Influence of Interval Between Successive Power-Offs on Extreme Value of Hydraulic Parameters in a Pumped Storage System with Fixed and Variable Speed Units

Focusing on the complex hydraulic transient problems caused by successive power-offs in the pump mode of variable speed pumped storage units, the study takes a pumped storage system with a hybrid layout of fixed and variable speed units (FVSU) as the research subject. The objective is to investigate the impact of initial speed and the interval between successive power-offs on the extreme values of dynamic hydraulic parameters, such as speed, pressure, and discharge. The results indicate initial speed exhibits a negative correlation with some dynamic response parameters, such as the duration in the pump zone, the maximum speed reversal rate. Specifically, at 96% nr (rated speed), the unit that loses power later exhibits the maximum speed reversal rate of 63.07%. When the interval between power-offs is 4s and the initial speed is at 96% nr, the first power-off unit experiences the maximum spiral case pressure of 615.5 meters. The results indicate that a lower initial speed is more likely to lead to unsafe operating conditions. When successive power outages occur while the first power-off unit is close to the pump braking zone, it is more likely to generate unsafe extreme values of hydraulic parameters. The research results can provide reference for the safe and stable operation of variable speed pumped storage unit (VSPSU).