Abstract: Pumped storage power plants usually adopt a layout of multi units sharing a common pipe system. During operation, two-stage load rejection often leads to an extremely low pressure in the draft tube and even liquid column separation. This study develops a mathematic model of hydraulic transients in such plants with a pipe connection between tailrace tunnels, and simulates the load rejection conditions using the parameters of the Xiamen pumped storage power station. We examine the mechanism of mitigating the draft tube’s low pressure by installing a connection pipe, and analyze its effect on the transients for the cases of different pipe diameters. The results show that the pipe connection between tailrace tunnels effectively improves the draft tube’s low pressure during two-stage load rejection and increasing the diameter of the pipe makes the flow in it more sensitive to the pressure difference. Thus, its effect on pressure balance between two tailrace tunnels is more evident, and the low pressure in the draft tube is significantly improved.

Key words: tailrace connection pipe, pumped storage power plants, hydraulic transients, two-stage load rejection, S-shaped characteristics of pump-turbine