Dynamic Characteristics Analysis Of Pump-Turbine In "S" Operating Zone

As the core of pumped storage power station, the pump-turbine often operates under complex operation conditions, which will induce unstable incidents of power net. As a result, it can make the characteristic curves of the first quadrant can be significant “S-Shaped” for some pump-turbines. Moreover, it leads to a poor stability and even challenge to synchronize with the electrical network safely. So it is necessary to conduct a detailed study on analysis of pump-turbine internal flow state.The research adopted the CFD code to carry out numerical simulations. Firstly,the unsteady simulations operating at the optimal condition, the turbine condition,the runaway condition and the brake condition were conducted respectively to analyze the evolutions of change of flow field and pressure signal using the SST turbulence model and SIMPLEC algorithm. Secondly, the unsteady simulation operating at GVOs from 9mm to 37 mm was conducted using dynamic mesh technique. Thirdly, the unsteady simulation was started in the turbine condition varing by reducing mass flow of the inlet to discuss the influence of flow rate on the development of vortex and the characteristics of flow field in the "S" operating zone.Last, the unsteady simulations with cavitation model or not were conducted in the turbine condition of GVO for comparing of the external characteristics and the internal flow field of the pump turbine in the calculation of the single phase and two phase.Dynamic analysis results showed that:(1)For GVO in 17 mm from the turbine to braking condition, the inlet vortex and reflux vortex in the runner are more and more large and extend to the outlet, and the secondary flow is formed at the tail of the blade. At the same time, the pressure side of the runner channel begins to produce the reflux, and the number and intensity of the reflux passage is gradually increased.(2)For GVOs from 9mm to 37 mm, the large scale vortex in the vaneless space extend to the local channels of the guide vane, where the vaneless space becomes smaller.(3)For GVO in 37 mm at large mass flow, the bubble shaped vortex rope in the draft tube is induced by the runner cone. When the mass flow continues to decrease, the double vortex ropes and the tri-vortex ropes appear.(4)For the cavitation simulation operating in the turbine condition of GVO in 37 mm,cavitation occurs only in the runner and mainly concentrates on the leading edge ofthe work surface near the inlet of the runner, where exists flow separation and reflux.The more serious phenomenon of flow separation and reflux, the lager range of cavitation zone.