Abstract: Due to the growing environmental awareness and demanding for the balancing power and frequency control, the renewable energy sources reversible pump-turbine seem to have the both characteristics of power generation and energy storage. While the Pump-turbine operates in the S characteristics, the difficulty in loading rejection process and unusual increase in water pressure will occur, which due to the machine vibration. In order to explore the pre-opening guide vane on the improvement of the S characteristics of pump-turbine. Firstly, uses the synchronization guide vane and pre-opening guide vane to do the energy test and the whole flow of CFD numerical flow; Secondly, by comparing the experimental data with the CFD results to analysis the S characteristics and its pressure pulsation; Then concluded that: by pre-opening part guide vanes the pump-turbine can avoid the influence of the S characteristic under the steady flow. While after pre-opening guide vanes, the increase of unit flow in the runway condition destroyed the symmetry of the flow in the runner, which due to the increase of the pressure pulsation and the unstable of the unit operation. So there is still some abuse improving the S characteristics by pre-opening part guide vanes.

Key words: Pre-opening guide vanes, Pressure pulsation, Pump-turbine, S-shaped characteristics