Hydraulic stability in high-load operation: a new model and its use in Francis turbine refurbishment

High load instability caused by draft tube surge in a Francis turbine may present a very bad surprise during commissioning, and lead to a limitation of the operational range. Although there are means for mitigation, it is clearly desirable to identify the risk of instability during the design phase when a change in design is still possible. This article gives insight into the modelling of the hydraulic system, including a full load vortex, and discusses the drawbacks of today’s most commonly used model. Several cases of refurbishment projects are known for which the risk of instability is strongly over-predicted, one of them being the Guri II refurbishment project which has been successfully commissioned in April this year, without any signs of high-load instability. A modification of the model is described and shown to yield far more realistic results. Although there is still research needed, the major elements of the improved model were confirmed by application to another recent refurbishment project.