Three-dimensional computational fluid dynamics analysis of the interaction mechanism of pump-turbines sharing one hydraulic system during successive load rejection transients

For pumped-storage hydropower stations (PSHSs) that have multiple pump-turbines sharing one hydraulic system, the most dangerous transient process with lowest draft-tube pressure happens during the successive rather than simultaneous load rejection transients. The lowest pressure mechanism in pump-turbines, along with the interaction mechanism among the pump-turbines and the hydraulic system during the transients, is not revealed. In this study, the successive load rejection transients with different delay times of two pump-turbines (PT1 and PT2, PT1 rejects load first and PT2 rejects load at a delay time) in a high-head PSHS are successfully simulated using one-dimensional and three-dimensional coupled (1D–3D) computational fluid dynamics method. The analysis based on rigid water hammer theory and pump-turbine dynamic characteristics indicates that the dangerous situation of lowest draft-tube pressures will occur in both pump-turbines if PT2 rejects load during the first turbine-braking mode period of PT1. The lowest draft-tube pressure in PT2 is induced by the delayed superposition of the first negative water hammer waves in the draft-tubes of PT1 and PT2, while the following lowest draft-tube pressure in PT1 is induced by the superposition of decreasing pressure caused by the enhanced pumping effect and the second negative water hammer wave in the draft-tube of PT1. The revealed lowest pressure mechanism provides a basis for further studying PSHS's safety in successive load rejection transients.