Mechanism and improvement methods for hydraulic instability in pump-turbines with large head variation

In order to achieve the goal of “Carbon Peaking and Carbon Neutrality”, and actively promo ting the development of clean energy, pumped storage technology has been widely used as an important means to maintain the security of the power grid. However, the operation of large head pump turbines may have problems such as vibration and noise caused by the “S” characteristic zone, so it is important to conduct an in-depth study of the characteristics of the “S” zone based on numerical calculations. The typical operating conditions of large head pump turbines based on numerical calculations were investigated, and the pressure distribution and vortex flow in the “S” zone were analyzed by steady-state numerical calculation method. It is found that when deviating from the design conditions, a water retaining ring will be formed in the bladeless area, which will hinder the water flow into the runner and cause the vortex to block the flow channel. Based on the revealed instability mechanism, a solution is proposed to improve the hydraulic characteristics by optimizing the impeller structure, which successfully and effectively controls the vortex flow in the flow channel, significantly reduces the flow separation phenomenon, and thus improves the operational stability of the unit. The research results can pro-vide theoretical guidance for the optimization design of pump-turbine with large head variation.