Z'mutt, Switzerland

Z’Mutt is the largest pumped storage hydropower plant of the Grand Dixence hydroelectric complex, featuring five units with a total pumping power of 88 MW. One of the units has recently been replaced by a 5 MW reversible Francis pump-turbine with full-size frequency (FSFC) converter to demonstrate variable speed operation on the full rotational frequency range in both pumping and generating mode. This will increase the flexibility and the operational performance of the plant.

In 2021, significant progress has been made on numerical flow and structural analysis by HES-SO of the Z’Mutt prototype. This aimed to select the best operating sequences to improve the response time of the unit during transient operations, such as the unit’s start-up sequence. To optimise transient operation, runner fatigue damage has been numerically characterised on a large area of the turbine operating quadrant. These results have then been used by PVE to optimise the turbine start-up sequence taking into account efficiency, runner and penstock fatigue with 1D hydraulic transient simulations.

Structural and flow simulation results for fatigue damage analyses of the Z’Mutt pump-turbine prototype with variable speed. Image credit to HES-SO.

EPFL also conducted a test campaign on a pump-turbine reduced scale model at EPFL’s test-rig facilities, to assess the behaviour of the runner under transient operations. On-board pressure transducers and strain gauges were mounted directly on the runner’s blades to get a complete overview of the dynamic phenomena taking place. The data collected during the test campaign is being processed and will be used to quantify the impact on the blades for the different start-up schemes.  

Due to technical challenges during the commissioning of the unit, the first on-site test campaign at the plant has been delayed and is now expected to be performed in winter 2022/23. The test campaign will implement technical solutions for investigations on components fatigue related to high flexibility variable speed operation, including strain gauge measurements on the runner, as well as guide vane torque and shaft torque measurement systems.

View the Z'Mutt video.