Transient simulation and analysis of the simultaneous load rejection process in pumped storage power stations using a 1-D-3-D coupling method

The load rejection imposes a danger in the pumped storage hydropower plants (PSPs), especially when two or more pump turbines reject their loads simultaneously. In this paper, the simultaneous load rejection scenarios in the PSPs are simulated and analyzed by using a 1-D, 3-D coupling method. The PSP pipe system is modeled by using the 1-D method of characteristics (MOC) and one pump turbine is modeled by using the 3-D computational fluid dynamics (CFD). The simulated flow and head are transmitted between the 1-D, 3-D regions through the interfaces between these two regions. By assuming that the installed pump turbines are of the same type and the corresponding branch pipes have the same properties, the variations of the transient pressures and the flowrates in different pump turbines will be identical. Therefore, only one pump turbine is modeled by the CFD in this study. A new branching junction boundary is proposed to assign the simulated dynamic pressures and flowrates obtained by the 3-D model to other pump turbines. The 1-D-3-D coupling method is validated by experiments with only one pump turbine rejecting its load. The simultaneous load rejection of two pump turbines is then simulated and validated by comparing the results with those of the 1-D simulation. By building only one pump turbine 3-D model, a large amount of computational resources can be saved. The simultaneous load rejection scenario is then analyzed and compared with the single load rejection scenario. Higher water hammer pressures and a larger rotational speed occur in the simultaneous load rejection scenario, which leads to larger pressure pulsations in the pump turbine. The larger pressure pulsations can be further explained by the flow patterns in the runner channels, in which heavier flow separations and vortexes can be observed in the simultaneous load rejection scenario.