Study of the dynamic behavior of Pelton Turbines

The future of hydropower is tied to the rapid increase of new renewable energies, such as photovoltaic and wind energy. With the growing share of intermittent electricity production, the operation of hydropower installations must be more flexible in order to guarantee the balance between supply and demand. As a result, turbines must increase their operating range and undergo more starts and stops, what leads to a faster deterioration of the turbine components, especially the runner. In the current scenario, condition monitoring constitutes an essential procedure to assess the state of the turbines while in operation and can help preventing major damage. Pelton turbines are used in locations with high heads and low discharges. The runner is composed by a disk with several attached buckets, which periodically receive the impact of high speed water jets. Buckets must thus endure large tangential stresses that can lead to fatigue problems and, in case the natural modes of the runner are excited, this problem can be severely aggravated. Therefore, a deep comprehension of the modal behavior and dynamics of Pelton turbines is required in order to keep track of the runner condition with monitoring systems. In this thesis, the dynamic behavior of Pelton turbines during different operating conditions has been studied in detail and the knowledge acquired has been used to upgrade the present condition monitoring. The first part of the document comprises the study of the modal behavior of Pelton turbines. A systematic approach has been followed with such purpose; first a single bucket has been analyzed, second the runner and then the whole turbine. With the help of numerical models and experimental tests the natural frequencies and mode shapes have been identified and classified. The effect of the mechanical design and the boundary conditions has also been discussed. The second part of the thesis is focused on determining the transmission of the runner vibrations to the monitoring locations. It is proved that these can be detected from the bearings and that the transmission depends on the mode type. In the third and last part, the analysis of Pelton turbines in operation is carried out. Two different machines have been studied during start-up and under different load conditions to determine which modes are excited, how the frequencies change in operation with respect to the still machine and how they are detected from different positions. The spectrum frequency bands corresponding to the runner modes and the overall vibration levels have been analyzed. Finally, the information obtained has been used to propose an upgrade of the current practice in condition monitoring. A case of damage has been analyzed with a numerical model and with historic data to illustrate the strategy.El futur de l'energia hidràulica està lligat al ràpid creixement de les noves energies renovables, tals com l'energia fotovoltaica i l'eòlica. A mesura que la porció d'energia intermitent que es produeix creix, el funcionament de les instal·lacions hidroelèctriques es veu forçat a ser més flexible per tal de garantir el balanç entre el subministrament i la demanda d'energia. Això es tradueix en un increment del rang de funcionament de les turbines i en més parades i arrancades, fet que contribueix a un deteriorament més ràpid dels seus components, especialment del rodet. En la situació actual, la monitorització de l'estat de les turbines és essencial per tal d'assegurar-ne les bones condicions de funcionament i evitar danys majors. Les turbines Pelton s'utilitzen en emplaçaments amb salts elevats i cabals reduïts. El rodet està compost per un disc amb diverses culleres que reben periòdicament l'impacte de raigs d'aigua a molta velocitat. Com a conseqüència, les culleres han de suportar grans tensions en direcció tangencial, les quals comporten seriosos problemes de fatiga a l'estructura. En cas que els modes naturals del rodet també s'excitin pels rajos d'aigua, aquest problema és altament agreujat. Així, és necessari tenir un coneixement profund del comportament modal i dinàmic de les turbines Pelton per tal de controlar l'estat del rodet amb sistemes de monitorització. En aquesta tesi s'ha estudiat en detall el comportament dinàmic de turbines Pelton en diferents condicions d'operació. El coneixement adquirit s'ha utilitzat per a millorar el sistema de monitorització actual. La primera part del document comprèn l'estudi del comportament modal de turbines Pelton. Amb tal propòsit s'ha abordat el problema de manera sistemàtica: primer s'han analitzat els modes d'una sola cullera, després els del rodet sencer i per últim els de tota la turbina. Amb l'ajuda de models numèrics i de proves experimentals s'han identificat i classificat les corresponents freqüències naturals i formes modals. A més a més s'ha estudiat l'efecte del disseny mecànic i de les condicions de contorn. La segona part d'aquesta tesi està centrada en determinar la transmissió de les vibracions del rodet a les posicions de monitorització. S'ha demostrat que aquestes es poden detectar des dels coixinets i que la qualitat de la transmissió depèn del tipus de mode. A la tercera i última part s'ha dut a terme l'anàlisi de turbines Pelton en funcionament. S'han estudiat dues màquines diferents durant el transitori de posta en marxa i sota diferents càrregues per tal de determinar quines modes s'exciten, com canvien les freqüències de la turbina en funcionament respecte la màquina parada i com es detecten des de les diferents posicions. Les bandes de freqüència de l'espectre de vibració corresponents als diferents modes del rodet i els nivells de vibració s'han analitzat. Finalment, la informació obtinguda ha estat utilitzada per a fer una proposta de millora de l'actual procediment de monitorització. Un cas de dany en un rodet ha estat analitzat amb un model numèric i amb l'històric de vibracions per tal d'il·lustrar l'estratègia a seguir en un futur

Study of the dynamic behavior of Pelton Turbines

The future of hydropower is tied to the rapid increase of new renewable energies, such as photovoltaic and wind energy. With the growing share of intermittent electricity production, the operation of hydropower installations must be more flexible in order to guarantee the balance between supply and demand. As a result, turbines must increase their operating range and undergo more starts and stops, what leads to a faster deterioration of the turbine components, especially the runner. In the current scenario, condition monitoring constitutes an essential procedure to assess the state of the turbines while in operation and can help preventing major damage. Pelton turbines are used in locations with high heads and low discharges. The runner is composed by a disk with several attached buckets, which periodically receive the impact of high speed water jets. Buckets must thus endure large tangential stresses that can lead to fatigue problems and, in case the natural modes of the runner are excited, this problem can be severely aggravated. Therefore, a deep comprehension of the modal behavior and dynamics of Pelton turbines is required in order to keep track of the runner condition with monitoring systems. In this thesis, the dynamic behavior of Pelton turbines during different operating conditions has been studied in detail and the knowledge acquired has been used to upgrade the present condition monitoring. The first part of the document comprises the study of the modal behavior of Pelton turbines. A systematic approach has been followed with such purpose; first a single bucket has been analyzed, second the runner and then the whole turbine. With the help of numerical models and experimental tests the natural frequencies and mode shapes have been identified and classified. The effect of the mechanical design and the boundary conditions has also been discussed. The second part of the thesis is focused on determining the transmission of the runner vibrations to the monitoring locations. It is proved that these can be detected from the bearings and that the transmission depends on the mode type. In the third and last part, the analysis of Pelton turbines in operation is carried out. Two different machines have been studied during start-up and under different load conditions to determine which modes are excited, how the frequencies change in operation with respect to the still machine and how they are detected from different positions. The spectrum frequency bands corresponding to the runner modes and the overall vibration levels have been analyzed. Finally, the information obtained has been used to propose an upgrade of the current practice in condition monitoring. A case of damage has been analyzed with a numerical model and with historic data to illustrate the strategy.El futur de l'energia hidràulica està lligat al ràpid creixement de les noves energies renovables, tals com l'energia fotovoltaica i l'eòlica. A mesura que la porció d'energia intermitent que es produeix creix, el funcionament de les instal·lacions hidroelèctriques es veu forçat a ser més flexible per tal de garantir el balanç entre el subministrament i la demanda d'energia. Això es tradueix en un increment del rang de funcionament de les turbines i en més parades i arrancades, fet que contribueix a un deteriorament més ràpid dels seus components, especialment del rodet. En la situació actual, la monitorització de l'estat de les turbines és essencial per tal d'assegurar-ne les bones condicions de funcionament i evitar danys majors. Les turbines Pelton s'utilitzen en emplaçaments amb salts elevats i cabals reduïts. El rodet està compost per un disc amb diverses culleres que reben periòdicament l'impacte de raigs d'aigua a molta velocitat. Com a conseqüència, les culleres han de suportar grans tensions en direcció tangencial, les quals comporten seriosos problemes de fatiga a l'estructura. En cas que els modes naturals del rodet també s'excitin pels rajos d'aigua, aquest problema és altament agreujat. Així, és necessari tenir un coneixement profund del comportament modal i dinàmic de les turbines Pelton per tal de controlar l'estat del rodet amb sistemes de monitorització. En aquesta tesi s'ha estudiat en detall el comportament dinàmic de turbines Pelton en diferents condicions d'operació. El coneixement adquirit s'ha utilitzat per a millorar el sistema de monitorització actual. La primera part del document comprèn l'estudi del comportament modal de turbines Pelton. Amb tal propòsit s'ha abordat el problema de manera sistemàtica: primer s'han analitzat els modes d'una sola cullera, després els del rodet sencer i per últim els de tota la turbina. Amb l'ajuda de models numèrics i de proves experimentals s'han identificat i classificat les corresponents freqüències naturals i formes modals. A més a més s'ha estudiat l'efecte del disseny mecànic i de les condicions de contorn. La segona part d'aquesta tesi està centrada en determinar la transmissió de les vibracions del rodet a les posicions de monitorització. S'ha demostrat que aquestes es poden detectar des dels coixinets i que la qualitat de la transmissió depèn del tipus de mode. A la tercera i última part s'ha dut a terme l'anàlisi de turbines Pelton en funcionament. S'han estudiat dues màquines diferents durant el transitori de posta en marxa i sota diferents càrregues per tal de determinar quines modes s'exciten, com canvien les freqüències de la turbina en funcionament respecte la màquina parada i com es detecten des de les diferents posicions. Les bandes de freqüència de l'espectre de vibració corresponents als diferents modes del rodet i els nivells de vibració s'han analitzat. Finalment, la informació obtinguda ha estat utilitzada per a fer una proposta de millora de l'actual procediment de monitorització. Un cas de dany en un rodet ha estat analitzat amb un model numèric i amb l'històric de vibracions per tal d'il·lustrar l'estratègia a seguir en un futur.Postprint (published version

Detection of hydraulic phenomena in francis turbines with different sensors

Nowadays, hydropower is demanded to provide flexibility and fast response into the electrical grid in order to compensate the non-constant electricity generation of other renewable sources. Hydraulic turbines are therefore demanded to work under o -design conditions more frequently, where di erent complex hydraulic phenomena appear, a ecting 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 this paper, a large medium-head Francis turbine was selected for this purpose. This prototype is instrumented with several sensors such as accelerometers, proximity probes, strain gauges, pressure sensors and a microphone. Results presented in this paper permit knowing which hydraulic phenomenon is detected with every sensor and which signal analysis technique is necessary to use. With this information, monitoring systems can be optimized with the most convenient sensors, locations and signal analysis techniquesPostprint (published version

Sensor-based optimized control of the full load instability in large hydraulic turbines

Hydropower plants are of paramount importance for the integration of intermittent renewable energy sources in the power grid. In order to match the energy generated and consumed, Large hydraulic turbines have to work under off-design conditions, which may lead to dangerous unstable operating points involving the hydraulic, mechanical and electrical system. Under these conditions, the stability of the grid and the safety of the power plant itself can be compromised. For many Francis Turbines one of these critical points, that usually limits the maximum output power, is the full load instability. Therefore, these machines usually work far away from this unstable point, reducing the effective operating range of the unit. In order to extend the operating range of the machine, working closer to this point with a reasonable safety margin, it is of paramount importance to monitor and to control relevant parameters of the unit, which have to be obtained with an accurate sensor acquisition strategy. Within the framework of a large EU project, field tests in a large Francis Turbine located in Canada (rated power of 444 MW) have been performed. Many different sensors were used to monitor several working parameters of the unit for all its operating range. Particularly for these tests, more than 80 signals, including ten type of different sensors and several operating signals that define the operating point of the unit, were simultaneously acquired. The present study, focuses on the optimization of the acquisition strategy, which includes type, number, location, acquisition frequency of the sensors and corresponding signal analysis to detect the full load instability and to prevent the unit from reaching this point. A systematic approach to determine this strategy has been followed. It has been found that some indicators obtained with different types of sensors are linearly correlated with the oscillating power. The optimized strategy has been determined based on the correlation characteristics (linearity, sensitivity and reactivity), the simplicity of the installation and the acquisition frequency necessary. Finally, an economic and easy implementable protection system based on the resulting optimized acquisition strategy is proposed. This system, which can be used in a generic Francis turbine with a similar full load instability, permits one to extend the operating range of the unit by working close to the instability with a reasonable safety margin.Postprint (published version

Experimental measurements of the natural frequencies and mode shapes of rotating disk-blades-disk assemblies from the stationary frame

Determining the natural frequencies and mode shapes of rotating turbomachinery components from both rotating and stationary reference frames is of paramount importance to avoid resonance problems that could affect the normal operation of the machine, or even cause critical damages in these components. Due to their similarity to real engineering cases, this topic has been experimentally analyzed in the past for disk-shaft assemblies and rotor disk-blades assemblies (bladed-disk or blisk). The same topic is less analyzed for disk-blades-disk assemblies, although such configurations are widely used in centrifugal closed impellers of compressors, hydraulic pumps, pump-turbines, and runners of high head Francis turbines. In this paper, experimental measurements, varying the rotating speed of a disk-blade-disk assembly and exciting the first natural frequencies of the rotating frame, have been performed. The rotating structure is excited and measured by means of PZT patches from the rotating frame and with a Laser Doppler Vibrometer (LDV). In order to interpret the experimental results obtained from the stationary frame, a method to decompose the diametrical mode shapes of the structure in simple diametrical components (which define the diametrical mode shapes of a simple disk) has been proposed. It is concluded that the resonant frequencies detected with a stationary sensor correspond to the ones predicted with the decomposition method. Finally, a means to obtain equivalent results with numerical simulation methods is shown.Postprint (published version

Improved damage detection in Pelton turbines using optimized condition indicators and data-driven techniques

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 ReviewedPostprint (author's final draft

Condition monitoring of a prototype turbine. Description of the system and main results

The fast change in new renewable energy is affecting directly the required operating range of hydropower plants. According to the present demand of electricity, it is necessary to generate different levels of power. Because of its ease to regulate and its huge storage capacity of energy, hydropower is the unique energy source that can adapt to the demand. Today, the required operating range of turbine units is expected to extend from part load to overload. These extreme operations points can cause several pressure pulsations, cavitation and vibrations in different parts of the machine. To determine the effects on the machine, vibration measurements are necessary in actual machines. Vibrations can be used for machinery protection and to identify problems in the machine (diagnosis). In this paper, some results obtained in a hydropower plant are presented. The variation of global levels and vibratory signatures has been analysed as function as gross head, transducer location and operating points.Postprint (published version

Exploring the Regulation Reliability of a Pumped Storage Power Plant in a Wind–Solar Hybrid Power Generation System

In the coming decades, the proportion of wind–solar energy in power system significantly increases, resulting to uncertainties of power fluctuation in abundant wind–solar energy regions. The flexibility operation of Pumped Storage Power Plants (PSPPs) has already been widely recognized to regulate wind–solar power fluctuations; however, less is known about the regulation reliability of the PSPP affected by them. It is a challenge, since various uncertainties exist during this regulation process. Here, a mathematical model with a solar–wind–hydro hybrid power generation system is adopted to investigate the regulation reliability of PSPP. The uncertainties and limitations of model parameters are considered during this process. Five regulation indexes, i.e., rise time, settling time, peak value, peak time and overshoot of the reactive power generator terminal voltage, guide vane opening and angular velocity, are extracted to evaluate the PSSP’s regulation quality. Finally, the PSPP reliability probability affected by parametric uncertainties is presented. The obtained results show that the inertia coefficient is the most sensitivity parameters for the settling time, peak value and peak time with sensitivity index 33.7%, 72.55% and 71.59%, respectively. The corresponding total contribution rate of the top 10 sensitive parameters are 74.45%, 93.45% and 87.15%, respectively. Despite some types of uncertainties not being considered, the results of this research are important for the regulation reliability evaluation of PSPPs in suppressing power fluctuations of wind and solar generation.Peer ReviewedPostprint (published version

Failure investigation of a Kaplan turbine blade

In this paper, an uncommon failure of a Kaplan turbine blade has been analyzed. After the monitoring system detected a sudden increase in vibration levels, the turbine was inspected. The inspection showed that a large crack occurred on one blade starting from the leading-edge side root hole. Moreover, the tip of the blade was also found broken. A comprehensive analysis of the crack surfaces, broken parts and the vibration data revealed a fatigue problem probably caused by rubbing between the blade and the nearby wall. The effect of contacts on the stress distribution of the blade was investigated through a numerical model of the turbine blade. To validate this numerical model an experimental modal analysis was done on a nondamaged blade and the results showed a good agreement with the numerical results. Stress distribution analysis showed that when applying a contact force, the stress at the leading-edge root hole increases drastically. Therefore, experimental evidences as well as numerical simulations confirm that the crack occurred due to rubbing between the blade tip and the nearby wall.Peer ReviewedPostprint (published version

Experimental investigation on the dynamic response of Pelton runners

An effective condition monitoring is essential to increase the availability of Pelton turbines and to avoid unexpected damage. The main challenge consists in determining the deformations and the stresses in the runner while in operation. To do so, it is essential to understand the dynamic response of the machine, and especially of the runner. Moreover, the monitoring locations have to be selected as to detect this response optimally. Having a deep knowledge of the natural modes of the runner, and how these can be excited while it is operating is mandatory to detect damage. The modal behavior of a Pelton runner can either be studied by means of numerical simulation or by experimental modal analysis (impact testing). However, none of these methods alone is able to describe with accuracy the dynamic behavior of the machine under real operating conditions. In this study, an experimental investigation of an existing Pelton turbine unit has been carried out. The modal response of the runner suspended, attached to the shaft and with the machine in operation has been studied. To do so, accelerometers, acoustic emission sensors and other types of sensors have been used. Signals were acquired during transients and with the machine operating at different loads. For a better understanding, a numerical model based on Finite Element Method (FEM) was created to represent the behavior of this type of turbines. At the end, the natural frequencies detected with the machine in operation were compared to those obtained numerically and by impact testing. Therefore, the effect of different factors on the runner response was analyzed. In addition, the best sensor and optimal location in describing the state of the machine were chosen. From the results obtained a possible improvement of the condition monitoring for these turbines is presented.Peer ReviewedPostprint (published version