Hydroelectric System Response to Part Load Vortex Rope Excitation

The prediction of pressure and output power fluctuations amplitudes on Francis turbine prototype is a challenge for hydro-equipment industry since it is subjected to guarantees to ensure smooth and reliable operation of the hydro units. The European FP7 research project Hyperbole aims to setup a methodology to transpose the pressure fluctuations induced by the cavitation vortex rope on the reduced scale model to the prototype generating units. A Francis turbine unit of 444MW with a specific speed value of ν = 0.29, is considered as case study. A SIMSEN model of the power station including electrical system, controllers, rotating train and hydraulic system with transposed draft tube excitation sources is setup. Based on this model, a frequency analysis of the hydroelectric system is performed to analyse potential interactions between hydraulic excitation sources and electrical components

Identification of the wave speed and the second viscosity of cavitation flows with 2D RANS computations - Part I

1D hydro-electric models are useful to predict dynamic behaviour of hydro-power plants. Regarding vortex rope and cavitation surge in Francis turbines, the 1D models require some inputs that can be provided by numerical simulations. In this paper, a 2D cavitating Venturi is considered. URANS computations are performed to investigate the dynamic behaviour of the cavitation sheet depending on the frequency variation of the outlet pressure. The results are used to calibrate and to assess the reliability of the 1D models

RANS computations for identification of 1-D cavitation model parameters ::application to full load cavitation vortex rope

Due to the massive penetration of alternative renewable energies, hydropower is a key energy conversion technology for stabilizing the electrical power network by using hydraulic machines at off design operating conditions. At full load, the axisymmetric cavitation vortex rope developing in Francis turbines acts as an internal source of energy, leading to an instability commonly referred to as self-excited surge. 1-D models are developed to predict this phenomenon and to define the range of safe operating points for a hydropower plant. These models require a calibration of several parameters. The present work aims at identifying these parameters by using CFD results as objective functions for an optimization process. A 2-D Venturi and 3-D Francis turbine are considered

Turbine mode start-up simulation of a variable speed Francis pump-turbine prototype ::Part II: 3-D unsteady CFD and FEM

The Z'Mutt pumping station, part of the Grande Dixence hydroelectric scheme, is one of the demonstrators of the XFLEX HYDRO project. A 5 MW reversible pump-turbine prototype equipped with a full-size frequency converter (FSFC) is used to investigate dynamic variable speed operation in pumping and generating mode. Since the FSFC converter is always connected to the electrical grid, the full rotational speed range of the motor-generator can theoretically be exploited. Furthermore, this technology enables fast operating point transitions and therefore increased grid regulation capacities. The advantages of the FSFC technology in generating mode are compared to a conventional fixed speed start-up with a variable speed start-up. The operating point trajectories are extracted from 1-D hydraulic transient simulations. Detailed hydrodynamic and structural aspects of the pump-turbine during the two start-up scenarios are further studied. Simplified unsteady 3-D CFD simulations and transient structural FEM analyses of the pump-turbine prototype are carried out to assess the harshness of the flow and to anticipate runner fatigue. The present work aims to point out potential mitigation of partial runner damages during start-up in generating mode using FSFC technology

ACOUSTIC RESONNANCE IN CAVITATION FREE AND CAVITATING FLOWS

At part load operation of Francis turbine, the swirl in the draft tube leads to flow instability known as vortex breakdown. This flow instability can interact with the rest of the hydraulic circuit through axially propagating plane-waves. Moreover, at low cavitation index , the gaseous rope is suspected to modify locally the propagation velocity. Acoustic models have been commonly used to tackle this problematic. In order to validate the parameters of those models, an experiment with equivalent phenomenology has been setup. The experiment is designed so that the flow characteristic are similar to draft tube surge, but with strong simplification in order to facilitate the study of the flow instability and its interaction with the acoustic field. The hydraulic circuit consists in a square pipe connecting two constant pressure reservoirs. The excitation mechanism is obtained by the shedding of vortices in the wake of a bluff body placed at ¾ of the pipe length. The excitation frequency can be adjusted through the flow velocity. To examine the influence of vapor cavity formed in the wake of the obstacle, the mean pressure inside the pipe is also adjustable. Without cavitation, the analyses of the pressure field along the circuit highlights the acoustic modes shapes and Eigen frequencies of the system. It is also demonstrated that the amplitudes along the circuit are strongly increased as the excitation frequency matches the Eigen frequency of the system. In a second step, the relation of the cavitation index with the Eigen mode shapes and frequencies has been systematically analyzed. A direct influence of the vapor cavity on the local propagation velocity is shown. Finally, at particular cavitation index, important amplification of fluctuations has been noticed. From the author’s point of view, it is a consequence of the vapor volume unsteadiness

Turbine mode start-up simulation of a FSFC variable speed pump-turbine prototype ::Part I: 1D simulation

Variable speed hydroelectric units equipped with full size frequency converter (FSFC) offer high operational flexibility enabling fast operating point transitions which increase grid regulation capacities. The XFLEX HYDRO H2020 European research project aims to demonstrate flexibility of such technology at prototype scale. The Z'Mutt pumping station, part of the Grande Dixence hydroelectric scheme located in Switzerland, is one of the demonstrators focused on the FSFC technology with a new 5 MW reversible Francis pump-turbine which will be commissioned in 2021. This paper, divided in two parts, aims to simulate the turbine mode fast start-up sequence made possible with the use of a FSFC and to assess the unit damage by means of 1D and 3D CFD simulations. The part I of this paper presents the 1D hydraulic transient simulation results of start-up sequences of unit U5 considering both conventional fixed speed technology and variable speed technology. The time evolution of the unit's operating point is used as input data for 3D CFD simulations of part II, aiming to assess the impeller damage. Different control strategies to use the FSFC for turbine mode start-up sequence are analysed. Advantages and limits of each strategy are discussed, and recommendation is made for the Z'Mutt prototype demonstrator

Numerical simulations of Pelton turbine flow to predict large head variation influence

In the framework of the new feed-in-tariff system in Switzerland for Small Hydropower Plants (SHP), the aim of the SmallFLEX project, led by HES-SO Valais and performed in collaboration with EPFL, WSL, EAWAG, PVE, and FMV, is to show how SHP can provide winter peak energy and ancillary services, whilst remaining eco-compatible. The pilot and demonstrator site selected is the new small hydropower plant of Gletsch-Oberwald (KWGO) owned by FMV and commissioned end of 2017. This run-of-river power plant is equipped with two six-jet Pelton turbine units featuring a maximum power of 7 MW each. The addition of flexibility can be reached by using existing volumes of the power plant: the settling basin, the forebay chamber as well as part of the headrace tunnel. By consequence, the turbine head will undergo variations. To ensure that these variations will not cause any damages to the Pelton runner, the influence of the available head is investigated by numerical simulations. The simulations are carried out using two different software. OpenFOAM, which is based on a Finite Volume Method (FVM), is used for computing the flow inside the distributor until the jet. GPU-SPHEROS, which is based on Arbitrary Lagrangian-Eulerian (ALE) Finite Volume Particle Method (FVPM) is utilized to compute the interaction between the jet and Pelton buckets. Mixing the two aforementioned approaches, i.e. mesh-based FVM and particle-based FVPM, the overall torque T of the present six-jet Pelton runner can be reconstructed for different rated heads and discharges. The predicted torque fluctuations can then be considered for an estimation of the structural fatigue damage of the runner