The impact of hump characteristics on variable speed pumped storage units under pump mode and improvement measures

The input power of variable speed pumped storage units (VSPSUs) under pump mode is adjustable, which effectively increases the units' frequency regulation capacity and ability during low demand periods in the power system, providing strong support for the stable operation of the grid. However, the operation of the VSPSU under pump mode is affected by the hump characteristics, which limits the regulation capability of the unit. Therefore, this paper analyzes the mechanism of the impact of the hump characteristics on VSPSUs and proposes an effective improvement measure. Firstly, on the basis of the pump on-cam curves, a simple and effective efficiency optimization method of the VSPSU under pump mode is proposed. Then, based on the characteristics of the hump region, the relationship between the optimal efficiency points and the hump characteristics is obtained, revealing that the hump characteristics affect the operating conditions of the variable speed units under any head. Subsequently, a novel nonlinear model of the pump-turbine is proposed based on the basic characteristic curves of various openings under pump mode to solve the multi-value phenomenon, and the influence mechanism of hump characteristics on pump mode of VSPSU from a control perspective is obtained. Finally, an improvement measure is proposed to modify the optimal efficiency points by setting operating safety margins, and the sub-optimal efficiency point that is not affected by the hump characteristic at the same input power is used to replace the optimal efficiency point that is affected by the hump characteristic to avoid the impact of the hump characteristic. Compared to the method of setting hump characteristic limit, the adjustment range of the VSPSU under pump mode is greatly increased. •A simple and effective efficiency optimization method of the VSPSU under pump mode was proposed.•A novel nonlinear model of the pump-turbine is proposed under pump mode to solve the multi-value phenomenon.•The impact of the hump region on the operation of VSPSU was analyzed.•An improvement measure is proposed to modify the optimal efficiency points by setting operating safety margins.•The adjustment range of the VSPSU under pump mode is increased.