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Con el incremento de presencia de energías renovables en la red eléctrica, mayormente solar y eólica, surge la necesidad de dotar a la red de mayor flexibilidad y estabilidad. Es aquí donde las centrales hidroeléctricas son de gran utilidad, especialmente aquellas con turbinas Kaplan, ya que permiten estar regulando la potencia entregada continuamente con relativa facilidad ajustando sus directrices y álabes. Sin embargo, que las turbinas operen ofreciendo este servicio implica una mayor fatiga y desgaste en los elementos móviles del conjunto. En este proyecto se realiza un estudio sobre la magnitud de las pulsaciones de presión y las tensiones que éstas generan en los álabes del rodete de una turbina Kaplan real de 35 MW de potencia en diferentes puntos de funcionamiento. La turbina trabaja con un salto bruto medio de 12 m y cuenta con 12 directrices fijas y 24 directrices móviles. Para ello, primeramente, se realizan simulaciones CFD mediante ANSYS CFX, cuyos resultados, después de ser correlacionados con datos experimentales, se importarán a ANSYS Mechanical para realizar un análisis estructural. La selección de los casos a simular (un total de 3 casos) se realiza a partir de los datos experimentales de la turbomáquina. Se escogen 3 puntos de operación que se encuentren sobre la curva de funcionamiento de la máquina (curva “on-cam”). El primer caso corresponde a una apertura de los álabes directrices y rodete del 100%, correspondiente al de máxima potencia, y a partir de este punto, se crean 2 casos más con diferentes grados de cierre. Finalmente, se obtienen resultados dentro de lo esperado respecto a pulsaciones de presión en turbinas Kaplan. En las simulaciones CFD se observa como la magnitud de las fluctuaciones sobre el rodete son pequeñas, haciendo que en los análisis estructurales la amplitud de la fluctuación de las tensiones máximas sea de alrededor de un 6%.

The present paper investigates the rotating vortex rope (RVR) mitigation on an axial turbine model by the radial protrusion of four cylindrical rods into the draft tube. RVR mitigation is of particular interest due to the unfavorable pressure pulsations it induces in the hydraulic circuit that can affect turbine life and performance. The protrusion lengths, which were the same among the four rods, were varied according to a predefined sequence. The experiments were performed under four part-load regimes ranging from upper part load to deep part load. Time-resolved pressure measurements were conducted at two sections on the draft tube wall along with high-speed videography and efficiency measurement to investigate the effect of the mitigation technique on the RVR characteristics and turbine performance. The recorded pressure data were decomposed and studied through spectral analyses, phase-averaging, and statistical analyses of the RVR frequency and peak-to-peak pressure amplitude distributions. The results showed different levels of pressure amplitude mitigation ranging from approximately 10% to 85% depending on the operating condition, protrusion length, and the method of analysis. The hydraulic efficiency of the turbine decreased by a maximum of 3.5% that of the best efficiency point (BEP) with the implementation of the mitigation technique. The variations in the obtained mitigation levels and efficiencies depending on protrusion length and operating condition indicate the need for the implementation of a feedback-loop controller. Thus, the protrusion length can be actively optimized based on the desired mitigation target., Validerad;2024;Nivå 2;2024-04-04 (joosat);Full text: CC BY license

The present paper investigates the rotating vortex rope (RVR) mitigation on an axial turbine model by the radial protrusion of four cylindrical rods into the draft tube. RVR mitigation is of particular interest due to the unfavorable pressure pulsations it induces in the hydraulic circuit that can affect turbine life and performance. The protrusion lengths, which were the same among the four rods, were varied according to a predefined sequence. The experiments were performed under four part-load regimes ranging from upper part load to deep part load. Time-resolved pressure measurements were conducted at two sections on the draft tube wall along with high-speed videography and efficiency measurement to investigate the effect of the mitigation technique on the RVR characteristics and turbine performance. The recorded pressure data were decomposed and studied through spectral analyses, phase-averaging, and statistical analyses of the RVR frequency and peak-to-peak pressure amplitude distributions. The results showed different levels of pressure amplitude mitigation ranging from approximately 10% to 85% depending on the operating condition, protrusion length, and the method of analysis. The hydraulic efficiency of the turbine decreased by a maximum of 3.5% that of the best efficiency point (BEP) with the implementation of the mitigation technique. The variations in the obtained mitigation levels and efficiencies depending on protrusion length and operating condition indicate the need for the implementation of a feedback-loop controller. Thus, the protrusion length can be actively optimized based on the desired mitigation target., Validerad;2024;Nivå 2;2024-04-04 (joosat);Full text: CC BY license

A Kaplan turbine unit from a new powerplant experienced high levels of vibrations and noise during the acceleration of the unit to nominal speed. These levels remained very high at speed no load. A first analysis seemed to indicate a possible blade resonance phenomenon as the highest vibrations were measured on the head cover. Nevertheless, after some modifications on the root of the blades, the vibrations remained high and the commissioning of the unit was delayed several months. To determine the origin of these vibrations, an array of sensors was installed in the machine. Different types of tests were performed. Experimental modal analyses of the runner head cover were conducted. Furthermore, experimental tests to determine the transmission of vibrations from the runner blades to the rest of the unit were carried out. Finally, the unit was tested during the acceleration phase to speed no load. Thanks to these tests, the origin of the high amplitude vibrations and loud noise perceived close to the head cover of the turbine was found. A solution to mitigate this problem was implemented by modifying the diameters of the air admission openings and their distribution, according to the up-to-date design rules. Both vibrations and noise amplitudes were drastically decreased. After this, the subsequent steps of the commissioning tests were performed and the unit started the commercial operation without major issues., Alexandre Presas and David Valentin want to acknowledge the Serra Hunter Program of Generalitat de Catalunya. The authors want to express their gratitude to all ELECNOR staff in the analysed powerplant, which showed their highest commitment with the tests performed there. Greco Moraga received funding from the National Agency for Research and Development (ANID) / Scholarship Program / DOCTORADO BECAS CHILE/2020 – 72210054 for his PhD scholarship., Peer Reviewed, Postprint (published version)

Nowadays, hydropower plants are being used to compensate for the variable power produced by the new fluctuating renewable energy sources, such as wind and solar power, and to stabilise the grid. Consequently, hydraulic turbines are forced to work more often in off-design conditions, far from their best efficiency point. This new operation strategy increases the probability of erosive cavitation and of hydraulic instabilities and pressure fluctuations that increase the risk of fatigue damage and reduce the life expectancy of the units. To monitor erosive cavitation and fatigue damage, acoustic emissions induced by very-high-frequency elastic waves within the solid have been traditionally used. Therefore, acoustic emissions are becoming an important tool for hydraulic turbine failure detection and troubleshooting. In particular, artificial intelligence is a promising signal analysis research hotspot, and it has a great potential in the condition monitoring of hydraulic turbines using acoustic emissions as a key factor in the digitalisation process. In this paper, a brief introduction of acoustic emissions and a description of their main applications are presented. Then, the research works carried out for cavitation and fracture detection using acoustic emissions are summarised, and the different levels of development are compared and discussed. Finally, the role of artificial intelligence is reviewed, and expected directions for future works are suggested., Peer Reviewed, Postprint (published version)

© The Author(s) 2023., The current renewable energy market forces hydraulic turbines to operate for longer periods of time at off-design and transient conditions. Their life expectancy is then decreased due to the wear provoked by flow instabilities and stochastic flow excitations. This study presents an experimental investigation into the fatigue damage induced on the runner blades of a Kaplan turbine model when working at speed-no-load (SNL), part load (PL) and during ramps of load. The unit was equipped with on-board sensors on the blades and the shaft as well as with off-board sensors installed on the supporting structure and the draft tube cone. The results reveal that operation at SNL induces more fatigue damage on the runner blades than at PL. The damage is then mainly induced by stochastic flow excitations at SNL and by the rotating mode of the rotating vortex rope (RVR) at PL. The ramps of load, when crossing each operating condition, cause levels of damage similar to those found in stationary operation. Finally, it was proved that the blade fatigue damage can be estimated from on-board shaft measurements at any condition. However, the blade fatigue damage can only be estimated using off-board measurements when the RVR is fully developed at PL., This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 814958. Additionally, the present authors would like to acknowledge the technical support received from Vattenfall AB., Peer Reviewed, Postprint (author's final draft)

The operation of submerged rotating machines, such as marine current or tidal turbines, can present deleterious fluid phenomena that may provoke extreme structural vibrations. To predict their dynamic responses, it is necessary to know the added modal coefficients of their runners under a whirling motion. For that purpose, a bespoke test rig was designed to investigate the added modal coefficients of a submerged cylinder, which could rotate at different speeds both in air and completely submerged in water inside a cylindrical tank. First, the modes of vibration were experimentally measured by exciting the cylinder with a push-release method during steady tests or with ramps in rotating speed during transient tests. The calculated natural frequencies and damping ratios were then used in a mathematical model of the dynamic system to calculate the added modal coefficients. During steady tests, the natural frequencies and damping ratios of the whirling modes changed significantly as a function of the rotating speed. Additionally, a whirling mode was observed to change its direction at a given rotating speed. During transient tests, rotating speed ramps with high accelerations were found to present lower lock-in amplitude and frequencies. Moreover, fast downward ramps presented lock-in amplitudes four times higher than fast upward ramps. Consequently, the added modal coefficients changed accordingly as a function of the rotating speed, ramp acceleration, and ramp direction. For these reasons, it was confirmed that the modal responses of submerged rotating bodies must be calculated for each operational rotating speed, even at low velocities, and for each transient event in order to precisely predict their vibration behaviors., This research was funded by the European Union’s Horizon H2020 research and innovation program under grant agreement No. 814958., Peer Reviewed, Postprint (published version)

Nowadays, in order to mitigate the global warming effects, there is a need of renewable energy generation for the objective of carbon neutrality. In this context, hydropower plays a key role not only because the amount of renewable energy generated but also because it is a fundamental player to ensure the stability of the electrical grid, as one of the main dispatchable sources. Nevertheless, hydropower is facing nowadays with the climatological problem of the extreme droughts, that are expected to be more common in the upcoming years. Therefore, it has become more important than ever to use the water flowing through the rivers properly. In this paper, the potentiality of using variable speed operation with the aim of increasing the overall efficiency of Francis turbines, which are the most widely used hydro turbines worldwide, is numerically explored. This implies to use less water to produce the same amount of electrical power. The study is based on an accurate modelling with real prototype data which has been made available for this study. It is shown that variable speed could improve the overall efficiency of the unit with respect to the constant speed generator, typically used in hydropower. While this idea has been mentioned in some previous studies, in this paper we also consider the electrical efficiency decrease of the variable speed technologies and restrictions of the unit regarding the electrical power generated. Results show that when the unit operates at some specific operating conditions, namely low heads at part load operations and high heads at maximum power, variable speed technologies could be used to save more than 2% of water with respect to the fixed speed unit. Main results and models of this paper can be used as a reference for future studies with similar type of units., First of all, Authors would like to acknowledge XFLEX Hydro [20]. This study is a direct output of the technologies being studied in this project. The Hydropower Extending Power System Flexibility (XFLEX HYDRO) project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 857832. Alexandre Presas would like to acknowledge, Nicolas Hugo from ALPIQ, Thomas Hildinger from VOITH and Josep Bordonau from UPC for his expertise and valuable comments regarding the FSFC and DFIG models and also Jo˜ao Delgado for his valuable contributions and ideas during part of this project. Alexandre Presas and David Valentín acknowledge the Serra Húnter program of Generalitat de Catalunya., Postprint (published version)

Part 2 of this work presents a numerical methodology, validated using the experimental results presented in Part 1, to calculate the added modal coefficients of a submerged cylinder in water both when it oscillates and when it rotates with a whirling motion. The numerical methodology is based on computational fluid dynamic simulations that obtain the added modal forces on the cylinder when it is forced to vibrate with mode shapes calculated using acoustic-structural modal analysis. Then, these forces are processed with a curve-fitting algorithm to extract all the coefficients. Most numerical coefficients presented a close agreement with the corresponding experimental ones, although the added modal damping was overestimated. In general, the added modal mass was found to be independent of both the rotating speed and the whirling frequency except for low whirling frequencies when it increased. The added modal damping was found to depend on both parameters, and the rest of the coefficients were independent of the whirling frequency and only depended on the rotating speed. As a conclusion, this numerical approach has permitted the study of particular conditions that could not be experimentally tested and thus broadened the knowledge of the behavior of the added modal coefficients of rotating submerged cylinders., This research was funded by the European Union’s Horizon H2020 research and innovation program under grant agreement No. 814958., Peer Reviewed, Postprint (published version)

The growing installation of intermittent renewable energy sources is forcing hydraulic turbines to work more frequently at speed no load when dangerous vaneless space vortex structures and stochastic flow phenomena can occur. An experimental campaign has been performed in a reduced-scale Kaplan turbine model at speed no load for different combinations of guide vane and runner blade angles under non-cavitation and cavitation conditions. Several simultaneous vaneless space vortex structures, all of them inducing torsional rotor vibrations, have been observed. Nonetheless, only one of them has been found to dominate over the rest depending on the blade and guide vane angles. Off-board pressures, torques and vibrations as well as on-board strains have been measured to characterize their nature, intensity, dynamic behavior and induced structural response. Their precession frequencies have been found to depend on the discharge factor, the number of vortices and their location inside the vaneless space. Under cavitation conditions, their dynamic behavior has not been significantly altered but the induced structural response has increased at the low-pressure side of the turbine., This research was funded by the European Union’s Horizon H2020 research and innovation program under grant number 814958., Peer Reviewed, Postprint (published version)

The structural dynamic response of hydraulic turbines needs to be continuously monitored to predict incipient failures and avoid catastrophic breakdowns. Current methods based on traditional off-board vibration sensors mounted on fixed components do not permit inferring loads induced on rotating parts with enough accuracy. Therefore, the present paper assesses the performance of fiber Bragg grating sensors to measure the vibrations induced on a rotating shaft–disc assembly partially submerged in water resembling a hydraulic turbine rotor. An innovative mounting procedure for installing the sensors is developed and tested, which consists of machining a thin groove along a shaft line to embed a fiber-optic array that can pass through the bearings. At the top of the shaft, a rotary joint is used to extract, in real time, the signals to the interrogator. The shaft strain distribution is measured with high spatial resolution at different rotating speeds in air and water. From this, the natural frequencies, damping ratios, and their associated mode shapes are quantified at different operating conditions. Additionally, the change induced in the modes of vibration by the rotation effects is well captured. All in all, these results validate the suitability of this new fiber-optic technology for such applications and its overall better performance in terms of sensitivity and spatial resolution relative to traditional equipment. The next steps will consist of testing this new sensing technology in actual full-scale hydraulic turbines., This work was supported by the funding received from the European Union’s Horizon H2020 research and innovation program (grant agreement no: 814958) and by the I+D+i project SYNERGY PID2020-118310RBI00 funded by the MCIN/AEI/10.13039/501100011033., Peer Reviewed, Postprint (published version)

The current paper presents an investigation into novel modal testing methods applied to a disk–shaft structure at different rotating speeds in air and water. The structure was excited using three different methods: an instrumented hammer, a piezoelectric PZT patch glued on the disk and a transient ramp-up. The structural response was measured using an accelerometer and strain gauges mounted on board as well as accelerometers and displacement lasers mounted off board. The potential to excite the natural frequencies using each excitation method and to detect natural frequencies with each sensor was analyzed and compared. Numerical structural and acoustic– structural modal and harmonic analyses of the non-rotating disk in air and water were also performed, taking into consideration the PZT patch. The numerical results showed a close agreement with the experimental ones in both air and water. It was found that the rotating speed of the disk modified the detected natural frequencies, depending on the frame of reference of the sensor. Finally, the PZT patch and the transient ramp-up were proven to be reliable methods to excite the natural frequencies of the current test rig and to be potentially applicable in full-scale hydraulic turbines under operating conditions., This research was funded by the European Union’s Horizon H2020 research and innovation program under grant agreement No. 814958., Peer Reviewed, Postprint (published version)

In the current energy market, hydraulic turbines are increasingly demanded to work in Frequency containment reserve (FCR) mode to compensate the constant frequency fluctuations in the electrical grid. To do so, hydraulic turbines change their generating power continuously which implies to regulate the flow rate. Kaplan turbines are double regulated machines that change the position of both guide vanes and runner blades to regulate the flow rate maximizing their efficiency. Therefore, guide vanes and runner blades are continuously moving when they provide FCR, leading to high wear and tear of the regulation system components. Within the frame of the European project XFLEX Hydro, a new technology to reduce the wear and tear of the regulation system in FRC have been implemented. This technology consists in the hybridization of the unit with a battery system. In that way, the battery is the one in charge of providing part of the power fluctuations to the grid, reducing the movements of guide vanes and runner blades of the turbine. The battery system was successfully installed in August 2021 in one unit of the Vogelgrun powerplant, in France. Since that moment, the unit has been working in hybrid mode. A monitoring system was installed in the power plant in two different units, the one hybridized and another without hybrid system. Several sensors were installed and different parameters were measured simultaneously to calculate the wear and tear of the different components. In this paper, a comparison between the hybrid mode and the standard mode (non-hybrid) is performed in terms of mileage and wear and tear of the guide vanes and runner blades servomotors., The authors would like to acknowledge the XFLEX project and the partners involved in WP9. Special thanks to EDF for their implication during the installation of the monitoring system and the tests. David Valentín and Alexandre Presas would like to acknowledge the Serra Húnter program too., Peer Reviewed, Postprint (published version)

Understanding and simulating the dynamic characteristics of the hydro-turbine governor system is essential to guarantee the safety of the hydro-power systems. This study proposes a modification method of the water head at the draft tube inlet, which is to discretize the draft tube and introduce the excitation term of the vortex rope. The proposed approach is applied for the modeling of the hydro-turbine governor system. The proposed model (Model I) is verified by means of a comparison with a second model (Model II) of the hydro-turbine governor system model without considering the vortex rope and verified against the experimental data during the load reduction. The model developed is accurate providing errors of the water head, inlet water head and flow of the hydro-turbine for Model ¿ are within ±3.3%, ±2% and ±5%, respectively. The proposed model overcomes the limitations of Model ¿ in the time-domain and frequency-domain since that only provides the average pressure at the draft tube, which do not reflect the pressure fluctuation and energy fluctuation. Instead the proposed model allows to capture and model the energy fluctuation during the load reduction as shown in the experimental results. The proposed model promotes the development of the hydro-turbine governor system model., Peer Reviewed, Postprint (author's final draft)

This paper presents the design and evaluation of an active control system to reduce vibrations and strains of submerged structures. For that, a metallic disk mounted in a test rig has been equipped with on-board waterproof strain gauges and accelerometers, to measure its structural response, and with an on-board PZT actuator patch, to provide the required control force. This control force is computed in real-time by an optimal algorithm that determines the exact voltage to be supplied to the PZT patch in order to mitigate any increase of vibrations and/or strains detected by the sensors. In order to capture the dynamics of the structure when it is excited both in air and submerged in water, a plant model has been identified from the Frequency Response Functions (FRFs) obtained experimentally from the measured vibrations and numerically from simulated strains. With these plant models, two different control techniques based on a Linear Quadratic Gaussian (LQG) controller and on an H8 controller have been implemented and simulated under different types of excitations. The obtained results are satisfactory and support the need to continue the research to validate them experimentally., This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 814958., Peer Reviewed, Postprint (published version)

The natural frequencies of a turbine can be calculated from numerical methods. By comparing these natural frequencies with excitation sources, one can know the danger of a resonance and a possible failure in a component of the turbine. Therefore, it is often very important to have an accurate numerical model of the turbine to determine these natural frequencies. There are not many publications on the determination of the natural frequencies of reduced-scale models of Kaplan turbines. More papers exist for pump turbines or Francis turbines. For real Kaplan turbines, very few experiments can be found to determine mode shapes and natural frequencies. In this paper a Kaplan turbine of 37MW (maximum power), 12.5m (maximum head) and 50 m3 /s (maximum flowrate) was tested. The turbine was equipped to determine the natural frequencies of the runner in air. For this purpose, one accelerometer in each blade of the runner was installed and a total of 16 impacts were done in each blade. Frequencies and mode shapes were obtained. In parallel, a numerical model was obtained. Numerical and experimental results were compared and an accurate numerical model is presented. With this numerical model the natural frequencies of the runner in water were calculated., This research activity is framed within the context of the XFLEX HYDRO project. The project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 857832. No. 857832)., Peer Reviewed, Postprint (published version)

To cope with the intermittent power supply of the new renewable energies and demand fluctuations, Francis turbines are required to operate more and more in an extended operating range, far away from the design point. With this operating behavior, it is very complex to interpret the trend of vibration parameters typically used in Condition Monitoring and to define reasonable alarm and trip levels valid for all the operating range of the unit working in steady conditions. As in the efficiency curves of Francis turbines represented as a function of net head and load (Hill Chart), in this paper we propose to represent the most relevant vibration parameters in surfaces, called Vibrational Hill Charts, which allow a more accurate evaluation of the indicators and their trends and a better classification of abnormal values. To show the potential of Vibrational Hill Charts, a complete database obtained after 2 years of monitoring a large Francis Unit (444 MW rated power) has been used. The mapping of the relevant vibration parameters has been performed by means of Artificial Neural Networks. It is shown that by setting the action levels based on the resulting maps, rather than a constant value, a better diagnosis capacity is achieved as the Receiver Operating Characteristic will be improved. Furthermore, phenomena such as erosive cavitation, which is hard to be detected, could be also assessed with the use of multidimensional analysis based on the Vibrational Hill Chart. As a conclusion, with the Vibrational Hill Chart, the condition monitoring and diagnosis of hydraulic turbines could be improved., Postprint (author's final draft)

This paper presents the failure analysis and diagnostic of a hydraulic turbine. Shortly after a maintenance revision the thrust bearing of the turbine was destroyed when the turbine was put into operation. The damaged bearing was examined and the possible causes discussed. To identify the problem that led to the failure of the thrust bearing, a comprehensive on-site measurement campaign was done. Vibrations, pressures, temperatures and operating parameters were acquired at different operating conditions of the turbine. The analysis of the data allowed to determine the source of the problem and to implement a solution., The authors would like to thank the company Endesa Generación and the XFLEX HYDRO 2020 research and innovation program under grant agreement No 857832FLEX., Peer Reviewed, Postprint (published version)

The rotating vortex rope, which can be decomposed in the rotating and the plunging modes, is the origin of pressure fluctuations in the draft tube cone when hydraulic turbines operate at part load, compromising the structural integrity and limiting the output load. A measurement campaign was carried out in a Kaplan turbine model which is a replica of the experimental 10 MW Porjus U9 prototype machine along a propeller curve to study the rotating vortex rope’s excitation levels and the induced structural responses. A complete set of sensors mounted on-board and off-board was used to measure pressures, forces, torques, accelerations, displacements, and strains. The characteristic frequencies and amplitudes of the pressure fluctuations and of the corresponding induced loads and vibrations associated with the two modes were quantified in a wide range of operating conditions at part load. The two modes are detected at different frequencies depending on the sensor position. Moreover, their frequencies change depending on the discharge and present different amplitudes depending on the mode. Particularly, the rotating mode shows higher amplitudes than the plunging mode in the majority of positions and directions measured., This research received funding from the European Union’s Horizon H2020 research and innovation program under grant agreement No814958., Peer Reviewed, Postprint (published version)

Kaplan turbines are nowadays used to provide Frequency Containment Reserve (FCR) to the grid due to their fast capacity to regulate their power maintaining high efficiency. However, this continuous power regulation increases the wear and tear of the regulation system considerably. To reduce the amount of movements in the regulation servomotors, and thus their wear and tear, a new technology is being investigated within the frame of the European project XFLEX Hydro. This new technology is based on hybridizing the hydro unit with a small size battery in parallel, this one being in charge of compensating the small frequency fluctuations in the grid by providing or absorbing power. In this paper, the benefits of the implementation of this new technology are evaluated. A Kaplan turbine prototype located in Vogelgrun, France, has been hybridized and different parameters have been monitored while the unit was working in hybrid mode and in normal standalone hydro mode. Wear and tear of the regulation system have been compared for both hybrid and standalone hydro modes. A reduction of about 25% in servomotors mileage and of 50% in fatigue damage have been obtained by hybridizing the unit., Peer Reviewed, Postprint (published version)

To prevent lifetime shortening and premature failure in turbine runners, it is of paramount importance to analyse and understand its dynamic response and determine the factors that affect it. In this paper, the dynamic response of a Kaplan runner is analysed in air by numerical and experimental methods. First, to start the analysis of Kaplan runner mode shapes, its geometry is simplified and modelled as a bladed disk. Bladed disks with different blade numbers are investigated, by numerical simulation, in order to understand the influence of this parameter on its modal characteristics. Then, mode shapes extracted are characterized and a classification is proposed. Second, an existing Kaplan runner is simulated by Finite Elements Method (FEM) and its mode shapes are extracted. The obtained results are contrasted with the bladed disks mode shapes, in order to validate the classification proposed. The simulated Kaplan runner is also experimentally studied. A numerical modal analysis is carried out in the real runner. Different, global and local, mode shapes are identified. The global mode shapes extracted by numerical and experimental modal analysis are compared and discussed. Finally, the local mode shapes identified are commented and explained by means of numerical simulation., Peer Reviewed, Postprint (published version)

An experimental investigation of frequential protrusion of four solid rods into the draft tube of a propeller turbine operating under partial discharge has been undertaken. The effectiveness of mitigating the pressure fluctuations associated with the rotating vortex rope (RVR) has been quantified using pressure measurements on the wall of the draft tube cone. Three azimuthal configurations of the phase difference between the rods, and four protrusion lengths were investigated. It is shown that the rotating component of the RVR decreases, irrespective of the azimuthal configuration and protrusion length, with imposed phase differences in the same direction as the runner rotation being the most effective, reducing the amplitude of the rotating component by a maximum of 62%. However, for each azimuthal configuration, the plunging mode of the RVR is amplified for large protrusion lengths, with the smallest amplification occurring for the case of 180 degrees phase difference between protrusions. Therefore, to quantify the most efficient configuration in mitigating the harmful effects of the RVR, an overall assessment of its effects on the entire turbine must be made before a conclusion can be drawn.

One of the most critical operating conditions in hydraulic turbines is the start-up transient. This may be characterized by the presence of several resonances, which increase the vibration and stress levels on the runners. At present, its study is of interest due to the more frequent starts and stops of the turbines in the last years. The present study is focused on analysing the start-up transient of Pelton turbines. To do so, an experimental investigation was carried out in a horizontal shaft prototype unit. The procedure started by determining the natural frequencies and mode shapes of the turbine by means of impact testing. Several accelerometers were placed on the runner, on the shaft and on the bearings. With experimental modal analysis the main vibration modes of the rotor and the runner were found. Once the structural response of the turbine was determined, the start-up transient was studied. For the tests, different types of sensors were installed on the machine. Accelerometers, proximity probes and acoustic emission sensors were placed on the bearings and on the turbine casing. Moreover, an accelerometer and a strain gauge were installed on the shaft with an on-board system. Different situations are identified during the start-up. At the very beginning of the transient, the initial collision of the water jet on the still runner produces large vibration levels due to the excitation of many bucket modes. Once the turbine starts rotating, the vibration levels increase every time the jet excitation at the bucket passing frequency meets a natural frequency of the structure. The effect of the transient regarding the change in the vibration levels and runner deformations is presented in this paper. The sensitivity of the different sensors to measure the runner vibrations is analysed in order to optimize the detection of the transient effects. The best locations to install the sensors are also identified., Peer Reviewed, Postprint (updated version)

In this study, the behavior of a platform for tidal energy is analyzed. The turbine of this platform is formed by two counter-rotating rotors aligned with the tidal current, such that one rotor is always under the wake of the upstream rotor. The platform has been tested by towing it with a tugboat to emulate tidal currents. The results of these tests are analyzed in this paper in order to estimate the generation capabilities of the platform., Peer Reviewed

With the current demand of flexibility to hydraulic turbines, these are working under off-design conditions more than ever. Under these conditions different complex hydraulic phenomena appear, especially in Francis turbines, affecting the machine stability as well as reducing the useful life of its components. Hence, it is desirable to detect in real-time these hydraulic phenomena to assess the operation of the machine. In addition, the position and type of sensor to detect these phenomena is a key point for the monitoring of Francis units. In this paper, a large medium-head Francis turbine was instrumented with several sensors such as accelerometers, proximity probes, strain gauges, pressure sensors and a microphone. Results obtained with the different sensors in different locations are compared to know which hydraulic phenomenon is detected with every sensor. Furthermore, different signal analysis techniques are also compared. With this information, monitoring systems can be optimized with the most convenient sensors, locations and signal analysis techniques, reducing its difficulty to install and economic cost., Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant, Postprint (author's final draft)

This paper presents an experimental modal test of a Kaplan turbine model and provides the corresponding analysis of the results. The modal test, which included both the runner and the shaft line, was performed using an impact hammer and a shaker as exciters and several accelerometers as sensors. Additionally, a numerical model of the Kaplan turbine has been built and validated in air. By comparing the numerical and experimental results, the main modes of vibration of the rotor and a runner blade in dry conditions have been identified and discussed. Finally, the rotor has also been tested in wet conditions (still water), and the results have been compared with those calculated from a numericalmodelincluding water as an acoustic domain., This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 814958. The authors also wish to acknowledge the technical support received from Vattenfall and Lightness by Design., Peer Reviewed, Postprint (published version)

Nowadays, one of the major mechanical issues of hydraulic turbines and particularly Francis turbines are the failures produced by fatigue. Due to the massive entrance of new renewable energies such as wind or solar, hydraulic turbines have to withstand off-design conditions and multiple transients, which greatly increase the risk of fatigue failures. Fatigue damage and crack propagation models are based on the static and dynamic stresses on the turbine blades. Therefore, an accurate and realistic determination of these stresses is of paramount importance although it is yet a challenging task. Numerical simulations have still limitations when predicting static and dynamic stresses in the most harmful conditions such as deep part load conditions and transients, which are highly stochastic. The installation of strain gauges on the blades gives accurate stress measurement but it involves long and very expensive measurement campaigns, as the turbine runner is submerged, confined and rotating. In this paper we propose a neural network-based method to determine the magnitude of static and dynamic stresses based on the measurements of stationary sensors, which greatly reduces the complexity and costs of strain gauge testing. Inputs of the neural networks are selected based on previous experience monitoring Francis turbines. The trained network can be implemented in advanced monitoring systems that could continuously evaluate the stress level of the turbine and determine the risk of possible fatigue damage., Peer Reviewed, Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant, Postprint (published version)

The Francis hydro-turbine generator unit (FHTGU) is a typical nonlinear system with the coupling hydraulic, mechanical and electric subsystems. It is a challenge to understand the reasons for its operational failures because the major reason for failures involves complex interactions of the three subsystems. Subsystems’ model interaction with the method of normal forms has been well developed and investigated, overcoming the linear methods used in the FHTGU’s stability analysis. However, these methods have not to quantify higher-order terms in a mathematically accurate type to capture dynamic modal interactions between subsystems. Due to the accelerating expansion of hydropower stations, stability of FHTGU shows singular nonlinear oscillations and new methods have to be upgraded to cope with this new situation. In this study, the nonlinear modal method is introduced to analyze the dynamic modal interactions between subsystems, and results given by the different methods are compared to verify the method’s feasibility. The effect of the second order modes is quantified to investigate its effect on the dynamic characteristics of FHTGU, and the vibration characteristics affected by the wind generation system are also investigated. The result shows that the intensity of modes can be effectively reduced to satisfy the stable requirements. All of these results provide a theoretical guidance for the stable operation of FHTGUs., Peer Reviewed, Postprint (published version)

The health condition of hydraulic turbines is one of the most critical factors for the operation safety and financial benefits of a hydro power plant. After the massive entrance of intermittent renewable energies, hydropower units have to regulate their output much more frequently for the balancing of the power grid. Under these conditions, the components of the machine have to withstand harsher excitation forces, which are more likely to produce damage and eventual failure in the turbines. To ensure the reliability of these machines, improved condition monitoring techniques are increasingly demanded. In this article, the feasibility of upgrading condition monitoring of Pelton turbines using novel vibration indicators and data-driven techniques is discussed. The new indicators are selected after performing a detailed analysis of the dynamic behavior of the turbine using numerical models and field measurements. After that, factor analysis is carried out in order to assess which are the most informative indicators and to reduce the dimension of the input data. For the validation of the proposed method, monitoring data from an actual Pelton turbine that suffered from an important fatigue failure due to a crack propagation on the buckets have been used. The novel condition indicators as well as classical indicators based on the spectrum and harmonics levels have been obtained while the machine was in good operation, during different stages of damage and after repair. All of these have been used to train an artificial neural network model in order to predict the evolution of the crack until failure occurs. The results show that using the improved monitoring methodology enhances the ability to predict the appearance of damage in comparison to typical condition indicators., Peer Reviewed, Postprint (author's final draft)

An unconventional tuft visualization method, along with an image processing technique adapted to the flow conditions, is proposed and implemented on a reduced-scale model of a Francis-type reversible pump-turbine in three different turbine stages such as turbine mode, runaway mode, and turbine break mode. The main goal of this technique is to visualize the complex flow developed during a rotating stall phenomenon. Fluorescent monofilament wires along with high-speed image processing and pressure sensors were installed in the narrow and vaneless gap between the impeller blades and guide vanes. Pressure fluctuations were analyzed along with tuft visualization to describe the flow with and without a rotating stall. The implemented tuft visualization method gives an adjusted qualitative description of the undergoing phenomena, making different flow behaviors such as backflow, high turbulences, recirculation, etc., evident. © 2012 by Begell House, Inc.

When operated far from their optimum conditions, pump-turbines may exhibit strong hydrodynamic instabilities, often called rotating stall, which lead to substantial increase of vibration and risk of mechanical failure. In the present study, we have investigated the flow filed in a model of radial pump-turbine with the help of tuft visualization, wall pressure measurement and structure-borne noise monitoring. As the rotation speed is increased, the machine is brought from its optimum operation to runaway with zero torque on the shaft. The runaway operation is characterized by a significant increase of pressure fluctuation at the rotor-stator interaction frequency. As the speed is further increased, the flow exhibits sub-synchronous instability, which rotates at 70% of the rotation frequency. Tuft visualization clearly shows that, as the instability evolves, the flow in a given distributor channel suddenly stalls and switches to reverse pumping mode in periodic way. We have also investigated the monitoring of the rotating stall with the help of vibration signals. A specific signal processing method, based on amplitude demodulation, was developed. The use of 2 accelerometers allows for the identification of the optimum carrier frequency by computing the cyclic coherence of vibration signals. This non-intrusive method is proved to be efficient in detecting the rotating stall instability and the number of stall cells. We strongly believe that it could be implemented in full scale pump-turbines. © 2014 Elsevier Ltd.

Recently, TOBB University of Economics and Technology was awarded a grant for a project on 'Design, Construction and Test of Hydraulic Turbines' from Turkish Ministry of Development. The project includes Computational Fluid Dynamics (CFD) aided hydraulic and structural design, geometry optimization, manufacturing and testing of hydraulic turbines. The main objective of the research presented here is to design a hydro turbine test stand which tests the performance and cavitation characteristics of turbine models according to standards. The paper describes the characteristics of the designed test stand. © 2013 IEEE., Abant Izzet Baysal Universitesi;Cankiri Karatekin Universitesi

Recently, TOBB University of Economics and Technology was awarded a grant for a project on 'Design, Construction and Test of Hydraulic Turbines' from Turkish Ministry of Development. The project includes Computational Fluid Dynamics (CFD) aided hydraulic and structural design, geometry optimization, manufacturing and testing of hydraulic turbines. The main objective of the research presented here is to design a hydro turbine test stand which tests the performance and cavitation characteristics of turbine models according to standards. The paper describes the characteristics of the designed test stand. © 2013 IEEE., Abant Izzet Baysal Universitesi;Cankiri Karatekin Universitesi

The development of a methodology to simulate numerically the steady state flow field in Francis turbines was carried out using the open source CFD software OpenFOAM along with a MRF and AMI approach. A structured mesh obtained from previous studies was used with some modifications to solve the presence of imbalance points in the simulation. Numerical simulations using the SIMPLE algorithm were performed together with two different turbulence models. It was confirmed that the results obtained reproduced the phenomenon with a great approximation and predicted the output load with only a deviation of around 5.02% against experimental data, Peer Reviewed, Postprint (published version)

The assessment of the remaining useful life due to fatigue in Francis turbine runners implies complex measurements with strain gauges that have to be installed in a submerged and rotating structure, which is excited with high pressure pulsations and strong turbulent flows. Furthermore, the conditioning, storage and transmission of these signals to the stationary frame involves complicated technical solutions. In order to avoid such complex and expensive measurements, in this paper we explore the feasibility of obtaining the strain on the runner with stationary sensors, which can be easily installed and used for a continuous monitoring of the machine. Based on the experimental strain tests performed in a Francis turbine unit, strain on the runner blade is correlated with relevant indicators obtained with stationary sensors. The correlation within indicators is obtained considering linear regression models and improved with artificial intelligence techniques., Peer Reviewed, Postprint (published version)

With the increasing entrance of wind and solar power for the generation of electricity, more flexibility is demanded to hydropower plants. More flexibility means that hydro turbines have to increase the operating range between minimum and maximum power. In Francis turbines the maximum power is limited by the appearance of a strong hydraulic excitation called overload instability. When the turbine operates at loads higher than design, the cavitating vortex rope that is generated in the draft tube may become unstable, producing huge pressure fluctuations, vibrations and power swing. Turbines are not allowed to operate under these conditions in order to avoid the destruction of the unit. The overload instability emerges abruptly, even when the machine is operating in a smooth condition. No visible transition can be detected by the monitoring system, so turbine operators have no margin to react. To avoid this phenomenon, operators limit the maximum power much before reaching this condition. By doing that, the maximum power is limited as well as the regulation capacity of the unit. In this paper, the feasibility of detecting the onset of this phenomenon is analyzed. Data-driven methods and artificial intelligence techniques, including principal component analysis, self-organizing map and artificial neural networks, are applied to the data available from experimental tests in a Francis turbine. The signals of vibration, pressure fluctuations and other parameters are combined and studied. The possibilities of a premature detection of the instability before it occurs are discussed. The method could be implemented in the monitoring system of the unit so that the operating range could be safely increased., Peer Reviewed, Postprint (published version)

An experimental investigation of frequential protrusion of four solid rods into the draft tube of a propeller turbine operating under partial discharge has been undertaken. The effectiveness of mitigating the pressure fluctuations associated with the rotating vortex rope (RVR) has been quantified using pressure measurements on the wall of the draft tube cone. Three azimuthal configurations of the phase difference between the rods, and four protrusion lengths were investigated. It is shown that the rotating component of the RVR decreases, irrespective of the azimuthal configuration and protrusion length, with imposed phase differences in the same direction as the runner rotation being the most effective, reducing the amplitude of the rotating component by a maximum of 62%. However, for each azimuthal configuration, the plunging mode of the RVR is amplified for large protrusion lengths, with the smallest amplification occurring for the case of 180 degrees phase difference between protrusions. Therefore, to quantify the most efficient configuration in mitigating the harmful effects of the RVR, an overall assessment of its effects on the entire turbine must be made before a conclusion can be drawn.

Hydroelectric units can be operated in synchronous condenser mode to provide reactive power to the electrical power system (EPS) for voltage regulation. In this operating mode, the hydraulic turbine consumes active power and in case of hydroelectric units equipped with a Francis turbine, pressurized air is injected in the draft tube to decrease the tailwater level below the runner for decreasing the friction losses. Several dynamic phenomena have been recorded during full-scale hydraulic turbine operation due to the air–water mixing causing pressure and torque fluctuations which may compromise the operation and safety of both the hydraulic machine and the EPS. In this paper, the study of the dynamic response of a Francis turbine prototype during synchronous condenser mode operation is performed to evaluate the transposition of the machine behaviour from the homologous reduced scale model to the full-scale prototype. Measurements of the pressure in draft tube cone, machine vibrations, and torque are performed in both full scale prototype and reduced scale model. The analysis of the dynamic behaviour of the hydraulic turbine focuses on the critical vibrations and power fluctuations. The onset of a sloshing wave in the draft tube cone is highlighted as predicted by the reduced scale model tests. Rotor–stator interactions (RSI) are analyzed and compared to the RSI during generating mode operation. The advantages of decreasing the pressure in the vaneless gap between the runner blades and the closed guide vanes by installing a draining pipe are emphasized to limit the amplitude of the fluctuations and to avoid unwanted two-phase flow phenomena in the vaneless gap which can perturb the machine operation and safety., Postprint (published version)

La finalitat d’aquest treball de fi de grau és estudiar mitjançant tècniques de mètodes numèrics els efectes produïts per l’aparició de cavitació en una turbina Francis. Per tal de reduir el cost computacional s’utilitza un model geomètric semblant a escala reduïda de les dimensions i característiques de treball de la turbina Francis de la central hidroelèctrica de Sallente. Mitjançant el software ANSYS CFX 16.2 es dur a terme la simulació del flux multi fàsic de líquid i vapor amb el model Zwart-Gerber-Belamri (ZGB) a partir dels resultats adquirits de la resolució per aproximació de les equacions de Navier-Stokes (RANS) el model de turbulència k-ω Shear-Stress Transport (SST) en les mateixes condicions. Per detectar els efectes s’han utilitzat gràfics de contorns que representen visualment diferents propietats, a més de gràfics de pressions en el temps, comparant els valors de la simulació sense utilitzar el model de cavitació respecte a la seva utilització. Obtenint resultats de comportament completament diferents en les dues simulacions.

La integración de energías renovables y la evolución en la demanda suponen nuevos desafíos en la operación del sistema eléctrico de potencia. En esta línea, surge la necesidad de una mayor flexibilidad con el objetivo de asegurar la calidad del servicio. Así, las centrales hidroeléctricas toman relevancia a la hora de proporcionar dichos servicios. Sin embargo, generalmente la capacidad de flexibilidad operacional de las centrales hidroeléctricas condiciona a las unidades a trabajar fuera de su punto de diseño. Esto conlleva una disminución de su rendimiento y la generación de fuerzas excitadoras que comprometen su vida útil. Este proyecto sigue los objetivos marcados en el proyecto de innovación energética XFLEX HYDRO (Hydropower Extending Power System Flexibility), los cuales se centran en el aumento del potencial hidroeléctrico en cuanto a eficiencia de la planta, disponibilidad y provisión de servicios de flexibilidad para el sistema eléctrico. La finalidad de este proyecto, así como su motivación, es la de estudiar algunos de los síntomas más importantes que sufren las máquinas hidráulicas, concretamente las turbinas Francis, al trabajar fuera de su punto de diseño. También se desarrollan brevemente las medidas correctivas, tanto activas como pasivas, para la cavitación de antorcha. Dicho análisis es teórico y aborda documentos y tecnologías actuales que aún se encuentran en una fase de investigación y otros proyectos desarrollados en una etapa más estable y operativa. El conocimiento de la sintomatología operacional de una máquina hidráulica al trabajar fuera de su punto de diseño y saber cómo evitarlo o reducirlo para maximizar su rendimiento es clave para poder responder a la flexibilidad que requiere el sistema. Finalmente, como parte de la metodología aplicada en el presente proyecto, se exponen tres casos de estudio de centrales hidroeléctricas reales de las cuales se tienen datos reales de operación. Dichos casos son protagonistas de la problemática oper

Al present treball es pretén conèixer el comportament de la turbina localitzada en la central Sallente, els àleps de la qual seran modificats en aquest projecte, amb l’objectiu de comprovar com influeix el dany dels àleps del distribuïdor al conjunt de la turbina.No obstant això, per a poder utilitzar la dinàmica de fluids computacional (CFD), s’ha d’obtenir una turbina a escala per reduir el tems de càlcul requerit, gràcies a la teoria de semblança de màquines podrem obtenir les característiques de la turbina a escala.Utilitzant el programari de Solidworks es realitzarà la modificació dels àleps, iel programari de Ansys per al mallat I simulació de la turbina, per a tenir constància del que ocorre dins de la turbina es crearan monitors que registraran les diferents variables.Originalment només es modificaria un àlab del distribuïdor, no obstant, el resultat obtingut va ser que el comportament de la turbina no es veia alterat a penes, I d’altra banda es van obtenir valors sense sentit físic en la sortida del rodet, per tant, es va realitzar una segona simulació modificant tots els àleps del distribuïdor iintentant corregir els valors sense sentit físic obtinguts en la sortida del rodet. Per a poder comprar els resultats obtinguts en totes dues simulacions utilitzarem les dades extretes de la simulació realitzada per Pablo Trigueros, on tots els àlabs estan en perfectes condicions.Manipulant les dades obtenim les freqüències de les variables registrades pels monitors, on podem concloure que la simulació original I la primera simulació guarden una gran semblança I per tant la turbina no veu minvat el seu rendiment. No obstant, en compararla simulació original amb la segona simulació s’observa l’aparició de múltiples freqüències a causa de la deterioració de tots els àleps del distribuïdor I com a conseqüència l’aparició de vibracions .

Reactive power shortage is one of the current issues that endanger the stability of the power grid systems. Some massive blackouts have been experienced in the past years due to the unbalance of reactive power. One of the solutions to provide the necessary reactive power to the grid is to operate generators as motors and supply or absorb reactive power when needed. Hydraulic turbines are the most suitable machines to perform this task since they can be easily and quickly dewatered to work as motors (Synchronous Condenser (SC) operation). Nowadays, hydraulic turbines are operating as SC for several hours per day, especially in grids like in US or Canada. The change from generating mode to SC and vice versa can be very fast, depending on the grid requirements. However, these fast changes may affect the mechanical behavior of the machine, reducing its useful life-time or increasing vibrations. In this paper, the effect in the mechanical system of SC operation in a Francis turbine prototype is evaluated. Strains in the runner and vibrations in the stationary parts are analyzed in detail in the fast changes between generating to SC and vice versa., Postprint (author's final draft)

A large crack was discovered on one blade of a Kaplan turbine prototype after the inspection of the machine. In order to know the origin of the crack, a failure analysis was previously carried out (Zhang et al., 2019). To know the modal behaviour of cracked structures is of interest in order to detect the crack while in operation. For this purpose, an experimental and numerical investigation of the damaged blade and of another non-damaged blade was performed. Results of natural frequencies and mode-shapes of both blades have been compared in order to identify the main differences in modal behaviour when a crack appears. The crack was so large than one part of the blade was deformed by its own weight, causing contact between the two surfaces of the crack. This fact gives an additional stiffness to the crack, which should be considered in the numerical models. For that, a stiffness change in this zone was included. The influence of this stiffness on the natural frequency values and mode-shapes has been studied in detail. In that way, a numerical model for estimating the modal behaviour of large cracks have been implemented and validated., Peer Reviewed, Postprint (published version)

Hydropower is nowadays essential for balancing the electrical grid providing flexibility and fast response. The role of hydraulic turbines has changed from working at their Best Efficiency Point (BEP) to work in the whole operating range when demanded. Francis turbines working at off-design conditions suffer from dynamic problems that affect the useful life of their components, especially of the runner. Reduced scale physical models of Francis turbines are widely used to determine their hydraulic behavior. Most of the hydraulic parameters obtained in model tests are transposable to prototype by using the similarity theories as long as they meet similitude requirements. However, from the mechanical behavior point of view, both model and prototype structures are not always similar and the mechanical behavior of the model is often not considered for transposition. In this paper, a transposition method for the mechanical behavior of Francis turbines models is presented and the results are compared with the real size prototype. The modal behavior of the runner, its stress and fatigue life estimation for different operating conditions are experimentally compared for both model and prototype. Results present the possibilities and limitations of transposing the mechanical parameters from model to prototype., Postprint (author's final draft)

Aquest projecte consisteix en l'estudi i simulació mitjançant CFD d'una turbina Francis amb danys en les vores de sortida dels àleps del rodet. El principal objectiu d'aquest projecte és veure quins efectes es presenten en el comportament de la turbomàquina quan aquesta conté danys respecte a l'original. Per a la modelització dels danys es modifica la geometria d'un àlep, extraient-li una porció de diferents mesures a la vora de sortida d'aquest, generant dos casos per comparar amb l'original: un cas amb una extracció gran i un altre amb una extracció petita. Es pretén observar aquests efectes en dues parts concretes de la turbomàquina: la zona d'unió entre el distribuïdor i el rodet, per veure si existeixen alteracions en els polsos de pressió deguts al fenomen RSI; i per altra banda, la zona del tub d'aspiració, en recerca de possibles modificacions en el comportament del fluid al llarg d'aquesta. Per dur a terme el comentat anteriorment, s'extrauran diferents representacions de contorns de pressió, línies de flux i altres paràmetres, a més de les dades obtingudes pels monitors ubicats en diverses posicions en tota la turbomàquina. Aquestes dades seran processades per poder dur a terme les comparacions, observant els senyals de pressió amb els seus respectius espectres de freqüències. Finalment, mitjançant l'obtenció i comparació de dades, es pot concloure que, pel tipus de dany simulat, la zona del tub d'aspiració és la més afectada, presenciant grans canvis en el comportament del flux, polsos de pressió i freqüències característiques. En general, la turbina amb els majors danys presenta una pèrdua de potència propera al 10%., Este proyecto consiste en el estudio y simulación mediante CFD de una turbina Francis con daños en los bordes de salida de los álabes del rodete. El principal objetivo de este proyecto es ver qué efectos se presentan en el comportamiento de la turbomáquina cuando esta contiene daños respecto a la original. Para la modelización de los daños se modifica la geometría de un álabe, extrayéndole una porción de diferentes tamaños en el borde de salida de este, generando dos casos para comparar con el original: un caso con una extracción grande y otro con extracción pequeña. Se pretende observar los efectos provocados por estos daños en dos partes concretas de la turbomáquina: la zona de unión entre el distribuidor y el rodete para ver si existen alteraciones en los pulsos de presión debidos al fenómeno RSI, y por otro lado la zona del tubo de aspiración, en busca de posibles modificaciones en el comportamiento del fluido a lo largo de este. Para llevar a cabo lo comentado anteriormente, se extraerán diferentes representaciones de contornos de presión, líneas de flujo y otros parámetros, además de los datos obtenidos por los monitores ubicados en diversas posiciones en toda la turbomáquina. Estos datos serán procesados para poder llevar a cabo las comparaciones observando las señales de presión con sus respectivos espectros de frecuencias. Finalmente, mediante la obtención y comparación de datos, se concluye que, para el tipo de daño simulado, la zona del tubo de aspiración es la que se ve más afectada, presenciando grandes cambios en el comportamiento del flujo, pulsos de presión y frecuencias características. En general, la turbina con los mayores daños presenta una pérdida de potencia cercana al 10%., This project consists of the study and simulation by CFD of a Francis turbine with damage to the trailing edges of the impeller blades. The main objective of this project is to see what effects occur in the behavior of the turbomachine when it contains damage compared to the original. It is intended to observe these effects in two specific parts of the turbomachinery: the junction area between the distributor and the impeller to see if there are alterations in the pressure pulses due to the RSI phenomenon, and on the other hand the area of the suction tube, in looking for possible changes in the behavior of the fluid along it. To carry out the above, different representations of pressure contours, flow lines and other parameters will be extracted, in addition to the data obtained by the monitors located in various positions throughout the turbomachine. This data will be processed in order to carry out the comparisons by observing the pressure signals with their respective frequency spectra. Finally, it is concluded that, for the type of simulated damage, the area of the draft tube is the most affected, witnessing large changes in flow behaviour, pressure pulses and characteristic frequencies. In general, the turbine with the greatest damage has a power loss of around 10%.

El present projecte de final de grau consisteix en l’estudi del comportament d’un fluid de treball a l’interior d’una turbina Francis, la qual presenta danys a la sortida del rodet a causa del fenomen de la cavitació. Aquest estudi s’ha dut a terme mitjançant una simulació de CFD emprant el software Ansys CFX, el qual ens permet reproduir d’una manera fidedigne el comportament del fluid al circular per l’interior de la turbina. La turbomàquina estudiada és una reproducció a escala reduïda de la turbina real que es troba a la central hidroelèctrica reversible de l’estany Gento i Sallente, ubicada a la província de Lleida. L’objectiu d’aquest estudi és comprendre el funcionament del fluid amb la finalitat de justificar i apreciar els diferents fenòmens que tenen lloc a l’interior de la turbina quan aquesta està malmesa. A la present memòria s’explica de quina manera s’ha dut a terme l’estudi, el procés que s’ha seguit i les posteriors conclusions que s’han extret., En el presente proyecto de final de grado consiste en el estudio del comportamiento de un fluido de trabajo por el interior de una turbina Francis, la cual presenta daños en la salida del rodete a causa de la cavitación. Este estudio se ha llevado a cabo mediante una simulación en CFD usando el software Ansys CFX, el cual nos permite reproducir de una manera fehaciente el comportamiento del fluido al circular por el interior de la turbina. La turbomáquina estudiada es una reproducción a escala reducida de una turbina real que se encuentra en la central hidroeléctrica reversible en el pantano Gento y Sallente, ubicada en la provincia de Lérida. El objetivo de este estudio es poder llegar a comprender mejor el funcionamiento del fluido con la finalidad de justificar y apreciar los diferentes fenómenos que se originan en el interior de la turbina cuando esta está dañada. En la presente memoria se explica de qué manera se ha llevado a cabo el análisis, el proceso que se ha seguido y, finalmente, las conclusiones a las que se ha llegado., The present final degree project is based on the study of the behavior of a fluid inside a Francis turbine, which presents some damage in the outlet of the impeller produced by cavitation. This study has been done using the CFD simulator in the Ansys CFX software, which allows to reliably reproduce and predict the behavior of the fluid circulating inside the turbine. The studied turbomachine is a reduced-scale reproduction of a real turbine that is located in the reversible hydroelectric plant in the Gento and Sallente reservoir, in the province of Lleida. The purpose of this study is to analyze the operation of the fluid in order to understand and justify the different phenomena that happen inside the turbine when damaged. The present document explains in detail this analysis, the process that has been followed and, finally, the conclusions that have been reached.

El factor de estrés más elevado en las turbinas-bombas, viene producido por la interacción entre el rotor y el estator, conocida dicha interacción como RSI (de sus siglas en ingles Rotor Stator Interaction). Dicho fenómeno, responsable de la generación de pulsos de presión, repercute en el daño por fatiga de la estructura del rotor, siendo las zonas más alejadas del centro de éstelas más propensas a sufrir los daños.En la actualidad, se conoce el hecho de que la presencia del RSI en las turbinas hidráulicas es capaz de producir la rotura de la corona del rotor mediante una geometría peculiar, la cual recuerda a una lágrima.En la presente memoria, se ha realizado un estudio del espectro de frecuencias de una turbina-bomba a la cual se le ha incorporado un rotor en el que se presentaba una rotura en la corona. Deesta forma, y mediante la comparación de los resultados obtenidos con los de un caso sin daño en el rotor, se han establecido relaciones entre las variaciones presentes en el espectro de frecuencias y la presencia de la rotura en la máquina.Finalmente, mediante la comparación de los espectros de frecuencias, se ha podido determinar que,la aparición de una rotura en la corona directriz del rotor de una turbina-bomba de 7 álabes en el rotor y 16 en el estator, produce una disminución de la amplitud de las frecuencias características de la máquina como consecuencia de una disminución de la presión en el interior de ésta

The paper reviews recent research and development activities in the field of hydropower technology. It covers emerging and advanced technologies to mitigate flow instabilities (active and passive approach) as well as emerging magneto-rheological control techniques. Recent research findings on flow instabilities are also presented, especially concerning fluid-structure interaction and transient operating conditions. As a great number of the existing large-scale hydroelectric facilities were constructed decades ago using technologies that are now considered obsolete, technologies to achieve the digitalisation of hydropower are also analysed. Advances in the electro-mechanical components and generator design are presented; their potential role to adapt hydropower to the current operating conditions is also highlighted. The text explores current efforts to advance hydropower operation, mainly in terms of European projects. It provides a detailed overview of the recent efforts to increase the operational range of hydraulic turbines in order to reach exceptional levels of flexibility, a topic of several recent research projects. Variable speed hydropower generation and its application in pumped storage power plants are presented in detail. Moreover, revolutionary concepts for hydroelectric energy storage are also presented with the analysis focusing on underwater hydro storage and hydropower's hybridisation with fast energy storage systems. Efforts to minimise hydropower's environmental footprint are also presented via the utilisation of small-scale and fish-friendly installations.

The paper reviews recent research and development activities in the field of hydropower technology. It covers emerging and advanced technologies to mitigate flow instabilities (active and passive approach) as well as emerging magneto-rheological control techniques. Recent research findings on flow instabilities are also presented, especially concerning fluid-structure interaction and transient operating conditions. As a great number of the existing large-scale hydroelectric facilities were constructed decades ago using technologies that are now considered obsolete, technologies to achieve the digitalisation of hydropower are also analysed. Advances in the electro-mechanical components and generator design are presented; their potential role to adapt hydropower to the current operating conditions is also highlighted. The text explores current efforts to advance hydropower operation, mainly in terms of European projects. It provides a detailed overview of the recent efforts to increase the operational range of hydraulic turbines in order to reach exceptional levels of flexibility, a topic of several recent research projects. Variable speed hydropower generation and its application in pumped storage power plants are presented in detail. Moreover, revolutionary concepts for hydroelectric energy storage are also presented with the analysis focusing on underwater hydro storage and hydropower's hybridisation with fast energy storage systems. Efforts to minimise hydropower's environmental footprint are also presented via the utilisation of small-scale and fish-friendly installations.

The increasing accommodation of intermittent renewable energies into power grids creates needs for additional flexibility in power generation. In the field of hydropower research, this has translated into significant attention paid to the larger control possibilities brought by power electronics converters. Through theoretical analysis and transient simulations, the present paper investigates the power control of a pump-as-turbine fed by a reservoir, and driving an induction machine, itself connected to the grid through a full-scale power converter. The modelling of each part of the system is thoroughly discussed and two control strategies are considered, namely the direct control of power through the electromagnetic torque, and the indirect control of power through the rotational speed. Theoretical analysis and transient simulations show that the first control strategy may lead to unstable operation, depending on the system and control parameters, while the second ensures the system stability.