Variable-speed operation of Francis turbines: A review of the perspectives and challenges

The paper presents the recent trends and ideas for flexible operation of Francis turbines using Full-Size Frequency Converter (FSFC) or Doubly-Fed Induction Machine (DFIM) technology for variable-speed operation. This technology allows for the speed of the runner to be adjusted in order to maximize the efficiency and/or reduce dynamic loads of the turbine according to the available head and power generation demands. Continuous speed variation of up to +/- 10% of the design rotational speed can be achieved with the DFIM technology, while for FSFC there is no such limit by the technology itself. For off-design operation of Francis turbines, depending on the variation of the head and the hydraulic design, the hydraulic efficiency gain from variable-speed operation compared to its synchronous-speed representative can go up to 10%. In addition, turbines operated at variable-speed can have significant improvement in the response times for power output variations, being able to utilize the flywheel effect from the rotating masses (also known as synthetic inertia). This review focuses on the investigations and the achievements done so far and does not tend to enter deeply into each multidisciplinary aspect of the technology itself. Possible further development directions are also disclosed, mainly towards the hydraulic design and optimization of variable-speed Francis turbines.