Surging of 140 MW Francis turbines at high load, analysis and solution

High load surging is a specially delicate side of Francis turbines' dynamic behavior. There is yet no sure method to predict the occurrence of pulsations on a full-size machine, either from model test results or theoretical analysis. Upon commissioning of the Saucelle-Huebra power plant, high load surging made it impossible to operate in the maximum power output range. The turbine designer brought the oscillations to an acceptable level by a modification of the runner cone, without air admission. Thorough lab testing on a scale model allowed a better understanding of the problem. Influences of the runner cone, test head and tailrace tunnels are highlighted. A modal analysis using the cavitation compliance evaluated from model tests provides an explanation for the surge in the plant.