Etude du comportement dynamique des turbines Francis

EPFL, 2000
Thèse EPFL

Résumé

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.

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https://infoscience.epfl.ch/record/32698?ln=fr

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EPFL, 2000
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/32698?ln=fr

À propos de ce résultat

Concepts associés (11)

Concepts associés

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Publications associées (59)

Publications associées

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Flows in hydraulic machinery show interactions between guiding and rotating components. The wakes of the fixed components will be part responsible for the pressure fluctuations sensed on the blades. Those fluctuations become more important when the distance between the fixed and rotating components decreases. Global instabilities induced by the rope in the draft tube will also be sensed all over the installation. Cavitation on the blades will react in some ways to those fluctuations. This work is devoted to the influence of flow unsteadiness on the cavitation phenomenon. Two different flow configurations are tested, both subject to macroscopic fluctuations of the flow variables. The first part of this work involves the use of a fixed and oscillating NACA hydrofoil in the test section of the High Speed Cavitation Tunnel. Different leading edge configurations corresponding to hydrodynamically smooth and rough surfaces are tested. Observations of cavitation figures are done in parallel with pressure measurements on the hydrofoil suction side. It is observed that the pressure field leads the hydrofoil motion. This positive phase lag increases with the oscillation reduced frequency. For a particular reduced frequency, the phase lag between the movement and the pressure signals increases from the leading to the trailing edge of the hydrofoil. The transition location of the boundary layer on the suction side is modified by the movement. This transition follows the hydrofoil movement if compared with the fixed incidence case. Leading edge cavitation on the hydrofoil suction side interacts with the fluctuating pressure field. Cavitation appearance becomes intermittent when the leading edge is smooth. It disappears over all oscillation cycles if the oscillation frequency is further increased. This phenomenon is not observed when the leading edge is rough. Cavitation is also observed on the smooth parts of the leading edge when a rough patch is put at mid span. Intermittence is therefore not related to the modification of the pressure field, but to the state of the laminar boundary layer in the low pressure region. The positive phase lag of the pressure field also act to lower cavitation aggressiveness when the leading edge is rough on the oscillating hydrofoil. Those observations are validated by the measured acceleration. The second part of this work is devoted to the study of cavitating flow field in Francis turbines. A model turbine is used for pressure measurements on the blades without cavitation. The operating conditions correspond to the best efficiency point, highest power output at highest head, part load at the lowest head and part load at the highest head. It is observed that pressure fluctuations measured on the blades have the highest amplitude at part load. Those pressure fluctuations are related to the swirl of the flow in the draft tube. Other pressure fluctuations sources are the distribution of velocity at runner inlet and the wakes of the guide vanes. Experiments with cavitation on the blades show that the distribution of the flow at the runner inlet has a strong influence on the pressure pulses measured. Those pulses are detected in the pressure signals before cavitation becomes visible on the blades. An increase in the Reynolds number of the flow acts to higher the incipient cavitation parameter. This observation suggests that cavitation tests should be done at the highest Reynolds number reachable with the model turbine.

EPFL2002

Chargement

Unstable Part-Load Operation of a Model Francis Turbine : Evaluation of Disturbance Magnitude

The low frequency pressure oscillations observed at the draft tube wall of a Francis turbine operating at part load may be split into two uniform components: a rotating pressure with a constant area-averaged value, and a synchronous pulse involving the whole draft tube section. The synchronous pulse is sometimes used to evaluate the magnitude of disturbances generated by part-load operation of the turbine. A more global way to evaluate the magnitude of these disturbances is to estimate the emission of acoustic power at the spiral case inlet. In this paper, both methods are used on model test data from a high specific speed turbine. A good correlation is found between results from the two approaches.

International Association For Hydraulic Research1994

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Methodology for Risk Assessment of Part Load Resonance in Francis Turbine Power Plant

Philippe Allenbach, François Avellan, Bob Greiveldinger, Christophe Nicolet, Jean-Jacques Simond

At low flow rate operation, Francis turbines feature a cavitating vortex rope in the draft tube resulting from the swirling flow of the runner outlet. The unsteady pressure field related to the precession of the vortex rope induces plane wave propagating in the entire hydraulic system. The frequency of the vortex rope precession being comprised between 0.2 and 0.4 times the turbine rotational speed, there is a risk of resonance between the hydraulic circuit, the synchronous machine and the turbine itself an acting as excitation source. This paper presents a systematic methodology for the assessment of the resonance risk for a given Francis turbine power plant. The test case investigated is a 1GW 4 Francis turbines power plant. The methodology is based on a transient simulation of the dynamic behavior of the whole power plant considering a 1D model of the hydraulic installation, comprising gallery, surge chamber, penstock, Francis turbine but also mechanical masses, synchronous machines, transformer, grid model, speed and voltage regulators. A stochastic excitation having energy uniformly distributed in the frequency range of interest is taken into account in the draft tube. As the vortex rope volume has a strong influence on the natural frequencies of the hydraulic system, the wave speed in the draft tube is considered as a parameter for the investigation. The transient simulation points out the key excitation frequencies and the draft tube wave speed producing resonance between the vortex rope excitation and the circuit and provide a good evaluation of the impact on power quality. The comparison with scale model tests results allows resonance risk assessment in the early stage of project pre-study.

2006

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EPFL2002