Abstract: Variable-speed pumped storage technology has become an emerging development and research direction in pumped storage industry in recent years, and research on the dynamic characteristics of variable-speed pumped storage power plants (VSPSPs) is of great significance. This paper focuses on the hydraulic-mechanical subsystem of a VSPSP and analyzes the rising trends of its unit speed during load rejection transient process, by using a nonlinear integrated model. We verify the correctness of the model by comparing its simulation results under load rejection against the on-set measurements of a real power station, analyze the dynamic response and speed rising of the unit under load rejection at different initial speeds, and find the speed rising can be explained through its momentum conversion mechanism and torque characteristic. The results demonstrate that under load rejection, if the initial speed of the variable-speed unit increases in a certain range, the maximum speed will not grow significantly, and there still exists a certain safety margin from the maximum allowable speed.

Key words: variable-speed pumped storage plant, characteristic curve of pump-turbine, large disturbance, speed rising trend, operation trajectory, speed rising and torque integral