**Zhiyan Yang 1, Zirui Liu 1, Yongguang Cheng 1,\*, Xiaoxi Zhang 2, Ke Liu <sup>1</sup> and Linsheng Xia <sup>3</sup>**


Received: 24 August 2020; Accepted: 5 October 2020; Published: 11 October 2020

**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.

**Keywords:** pump-turbine; flow patterns; pressure pulsations; similarities; differences; S-shaped characteristics; runaway transient process
