Strong Behavioral Similarities in Timed-Arc Petri Nets

The concept of process similarity has attracted the attention of many researchers in the recent literature, since it measures the degree of proximity of processes. Similarities are therefore useful for the efficient management of large repositories, since they allow us to find the appropriate process models among hundreds or thousands of possible candidates. However, time constraints are usually omitted when these similarity measures are defined, so in this paper two similarity measures are defined over a timed extension of Petri nets, the so-called timed-arc Petri nets, in which tokens are assigned an age indicating the time elapsed from creation, and PT-arcs (place to transition arcs) are labeled with time intervals that are used to restrict the age of the tokens that can be used to fire the adjacent transition

Overview of the experimental tests in prototype

Experimental tests in prototype are necessary to understand the dynamic behaviour of the machine during different operating points. Hydraulic phenomena as well as its effect on the structure need to be studied in o rder to avoid instabilities during operation and to extend the life - time of the different components. For this purpose, a complete experimental study of a large Francis turbine prototype has been performed installing several sensors along the machine. Pres sure sensors were installed in the penstock, spiral case, runner and draft tube, strain gauges were installed in the runner, vibration sensors were used in the stationary parts and different electrical and operational parameters were also measured. All the se signals were acquired simultaneously for different operating points of the turbine.Postprint (published version

Decibilidad de problemas sobre redes de Petri temporizadas

La tesis investiga cuestiones de decibilidad sobre distintos variantes de redes de Petri temporizados. Se detiene diversas extensiones de las propiedades clásicas de redes de Petri, a los modelos temporizados, se consideran, asimismo distintas hipótesis sobre el tipo de modelo temporal: discreto, racional, real etc.. El trabajo se encuadra en una línea de investigación de acrualidad y los resutados obtenidos son interesantes, numerosos y originale

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

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

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

Influence of the boundary conditions on the natural frequencies of a Francis turbine

Natural frequencies estimation of Francis turbines is of paramount importance in the stage of design in order to avoid vibration and resonance problems especially during transient events. Francis turbine runners are submerged in water and confined with small axial and radial gaps which considerably decrease their natural frequencies in comparison to the same structure in the air. Acoustic-structural FSI simulations have been used to evaluate the influence of these gaps. This model considers an entire prototype of a Francis turbine, including generator, shaft, runner and surrounding water. The radial gap between the runner and the static parts has been changed from the real configuration (about 0.04% the runner diameter) to 1% of the runner diameter to evaluate its influence on the machine natural frequencies. Mode-shapes and natural frequencies of the whole machine are discussed for all the boundary conditions testedPostprint (published version

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

A sound and complete proof system for probabilistic processes

n this paper we present a process algebra model of probabilistic communicating processes based on classical CSP. To define our model we have replaced internal non-determinism by generative probabilistic choices, and external non-determinism by reactive probabilistic choices, with the purpose of maintaining the meaning of the classical CSP operators, once generalized in a probabilistic way. Thus we try to keep valid, as far as possible, the laws of CSP. This combination of both internal and external choice makes strongly difficult the definition of a probabilistic version of CSP. In fact, we can find in the current literature quite a number of papers on probabilistic processes, but only in a few of them internal and external choices are combined, trying to preserve their original meaning. Starting with a denotational semantics where the corresponding domain is a set of probabilistic trees with two kinds of nodes, representing the internal and external choices, we define a sound and complete proof system, with very similar laws to those of the corresponding CSP