Over the past decade, the use of conventional one-dimensional numerical-simulation methods has been demonstrated to be inadequate in terms of their usefulness in investigations concerning transient processes in hydraulic machines systems—their theoretical analyses and engineering applications. Consequently, numerous three-dimensional numerical methods capable of accurately simulating transient processes in hydraulic-machine systems have been proposed and improved upon in recent years. Through use of these novel methods and strategies, many researchers have investigated transient characteristics of processes occurring within hydraulic-machine systems along with corresponding formation mechanisms. This study presents a comprehensive review of related experimental studies, novel numerical methods and strategies along with transient characteristics and formation mechanisms in hydraulic-machine systems. Based on this study, suggestions have been made concerning the selection of simulation methods to be used and directions for future research have been proposed.
Hydraulic power generation, among sustainable renewable-energy sources, has been recognized as a proven, extremely flexible, and welladvanced grid-regulating technology . In particular, hydropowerpumped storage has been recognized as, perhaps, the only commercially proven grid-scale energy-storage technique [2–4]. Hydraulic machines, without doubt, play a significant role in hydraulic power generation, and key devices include hydro-turbines , pump–turbines [6–10], and pumps [11–13]. The past few years have witnessed an increasing investment in renewable energy sources—wind and solar power. In order to achieve load levelling, grid-frequency regulation, and reserve spinning, hydraulic machines in hydropower stations frequently perform transient processes [14,15]. Transient processes of hydraulic machines involve a series of transitions, wherein hydraulic machines undergo a transformation in their operating mode from one stable state of operation to another. Pump–turbines, in particular, undergo all types of transient processes during their typical four-quadrant operations, as depicted in Fig. 1. Pump–turbines, during their operation as a pump, may undergo such processes as pump start-up, shutdown, and pump power failure. In the event of a pump power failure, if the guide vanes malfunction, pump–turbines perform a pump runaway process along a dynamic trajectory, as depicted by the hill chart in Fig. 2. In contrast, during operation as hydro-turbines, pump-turbines may undergo such processes as hydro-turbine start-up , shutdown , and load acceptance and rejection processes . At the end of hydroturbine load rejection, if guide vanes malfunction, the pump–turbine performs a runaway oscillation process, as depicted in Fig. 3 . During transient processes, especially near the speed-no-load condition (when hydraulic torque on runner equals zero), the pump–turbine demonstrates serious fluctuation characteristics, and a series of water-hammer phenomena may occur within water-conveyance pipeline systems. The large rise in pressure head and severe fluctuations caused by water-hammer phenomena directly impact the safe and stable operation of pumped-storage power stations. In fact, even rotating parts of hydraulic machines tend to be lifted by unbalanced axialreaction forces generated by leakage flows in the sidewall clearance as well as the reverse water hammer within the draft tube.