Abstract: The capability of Francis turbine units with wide-load stable operation is crucial for constructing a new electric power system. Currently, a great number of hydropower units suffer from poor stability under off-design conditions, which cannot meet the demand for flexible operation over a wide-load range. This paper presents a Francis turbine hydraulic optimization method for wide-load operation and its application to Unit No. 5 at the Dachaoshan hydropower station in Yunnan, using specific energy and gradient function as a quantified target. And developed a new type of runner, namely "Negative Inclination and Reverse C-Type" that can effectively improve the hydraulic stability of the unit and achieve a stable wide-load range. Numerical calculations and model tests show new design reduces significantly the draft vortex and the characteristic channel vortex in comparison to the original runner at Dachaoshan, eliminating high partial load pressure pulsations. It brings about a reduction of more than 50% in the maximum pressure fluctuation amplitude in the vaneless region and draft tube, meeting the requirements for long-term continuous safe and stable operation over a wide-load range.

Key words: Francis turbine, "Negative Inclination and Reverse C-Type" runner, wide-load stable operation, numerical simulation, model tests