| 31. |
- Atta, Khalid
(författare)
-
Extremum Seeking Control : Stability, Accuracy, and Applications
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In many applications, there is a variable that indicates the overall performance and that must be maximized, such as the output of a hydro power turbine or a mineral processing plant, or that must be minimized, such as CO2 emissions or the consumption of resources. Extremization of this variable (maximization or minimization) through adjusting the influencing manipulated variables is occasionally required without prior knowledge of the optimal values of the manipulated variables or of the optimized variable. Extremum seeking control (ESC), which is an on-line concept for the optimization of dynamic systems, can achieve this task.Many types of ESC have been proposed in the literature, and the majority of these approaches are based on the gradient descent optimization method. The most common type of ESC is the classic ESC, which is based on adding a sinusoidal perturbation to the manipulated variables followed by the use of a band pass filter (BPF) to find an estimate of the gradient of the output with respect to the input. The plant is then extremized by adjusting the manipulated variables to make this estimate approach zero.In this work, an alternative approach called phasor ESC is proposed, which is based on estimating the phasor of the plant output at the perturbation frequency rather than the gradient. Stability analysis of the phasor ESC is presented, including local and semi-global practical asymptotic stability for general non-linear dynamic plants. As an improvement of the existing stability analysis of the classic ESC, a less constrained semi-global practical asymptotic stability condition is also presented. In perturbation-based ESC, the output will continuously oscillate due to the input perturbation. As noted in the literature, the averaged system may not converge to the optimum point, and an average offset from the optimal value will emerge. This offset is negligible in the case of a small perturbation signal but will increase as the perturbation amplitude increases. In this work, a modification for the classic ESC and phasor ESC is proposed that improves the accuracy by reducing this offset. Two applications of ESC are considered: cone crushers and hydro power plants. Cone crushers are used for reducing the size of minerals and are considered to be a key component of many mineral processing plants. On-line optimization of the throughput is proposed based on the concept of ESC. A novel model of cone crushers is presented that can predict the flow and the size distribution of the output as a function of the primary manipulated variables, and simulations of ESC control on this model are presented.For hydro power plants, which are considered to be the most important source of renewable energy, phasor ESC is proposed for constructing and correcting the combinator, which ensures the optimal and efficient operation of the Kaplan turbine. Moreover, phasor ESC is proposed as a tool for maximum power point tracking in micro hydro plants by manipulating the turbine speed, and a test rig is used to conduct an experiment to validate this approach.
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| 32. |
- Axelsson, Ann
(författare)
-
Planing wood with twist
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- About half the total volume of sawlogs end up as sawn timber. The rest is lostdue to drying shrinkage or is turned into byproducts like wood chips, sawdustand shavings. As the raw material is a large expense for a sawmill, it is important to fully utilize the logs. The inherent properties of timber are such that warp, such as bow, cup, spring and twist, is inevitable, and extensive knowledge of whether and to what extent warp will appear is therefore important for managing the production. It is also important to develop strategies to handle warped timber, for example in the planing process.This thesis focuses on how twisted timber is affected by the planing process with regard to twist reduction, cross-sectional shape, planer misses and cutting depth. This was studied in three practical tests on sawn timber with differentapproaches. In one test, sawn pine timber with a large variation of twist withinthe group was planed with standard settings, and five evenly spaced crosssections along the length of the sawn timber were subjected to more detailed studies. In the second test, the main yield from spruce logs was planed. One sample board from each log was planed with the normal pressure settings of the planing mill, while the second sample was planed with a pressure either higher or lower than the normal settings. In this study, seven cross-sections were studied in more detail, three close to the top end, three close to the butt end, and one in the middle of the sawn timber. In the third test, sawn pine timber with a more moderate twist was planed with standard settings in another similar planer.
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| 33. |
- Axelsson, Sarianne Wiklund
(författare)
-
Prerequisites for sustainable life style changes among older persons with obesity and for ICT support
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The experiences from persons in the third age of lifestyle changes due to obesity are rarely described in research. Interventions regarding lifestyle changes and obesity show weak evidence for long-term effects. Information and communication technology (ICT) may add an important potential in interventions, but readiness among older persons and the actual needs for support must be explored. Therefore the overall aim for the thesis was to explore the prerequisites for sustainable lifestyle changes among older persons with obesity; and how this could be supported by ICT.Both qualitative and quantitative approaches were used. In paper I, 10 participants were individually interviewed about their experiences of lifestyle changes and in a multistage focus group study (paper IV), 6 participants expressed the support needed for sustainable lifestyle changes in relation to obesity. Qualitative content analysis was used. In a randomly selected population survey, 154 participants responded to an enquiry about general and health related ICT usage (paper II). They also rated scenarios for expected psychosocial impact on web based e-health services and mobile health applications (paper III).An integration of the results identified four main findings: The first finding, A never-ending process of vigilance and vulnerability, showed constant attempts of weight loss and pre-occupation on not to fail in an un-supportive environment. The second finding; Need for psychosocial support marked that the process of lifestyle changes was influenced by factors as sensibility to moods, social support and others’ attitudes. It was therefore essential for the persons to achieve emotional balance. The third finding, ICT as a possible support for adaptability, presented that older Swedes expected a positive psychosocial impact by ICT, especially in a future perspective. ICT was most valued in the domain of adaptability. The degree of usage of health related ICT, however, was low, although persons with overweight had more experience of this usage. Finally, the fourth finding; Reconstructions from weight to health expresses the needs of reconstructing focus towards health, rather than being pre-occupied with weight loss. Physical activity that was associated to joy and to have a functioning body, a body that would serve them as before, were motives to lose weight.In conclusion, this thesis illustrates that a lifestyle change due to obesity in third age is lined with vigilance not to fail and requires a great deal of endurance and effort. The potential of ICT to support lifestyle changes is likely to be substantiated by the positive expectations on ICT from a psychosocial impact perspective, especially in views of the future. Findings indicate a need for moving focus from weight towards health and to see opportunities, and that psychosocial support and enjoyable physical activity are important prerequisites for sustainable change.
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| 34. |
- Babu, Bijish, Tec. Lic. 1979-
(författare)
-
Mechanism-based flow stress model for Ti-6Al-4V : applicable for simulation of additive manufacturing and machining
- 2018
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ti-6Al-4V has remarkable properties such as high specific mechanical properties (viz. stiffness, strength, toughness, fatigue resistance), corrosion resistance, biocompatibility etc. These properties make it attractive for applications in aerospace, chemical industry, energy production, surgical implants, etc. Many of these applications have to satisfy high requirements on mechanical properties, which are directly affected by the microstructure. Therefore, it is essential to understand as well as to model the microstructure evolution during manufacturing as well as in-service. Furthermore, this alloy has a narrow temperature and strain rate window of workability.This work was initiated as part of a project aimed at performing finite element simulations of a manufacturing process chain involving hot forming, welding, machining, additive manufacturing and heat treatment of Ti-6Al-4V components within the aerospace industry. Manufacturing process chain simulations can compute the cumulative effect of the various processes by following the material state through the whole chain and give a realistic prediction of the final component. Capacity to describe material behavior in a wide range of temperatures and strain rates is crucial for this task.A material model based on the dominant deformation mechanisms of the alloy is assumed to have a more extensive range of validity compared to an empirical relationship. Explicit dislocation dynamics based models are not practically feasible for manufacturing process simulation, and therefore the concept of dislocation density, (length of dislocations per unit volume) developed by (Kocks1966; Bergström, 1970) is followed here. This mean field approach provides a representation of the average behavior of a large number of dislocations, grains, etc. Conrad (1981) studied the influence of various factors like solutes, interstitials, strain, strain rate, temperature, etc., on the strength and ductility of titanium systems and proposed a binary additive relationship for its yield strength. The first component relates to long-range interactions and second short-range relates to lattice resistance for dislocation motion. For high strain rate deformation, this short-range term is extended to include the effects of a viscous drag given by phonon and electron drag (Lesuer et al. 2001). Immobilisation of dislocation by pile-ups gives hardening and remobilization/annihilation by dislocation glide and climb gives restoration. Globularization is also considered to restore the material. The material model is calibrated using isothermal compression tests at a wide range of temperatures and strain rates. Compression tests performed using Gleeble thermo-mechanical simulator is used at low-strain rates and split-Hopkins pressure bar is used at high strain rates for calibration.During additive manufacturing depending on the temperature, heating/cooling rates, Ti-6Al-4V undergoes allotropic phase transformation. This transformation results in a variety of textures that can give different mechanical properties. Based on the texture (Semiatin et al., 1999b; Seetharaman and Semiatin, 2002; Thomas et al., 2005) identified few microstructural features that are relevant to the mechanical properties. The three separate alpha phase fractions; Widmanstatten, grain boundary, Martensite, and the beta-phase fraction are included in the current model. However, since the strengthening contributions of these individual alpha phases are not known, a linear rule of mixtures for the total alpha-beta composition is developed. This model is calibrated using continuous cooling tests performed by Malinov et al. 2001 with differential scanning calorimeter at varying cooling rates. This mechanism-based model is formulated in such a way that it can be implemented in any standard finite element software. In the current work, this is implemented as subroutines within MSC Marc and used for simulation of hot-forming and additive manufacturing.
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| 35. |
- Bach-Oller, Albert, 1989-
(författare)
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Alkali-enhanced gasification of biomass : laboratory-scale experimental studies
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gasification seeks to break carbonaceous materials into synthetic gas (CO+H2) which can be subsequently upgraded into valuable products. Thus gasification can be utilized to convert low grade biomass stocks into carbon-neutral chemicals heat and power. Nonetheless, gasification produces tar and soot as a by-product, impurities which deposit on cold surfaces thereby risking operation downstream of the gasifier. Cleaning the syngas after the gasifier is a conventional way to attenuate the problem, yet a complex and expensive one. Thus, tar and soot should preferably be addressed already in the gasifier. Given that these impurities are non-equilibrium species they could be targeted by using some sort of catalytic material. Alkali elements have precisely shown to possess catalytic activity on char gasification, besides they have also been associated with a decrease in tar and soot. Yet, to design a functional alkali-catalysed gasification process we need to investigate in more detail on what exact products does alkali show an activity on, on what stage, under what circumstances and, on the measure that it is possible, the mechanism. This was investigated on the basis of experimental work that approached the topic from two opposite sides. On the one hand, we studied the effects of diluting the alkali content of a Na-rich black liquor (BL) by blending it with pyrolysis oil (PO), and on the other hand, we investigated adding various amounts of alkali on more conventional types of biomass fuels. Most of the experiments were conducted on a laminar drop tube furnace but the reactivity of BL chars was also studied through thermogravimetric analysis.Alkali was found to catalyse heterogeneous gasification reactions (e.g. char) and to lead to much lower yields of C2 hydrocarbons, heavy tars and soot, favouring the presence of lighter species over large aromatic clusters. Alkali was hypothesized to reduce the quantity of soot by inhibiting the formation and growth of PAH, key intermediates on the road to soot. Besides, it was found that the initial contact between the alkali and the organic matrix was not critical, neither for gas impurities nor regarding char conversion, suggesting that the activity of alkali was a gas-induced phenomenon. The latter implied the existence of a vaporization-condensation cycle that could supply alkali into the char. Nonetheless, the beneficial effects by alkali were impaired by the affinity of Si to capture K and form potassium silicates which are inert. This interaction effect was particularly noticeable on char conversion as the silicates are not only inert but also liquid and viscous and prompt to encapsulate the char particles, thereby limiting mass transfer.The experiments with blends of BL and PO showed that the concentration of alkali in BL could be decreased by 30% without any sign of a decrease in the catalytic activity on char gasification, thus indicating the existence a saturation threshold. Furthermore, adding PO into BL lead to a further reduction on the quantities of tar and soot, this finding was attributed to changes in the fuel composition unrelated to alkali. In any case the experiments with BL-based fuels showed lower amounts of tar and soot than those from alkali-impregnated biomass powder. The difference was partially attributed to the content of S in BL. The subsequent investigation targeting the role of S confirmed that S possessed a soot inhibiting role similar to that of alkali, yet unlike K, it did not show a catalytic effect on char gasification.
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| 36. |
- Backe, Björn, 1978-
(författare)
-
An Integrated Development Approach for Monitoring and Simulation to Predict Functional Product Availability
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- For nearly two decades, business models such as Functional Products have been in focus within research and of interest in the manufacturing industry. Functional product offers consist of hardware, software, service -support systems and management of operation which, when developed in an integrated manner, together provide the customer with an agreed-upon function with a specified level of availability. Compared to product-oriented sales, this type of business model can provide added value to customers, usually through an increase in the service content. Due to the total care commitment, offering Functional Products requires management of reliability and maintainability in order to meet the availability requirement of the function provided. The development of the Functional Product must include holistic analysis and prediction of the functional product availability performance to reduce technical and economic risks and ensure that the function is delivered according to contract. The research performed in this thesis presents an integrated development approach for monitoring and simulation to predict functional product availability. It is shown how the constituents of a functional product can be modelled in an integrated manner in order to simulate and predict functional product availability. A part of this modelling strategy is demonstrated through a simulation case example to show that is possible through this approach to evaluate the availability of different functional product designs. To support the development of the monitoring capability needed for availability simulations it is shown how it is possible to develop fault detection and diagnosis methods for fault detection systems based on data stream management systems. It is also shown how data stream forecasting can be used to predict failures due to faults occurring at short notice. Different fault detection methods have been developed, tested and evaluated on real industrial applications to verify applicability as queries on data streams, managed by data stream management systems. The results from these tests have been evaluated for their predictive performance and detection accuracy. Finally, methodological and technological approaches to monitoring and analysis in functional product development and similar business models to functional products are reviewed. The results showed that few research contributions address the information perspective in functional product development and similar business models holistically. The integrated development approach presented is a pragmatic approach to functional product development which is based on the merged research results of the papers included and knowledge domain presented.
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| 37. |
- Backman, Ylva
(författare)
-
Students’ Reasoning about Learning and Well-being in School : On the epistemic privilege of Swedish early adolescent students
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The recent emphasis on performance in standardized testing of Swedish students is visible in reports from both national and international agencies. According to the reports, there has been a steep decline in the internationally measured performance of Swedish students in school since the 1990s. In fact, the performance of Swedish students in the Programme for International Student Assessment (PISA) has declined more dramatically than any other OECD country in the last decade. This has influenced a national debate about factors promoting knowledge acquisition. Alongside the drop in performance, young Swedish people’s mental problems have increased since the 1990s. The impact of one’s well-being on performance has been widely acknowledged in previous research, and two explicit targets in both current andprevious Swedish national curriculum for the compulsory school are learning and well-being. Meanwhile, the importance of encouraging early adolescents’ (10–14 years) participation in improving their own standards is acknowledged in several recent reports from influential non-governmental organizations such as the WHO and UNICEF. However, advancing students’ development requires a keen understanding of their current situation, something impeded by the fact that data on early adolescents is relatively scarce. Furthermore, a large amount of previous research has been focused on risk factors related to illness in children, rather than factors promoting well-being. The aim of this thesis is to explore early adolescent students’ reasoning about learning and well-being in school. A qualitative design with open-ended writing tasksand interview questions was constructed in order to facilitate the incorporation of students’ reasoning about learning and well-being into the research, in accordance with the study’s salutogenic point of departure and the specific standpoint epistemology that I propose and use in the thesis.The empirical data includes (1) written reflections by 200 students in grades 5–9 from 11 classes in four different schools (rural and urban), which were part of the Swedish compulsory school system and which were located in two municipalities in the northern part of Sweden, and (2) interviews with 24 students, from 12 to 15 years old, from two municipally run schools, which were also part of the compulsory school program in northern Sweden. Four sub-studies were conducted and presented in four journal articles. The data in sub-studies I, II, and IV consisted of the written reflections described above, while the data in sub-study III consisted of the interviews described above. The first sub-study concerned early adolescent students’ previous positive experiences and indicated that the students found aspects both within and beyond the classroom relevant for having a good time in school. In more detail, the students’ positive experiences concerned (i) interaction with teachers, (ii) freedom of choice regarding work and workmates, (iii) the atmosphere for discussions, (iv) school subjects and success, (v) learning processes in outings, (vi) friends, and (vii) primary (basic) needs. The results show a perceived complex relation between learning and well-being in school, which may well be studied further in future research. The second sub-study investigated students’ preferred states of affairs. The students emphasized a variety of views of what kind of structures, content, actions and attitudesmay have a positive impact on the learning environment in school. The variousreflections provided by the students were understood as falling under four themes: (i) influencing educational settings; (ii) striving for reciprocity; (iii) managing time struggles; and (iv) satisfying well-being needs. Besides providing perceived opportunities for change in school concurring with previous research, the results also point towards great variety and certain inconsistencies in the students’ perceptions about factors promoting learning.In the third sub-study, the interview data was analyzed using a distinction between decision methods and criteria of rightness, which has rarely been used in empirical research. Six forms of variety in the students’ moral reasoning were found, denoted as follows: (i) interpersonal variety in decision method dimension, (ii) intrapersonal variety in decision method dimension, (iii) interpersonal variety in criterion of rightness dimension, (iv) intrapersonal variety in criterion of rightness dimension, (v) interpersonal variety between the two dimensions, and (vi) intrapersonal variety between the two dimensions. The use of the distinction between decision methods and criteria of rightness enabled a nuanced understanding of varieties in students’moral reasoning, and may be useful in large-scale studies, for example, to explore the rate of occurrence of each respective form of variety. Future research could also examine the potential context dependence of the different forms of varieties. In the fourth sub-study, five perceived bidirectional crossovers of subjective wellbeing (i.e., perceived two-way transmissions between happiness and other aspects within the school domain) were noted in the students’ written reflections. These were: (i) happiness and learning, (ii) happiness and school engagement, (iii) happiness and appreciation of subjects or lesson content, (iv) happiness and others’ happiness, and (v) happiness and prosocial behavior. Besides providing novel hypotheses aboutbidirectional crossovers of subjective well-being, this sub-study supplied a systematic framework for interpreting qualitative outputs and provided conceptual as well as analytical direction for future research about happiness in education. I provide a definition of the formal expression “bidirectional crossovers” of subjective well-being, which may provide analytic categories for future research, while the more informal expression “circles of happiness” could be used in education, allowing teachers and students to conceptualize everyday events in the classroom. In the final parts of this thesis, students’ reasoning about the conditions for and effects of learning and well-being in school in all the four sub-studies are compared with previous empirical research through the use of established and novel definitions and theory. The comparison shows considerable correspondence between the students’ reasoning and previous research, providing evidence to the effect thatSwedish early adolescent students’ reasoning is trustworthy regarding the conditions for and effects of learning and well-being in school. There are also novel researchgenerating hypotheses formulated directly based on the students’ reasoning. The trustworthiness and novelty of the students’ reasoning about the conditions and effects of learning and well-being in school provides evidence for an epistemic privilege for the students participating in this study. This constitutes a starting point for futureresearch in which the claims about epistemic privilege in the standpoint epistemology proposed here can be further tested for Swedish early adolescent students in general.KEYWORDS: education, epistemic privilege, happiness, learning, moral reasoning, standpoint epistemology, well-being
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| 38. |
- Bagge, Niklas, 1987-
(författare)
-
Structural assessment procedures for existing concrete bridges : Experiences from failure tests of the Kiruna Bridge
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Assessing existing bridges is an important task in the sustainable management ofinfrastructure. In practice, structural bridge assessments are usually conducted usingtraditional and standardised methods, despite knowledge that these methods oftenprovide conservative estimates. In addition, more advanced methods are available, suchas nonlinear finite element (FE) analysis, that are used for research purposes and cansimulate the structural behaviour of bridges more accurately. Therefore, it would beuseful to develop practical and reliable procedures for refined assessments using theseadvanced techniques.Focusing on the ultimate load-carrying capacity of existing concrete bridges, this thesispresents a procedure for structural assessments. The fundamental idea is to improve theassessment successively, as necessary to predict bridges’ structural behaviour adequately.The procedure involves a multi-level assessment strategy with four levels of structuralanalysis, and an integrated framework for safety verification. At the initial level (Level 1)of the multi-level strategy, traditional standardised methods are used, no failures arecovered implicitly in the structural analysis and action effects are verified using localresistances calculated using analytical models. In the subsequent enhanced levels (Levels2 – 4), nonlinear FE analysis is used for stepwise integration of the verification of flexural,shear-related and anchorage failures into the structural analysis. The framework for safetyverifications includes partial safety factor (PSF), global resistance safety factor (GRSF) andfull probabilistic methods. Within each of these groups, verifications of desired safetymargins can be conducted with varying degrees of complexity.To demonstrate and evaluate the proposed structural assessment procedure, comparativestudies have been carried out, based on full-scale tests of a prestressed concrete bridge.This was the Kiruna Bridge, located in the northernmost city in Sweden, which was duefor demolition as part of a city transformation project, necessitated by large grounddeformations caused by the large nearby mine. Thus, it was available for destructiveexperimental investigation within the doctoral project presented in this thesis. The bridgehad five continuous spans, was 121.5 m long and consisted of three parallel girders with a connecting slab at the top. Both the girders and slab were tested to failure to investigatetheir structural behaviour and load-carrying capacity. Non-destructive and destructivetests were also applied to determine the residual prestress forces in the bridge girders andinvestigate the in situ applicability of methods developed for this purpose. The so-calledsaw-cut method and decompression-load method were used after refinement to enabletheir application to structures of such complexity. The variation of the experimentallydetermined residual prestress forces was remarkably high, depending on the sectioninvestigated. There were also high degrees of uncertainty in estimated values, and thusare only regarded as indications of the residual prestress force.Level 1 analysis of the multi-level assessment strategy consistently underestimatedcapacity, relative to the test results, and did not provide accurate predictions of the shearrelatedfailure observed in the test. With linear FE analysis and local resistance modelsdefined by the European standard, Eurocode 2, the load-carrying capacity wasunderestimated by 32 % for the bridge girder and 55 % for the bridge deck slab. At theenhanced level of structural analysis (Level 3), nonlinear FE analyses predicted thecapacities with less than 2 % deviation from the test results and correctly predicted thefailure mode. However, for existing bridges there are many uncertainties, for instance,the FE simulations were sensitive to the level of residual prestressing, boundaryconditions and assumed material parameters. To accurately take these aspects intoaccount, bridge-specific information is crucial.The complete structural assessment procedure, combining the multi-level strategy andsafety verification framework, was evaluated in a case study. Experiences from theprevious comparative studies were used in an assessment of the Kiruna Bridge followingthe Swedish assessment code. The initial assessment at Level 1 of the multi-level strategyand safety verification, using the PSF method, indicated that the shear capacity of one ofthe girders was critical. The most adverse load case (a combination of permanent loads,prestressing and variable traffic loads) was further investigated through enhancedstructural analyses implicitly accounting for flexural and shear-related failures (Level 3).Nonlinear FE analysis and safety evaluation using the PSF method, several variants of theGRSF method and the full probabilistic analysis for resistance indicated that the permittedaxle load for the critical classification vehicle could be 5.6 – 6.5 times higher than thelimit obtained from the initial assessment at Level 1. However, the study also indicatedthat the model uncertainty was not fully considered in these values. The modeluncertainty was shown to have strong effects on the safety verification and (thus)permissible axle loads. The case study also highlighted the need for a strategy forsuccessively improving structural analysis to improve understanding of bridges’ structuralbehaviour. The refined analysis indicated a complex failure mode, with yielding of thestirrups in the bridge girders and transverse flexural reinforcement in the bridge deck slab,but with a final shear failure of the slab. It would be impossible to capture suchcomplexity in a traditional standardised assessment, which (as mentioned) indicated thatthe shear capacity of the girder limited permissible axle loads. However, nonlinear FEanalyses are computationally demanding, and numerous modelling choices are required.Besides a strategy for rationally improving the analysis and helping analysts to focus oncritical aspects, detailed guidelines for nonlinear FE analysis should be applied to reduce the analyst-dependent variability of results and (thus) the model uncertainty. Clearly, toensure the validity of bridge assessment methods under in situ conditions, theirevaluations should include in situ tests. This thesis presents outcomes of such tests, therebyhighlighting important aspects for future improvements in the assessment of existingbridges.
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| 39. |
- Bagheri, Azam
(författare)
-
Artificial Intelligence-Based Characterization and Classification Methods for Power Quality Data Analytics
- 2018
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One of the important developments in the electric power system is the fast increasing amount of data. An example of such data is formed by the voltages and currents coming from power-quality measurements. Power quality disturbances like voltage dips, harmonics and voltage transient can have a serious negative impact on the performance of equipment exposed to such disturbances. Voltage dips, short duration reductions in voltage magnitude, are especially considered as important disturbances because they regularly lead to stoppages in industrial process installations and subsequently to high costs.The overall aim of this dissertation is the development of automatic analysis methods and other methods for extracting information from large amounts of power-quality data. This includes, methods to detect and extract event characteristics from recorded data and classify the events, for instance, based on their origins or their impact on equipment. The classification facilitates further analysis steps including reasoning and interpretation. Once the data corresponding to each class is available, a proper characterization method can be used to create more semantic data useful for information extraction. The resulting information can be used to improve the performance of the whole system, e.g., updating grid-codes, or immunity requirements of sensitive installations or processes.This dissertation proposes different methods to fulfil each one of the above-mentioned steps. It proposes particularly a space-phasor model (SPM) of the three phase-to-neutral voltages as basis for analytic methods. The SPM is especially suitable as it is a time-domain transform without loss of any information. Another important contribution of the work is that most of the developed methods have been applied to a large dataset of about 6000 real-world voltage dips measured in existing HV and MV power networks.The main contributions of this dissertation are as follows:A complete framework has been proposed for automatic voltage quality analysis based on the SPM. The SPM has been used before, but this is the first time it has been used in a framework covering a range of voltage quality disturbances. A Gaussian-based anomaly detection method is used to detect and extract voltage quality disturbances. A principal component analysis (PCA) algorithm is used for event characterization. The obtained single-event characteristics are used to extract additional information like origin, fault type and location. Two deep learning-based voltage dip classifier has been developed. In both classifier a 2D convolutional neural network (2D-CNN) architecture has been employed to perform automatic feature extraction task. The soft-max activation function fulfills supervised classification method in first classifier. The second classifier uses a semi-supervised classification method based on generative-discriminative model pairs in active learning context.The same SPM was shown to enable the effective extraction of dip characteristics for multi-stage voltage dips. Applying the k-means clustering algorithm, the event is clustered into its individual stages. For each stage of the dip, a logistic regression algorithm is used to characterize that stage. The proposed method offers a new solution to the problem with transition segments that is one of the main challenges of existing methods for characterization of multi-stage dips. It is also shown in the dissertation that the SPM is an effective method for voltage transient analysis. It is possible to extract corresponding sample data and get appropriate single-event characteristics.A systematic way has been developed and applied for comparing different sets of voltage dip characteristics. With this method, both measured and synthetic voltage dips are applied to generic models of sensitive loads. The best set of characteristics is the one most accurately reproducing the behavior of equipment when exposed to measured voltage dips.The dissertation further contains a number of practical applications of the before-mentioned theoretical contributions: a proposal to an international standard-setting group; energy storage for voltage-dip ride-through of microgrids; impact of different voltage dips on wind-power installations.
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| 40. |
- Banda, Sraj Umar, 1974-
(författare)
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Caving mechanisms for a non-daylighting orebody
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The sublevel caving mining method is a mass production method with potentially very low operational costs. The success of this method is dependent on, among other factors, the cavability of the orebody and the overlying rock mass. However, caving of the surrounding rock mass also results in deformations in the cap rock as well as on the ground surface above the orebody being mined. From this follows that any existing infrastructure on the ground surface must be relocated as not to be affected by the mining-induced deformations.This thesis work was undertaken to bring about a better understanding of the rock mass behavior in the cap rock of non-daylighting orebodies, with particular application to the Printzsköld orebody as part of the LKAB Malmberget Mine. Rock testing, field observations and underground mapping was conducted to characterize the rock mass in the caving environment. A methodology for identifying the caving front based on seismic monitoring data was derived by studying the Fabian orebody (which has caved to surface), and using laser scanning data for validation. The methodology was then applied to the Printzsköld orebody to identify the caving front.Numerical modeling was performed for various scenarios of the rock mass as mining proceeded. Modeling included (i) stress analysis to understand stress changes and their effects on the rock mass behavior, (ii) discontinuum numerical modeling to quantify the influence of large-scale geological structures on the cave progression, and (iii) discontinuum cave modeling to simulate possible cave mechanisms in the cap rock more explicitly. Laser scanning together with seismic event data were used to calibrate the numerical models.The numerical simulation results showed that as mining progresses, the cap rock and hangingwall were exposed to stress changes that resulted in yielding. Two failure mechanisms were predominantly at play (i) shear failure (dominant in the cap rock) and (ii) tensile failure (dominant in the hangingwall). The presence of the large-scale structures affected thenearfield stresses through slip along the cave boundaries. The effect of the structures on the far field stresses were less significant.Discontinuum modeling to explicitly simulate failure and caving involved simulating the rock mass as a jointed medium using Voronoi tessellations in 2D, and bonded block modeling (BBM) in 3D. Both the 2D and the 3D modeling results showed fair agreement when comparing the inferred boundary of the seismogenic zone, with that identified from seismic monitoring data. Predictive numerical modeling was conducted for future planned mining to assess future cave development in the cap rock. The results from 3D modeling indicated that cave breakthrough for the Printzsköld orebody is expected when mining the 1023 m level, corresponding to approximately year 2022, as per current mining plans. The 2D model was non-conservative with cave breakthrough predicted to occur when mining the 1109 m level, corresponding to the year 2028.The estimated boundary between the seismogenic and yielded zones, as defined in the Duplancic and Brady conceptual model of caving, was coinciding with, or was close to, the cave boundary in the Printzsköld orebody. This may imply that in some areas the yielded zone was not present and that the Duplancic and Brady model may not be universally applicable. Additional work is required to verify this indication, as well as to fine-tune the modeling methodology.
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