The variable speed pump-turbine is usually used to adjust the rotational speed to improve the efficiency in turbine mode and change the input power in pump mode because its rotational speed can vary within a certain range. In order to explore the evolutions of pressure pulsation and flow patterns caused by changes in the rotational speeds, the steady operating conditions under different rotational speeds in turbine and pump modes were investigated by using three-dimensional numerical simulations. The results show that as the pump-turbine operates with the highest efficiency at the rated rotational speed, the change in the rotational speed leads to the variation in macro-parameters, deterioration of the flow patterns, and increase in pressure pulsations. In addition, under a certain guide vane opening, with the increase in the rotational speed, the torque, power, and discharge increase in the turbine mode, while these parameters decrease in the pump mode. And when the rotational speed is too high or too low, it causes an obvious increase in pressure pulsations.

Pumped storage power stations are effective stabilizers and regulators of the power grids. However, during the transient process, especially the operating point entering the S-shaped region, the internal flow patterns and pressure pulsations in the pump–turbine unit change violently, seriously affecting the safety of the power stations, which requires enough optimizations in the design stage of the pump–turbine. In this paper, to explore the key factors which influence the evolutions of flow patterns and pressure pulsations during the runaway process, three pump–turbine runners with different inlet blade lean, including positive angle, no angle and negative angle, were selected to simulate by using the three-dimensional method. The results show that the changes in the inlet blade lean angles have significant effects on the variation periods and maximum values of the macro parameters during the runaway process, and especially the runner with no lean angle results in the smallest oscillation periods and pressure pulsations but enlarges the runner radial forces. In addition, backflows generate from the hub side under the cases with positive or no blade lean angle, while those occur from the shroud side due to the negative angle. The results provide a basic reference for the design of the pump–turbine.