigate the severe low draft tube pressure (DTP) commonly induced by SLR, this study proposes installing a tailwater connection pipe (TWCP) between the draft tubes of adjacent units. The mitigation mechanism and performance are investigated through theoretical modeling and three-dimensional numerical simulations. Results show that the first load-rejecting unit (FLRU) enters the reverse-pump region prematurely and effectively behaves as a normal-rotation pump. This accelerates the discharge reduction in the subsequent load-rejecting unit (SLRU), drives its operating point deeper into the reverse-pump region, and substantially worsens its minimum DTP. With the TWCP, the head difference drives compensating flow from the FLRU to the SLRU, alleviating transient water shortage, reducing discharge and pressure discrepancies between the two tailwater systems, and improving local turbulent flow. The mitigation effect is sensitive to structural parameters: a larger pipe diameter increases flow capacity, while installation closer to the pump-turbines increases the driving head difference. Both measures enhance DTP mitigation, whereas the TWCP has little influence on steady-state flow conditions.