Abstract
Frequent working condition conversions in pumped-storage power stations often induce stability problems, especially when the operating point enters the S-shaped region, during which flow transitions and pressure fluctuations are serious. The pump-turbines with different specific speed values show different characteristics, but their differences in stability features are still not clear. In this study, four different pump-turbines were selected to simulate the runaway processes from turbine modes. The similarities and differences of flow patterns and pressure fluctuations were analyzed. For the similarities, pressure pulsations increase gradually and fluctuate suddenly once the backflows occur at the runner inlets. For the differences, the evolutions of backflows and pressure pulsations are related to specific speeds and runner shapes. Firstly, it is easier for the lower specific speed turbines to enter the reverse pump mode. Secondly, the blade lean angle influences the position where backflows occur, because it determines the pressure gradient at the runner inlets. Thirdly, the runner inlet height influences pressure pulsations in the vaneless space, because the relative range of backflow transitions will be enlarged with the decrease of specific speed. Overall, investigating the mechanisms of flow pattern transitions and pressure variations is important for runner design and transient process control.
Highlights
With the substantial increase of electricity consumption and the rapid development of green sustainable energies, pumped-storage power undertakes the functions of peak load regulation, valley filling, frequency modulation, phase modulation, and emergency standby in the power grids [1,2]
The working point slides rapidly through the S-shaped characteristic region that is comprised of the high-speed turbine, turbine braking and reverse pump modes, and violent vibrations in the unit happen due to quick flow pattern transitions and strong pressure pulsations [1]
In order to fully reflect the characteristics of the pump-turbines during the transient process, the actual water conveyance systems were removed in the simulations to eliminate the impact of flow inertia in water conveyance systems [1]. This removal will affect the variation period and maximum value of macro parameters due to the flow inertia in pipelines, but we mainly focused on the evolutions of flow patterns and pressure pulsations, which are more affected by the pump-turbine unit
Summary
With the substantial increase of electricity consumption and the rapid development of green sustainable energies, pumped-storage power undertakes the functions of peak load regulation, valley filling, frequency modulation, phase modulation, and emergency standby in the power grids [1,2]. The runner will be driven only by the unceasing water power, and the unit will be accelerated to the runaway speed During this process, the working point slides rapidly through the S-shaped characteristic region that is comprised of the high-speed turbine, turbine braking and reverse pump modes, and violent vibrations in the unit happen due to quick flow pattern transitions and strong pressure pulsations [1]. The research discussed above shows that whether at the runaway point or during runaway process, flow blockages and severe pressure fluctuations are strong in the runner and vaneless space, which are the common features in pump-turbines. The paper will be arranged as follows: the Section 2 describes the basic simulation model and parameters; the Section 3 shows the resulting histories of macro parameters, and the evolutions of flow structures and pressure pulsations, along with their relations with specific speeds; the Section 4 explains the influences of runner shapes for the differences in the evolutions of flow structures and pressure pulsations; and conclusions are drawn in the Section 5
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