Hydraulic instability of pump-turbines during fast pump-to-turbine transition process
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Abstract:
The fast pump-to-turbine transition process of a pumped storage unit is one of the most complex and dangerous operating conditions, which involves complex hydraulic, mechanical, and electrical processes. It can be highly responsive to the electrical power system regulation demands, thus being a critical process for pumped storage power station. The correct prediction of this hydraulic transient process is crucial for not incurring in issues during operation.A three-dimensional numerical model of the transition process of a pumped storage hydraulic system was proposed and analysed. Numerical simulations were carried out to investigate the hydraulic instability during this transition process, and the study was carried out from three perspectives: the variations of external parameters, the pressure fluctuations within the pump-turbine unit, and the evolution of flow patterns in the pump-turbine.Results indicated that: (1) During the fast pump-to-turbine transition process, the opening and closing of the guide vanes have an instantaneous impact on the hydrodynamic performance within the pump-turbine unit. The variations in discharge, torque, and axial force exhibit stepwise changes during the movement of the guide vanes. Periods of significant oscillation and drastic fluctuations in torque are prone to occur during the pump braking operating condition and the reversal of the runner speed. (2) The pressure fluctuations within the vaneless region and runner exhibit significant variations during the transition process. Notably, the pressure fluctuations in the vaneless region are particularly pronounced. The temporal variations in pressure at monitoring points in the spiral casing, stay vanes, and guide vanes regions exhibit similar trends with the magnitude of pressure variations increasing from the spiral casing to the guide vanes. The draft tube region does not generate tailrace eddy, thus avoiding intense pressure fluctuations. (3) During the transition process, the inertia of the water flow and the blocking effect of the guide vanes, contributes to the occurrence of high-speed circulation in the vaneless region, which is the reason for the intense pressure fluctuations observed in the vaneless region. The internal flow radically alters and gets more complex when the guide vanes are closed, the flow rate goes to zero, or the runner speed drops to zero. The rapid changes in the guide vane opening, flow direction, and rotational direction become the main causes of the pump-turbine's hydraulic instability.In conclusion, a numerical three-dimensional model was reported for the entire flow system of a pumped storage station. Simulations were carried out to investigate the transitional process of the fast pump-to-turbine. The research delved into the variations of external parameters, the pressure fluctuations within the pump-turbine unit, and the evolution of flow patterns during the fast pump-to-turbine transition process. The variations in discharge, torque, and axial force exhibited stepwise changes during the movement of the guide vanes during the fast pump-to-turbine transition process. The pressure fluctuations within the vaneless region and runner exhibited significant variations during the pump braking operating condition. The rapid changes in the guide vane opening, flow direction, and rotational direction becomed the main causes of the pump-turbine's hydraulic instability. To the authors’ opinion, this research considerably contributes to the understanding of the hydraulic transient mechanisms during the fast pump-to-turbine transition process, and provides theoretical guidance for enhancing stability and optimizing control strategies during the transitional process.
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