To explore the cause of the S-shaped region of a revisable pump turbine

its full characteristics are compared with model test results. Then

we analyze the flow and vortex distribution in each of its flow components in this region using the Liutex vortex identification method

and reveal the variations in the corresponding internal flows with flow rate. We find that under the same rotational speed

flow stability in the volute is better and almost no vortices occur there. In the guide vane and runner sections

the flow is relatively uniform and has a good stability; only a small number of vortices are distributed in the vaneless section. Under the braking condition

the flow in the volute becomes more unstable and the vortices is concentrated on its outlet ring

where a large vortex is easy to develop and block the flow into the fixed guide vanes. Evident vortices and backflows are observed in the guide vane spaces

and cascade blockage is more severe in the guide vane and runner sections. In the reverse pump condition

spiral backflow occurs at the outlet of the draft tube

resulting in unsteady phenomena such as the rotating stall of water flow into the runner. Near the inlet edge of the guide vanes

part of the flow runs at a high velocity along its circumferential direction

and severe flow separation is generated with the densest vortices gathering around. In the vaneless section where the zones of great pressure gradient are possible

the vortices occupy nearly the whole flow channel. This is an operating region where the unit is easy to suffer large pressure fluctuations and even the runner could be damaged

and longtime operation in this region must be avoided.