Solutions for upgrading the hydraulic system of Tonstad HPP

The pressurized tunnel system of the 960 MW Tonstad hydropower plant (HPP) is a characteristic example of the Norwegian design philosophy. The use of a pressurized sand trap in the downstream end of the tunnel system allows the use of unlined tunneling in the long upstream headrace tunnel. The sand trap is constructed together with the surge tank equipped with a gate in the main riser. Investigations of upgrading of the installed capacity have been made for the power plant. The surge tank and the pressurized sand trap are identified as limitations for the upgrade. This paper describes the purpose and design of the surge tank and sand trap, and potential solutions to allow the power plant upgrade. These solutions include retrofitting of the surge tank and the sand trap, a digital twin protection system, and expansion of the tunnel cross section with a new tunnel in parallel.