HYDRAULICS OF THE TAIL RACE SURGE TANK OF GOUVÃES PUMPED-STORAGE HYDROPOWER W. Richter, G. Zenz, Ch. Nicolet, Ch. Landry, J.C. Vera Rodriguez, L. de la Torre Abietar Abstract: The 880 MW pumped storage hydro power plant Gouvães, part of the Alto Tâmega hydro power scheme from Iberdrola is currently under construction in the north of Portugal. The energy storage and grid regulating plant is equipped with 4 reversible Francis pump turbines with nominal power of 220 MW and a gross head of 660 m, the discharge in turbine mode is 160 m³/s and in pumping mode 128 m³/s. The current paper underlines the layout, the design criteria and the results of the physical small-scale test of the tail race surge tank in the hydraulic laboratory. Most unfavourable load cases were studied such as synchronous pump trip in resonance and multiple loading and unloading followed by full load rejection in generating mode. A collaborated transient investigation for water hammer and mass oscillation capturing was conducted and gives cross checking opportunity. 1 Introduction The 880 MW pumped storage power plant Gouvães, part of the Alto Tâmega hydro power scheme from Iberdrola is currently under construction in the north of Portugal. The cavern situation with complex civil works is shown in Fig. 1. The power plant is equipped with four reversible Francis pump turbines with nominal power of 220 MW and a gross head of 660 m, the discharge in turbine mode is 160 m³/s and in pumping mode 128 m³/s. The grid balancing and energy storing power plant utilizes the lower reservoir Daivões at the river Tâmega and the upper reservoir Gouvães at the river Torno. This paper presents the specifics of the hydraulic layout and design issues regarding the tail race surge tank and the results of the physical model test. Especially the final hydraulic design load cases for the upper and the lower chamber are visualized. Fig. 1 shows the placement of the tailrace surge tank right behind the transformer cavern. All four draft tube pipes are directly connected to the surge tank to allow quick transient response.