Etude du comportement dynamique des turbines Francis

Abstract

A common phenomenon in Francis turbines is draft tube surge. In these fixed-bladed machines, a strong runner outlet swirl can develop under off-design conditions. The action of the diffuser, and in particular the draft tube elbow, on the rotating flow field induces synchronous pressure and discharge fluctuations. These low frequency disturbances are of special interest, because they can easily propagate throughout the whole hydraulic system. The dynamic response of the system to the natural excitations can inhibit the normal usage of the powerplant. If the natural excitation occurs at frequencies close to an eigenfrequency of the hydraulic system, an important amplification of the hydraulic fluctuations can be expected. Passive measures, such as air admission or minor design modifications, are commonly taken as a last resort to try to reduce these phenomena. This work explores an active control approach to alleviate the problem. It is shown that the hydroacoustic fluctuations in a hydraulic installation can strongly be reduced by injecting the inverse signal of the turbine's natural excitation. While the main theme is the application of the active control approach to improve the operation stability of Francis turbines, the scope of this study is larger and addresses the following scientific topics: Modeling of an hydroelectric installation The dynamic behavior of an installation is studied based on an one-dimensional model. A compilation of the basic modeling tools was done. Using these tools, the dynamic model of a hydraulic installation and an external hydroacoustical source is presented. It is used to explain how the overall hydroacoustic fluctuations can be reduced using a hydraulic exciter mounted in the wall of a draft tube's cone. Improving the operation stability by active control The hydraulic fluctuations associated with off-design operating conditions of Francis turbines are often very periodic and characterized by the presence of a dominant frequency component. The aim is to cancel out this component for which the amplitudes are often excessive. An active control system, composed of a hydraulic exciter and its controller, has been developed for a Francis turbine. The exciter employs a pulsed flow injection in the draft tube to generate an antiexcitation at a given frequency, which is then synchronized with the turbine's natural excitation. A self-tuning extremum control algorithm optimizes the operating parameters of the exciter to minimize the overall fluctuations in the hydraulic installation. Laboratory tests show the efficiency of the active control system. The amplitude of the pressure and discharge fluctuations at the dominant frequency component are reduced to background noise levels. Multiple configurations of the exciter have been tested. A spectral analysis of the pressure fluctuations is presented, the system's energy balance has been performed and the control algorithm of the system is analysed. The energy balance is very encouraging: the exciter system needed only about 1 percent of the hydraulic power of the Francis turbine. Prediction of the operation stability of turbines based on model tests Laboratory tests on a scale model, homologous to the turbine, are an indispensable step in the development of a hydroelectric powerplant. Unlike the static characteristics, the dynamic behavior of a turbine installation is difficult to predict. An original identification method of the dynamic characteristics of a model turbine is proposed. The method is verified on a theoretical case study and experimentally tested. It constitutes the basis of the prediction of the operation stability of the powerplant based on model tests.