为满足电力系统动态服务需求，要求抽蓄系统快速完成工况转换，抽水快转发电过渡过程是抽蓄系统在工况转换过程中最为复杂和危险的工况之一，但目前相关研究较少。为明确该过程的水力不稳定性，建立抽蓄电站全过流系统三维模型，开展抽蓄系统抽水快转发电过渡过程的数值模拟计算，探究水泵水轮机组的外特性参数变化、压力脉动特性和内部流态演变。研究表明：抽蓄机组抽水快转发电过程中，流量、力矩及受力的变化皆随着活动导叶动作呈现出阶梯式变化；水泵制动运行工况及转轮转速发生转向期间为机组易产生较大摆动、力矩易发生剧烈波动的时段，且尤以无叶区的压力波动最为剧烈；导叶开度的快速变化以及流量和转速的变向成为机组内部危险流动的主要来源。研究结果可为提高抽水快转发电过渡过程中的稳定性和优化控制策略提供理论支撑。

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.