1,720,974 research outputs found
Grid Monitoring and Advanced Control of Distributed Power Generation Systems
Full text linkThe movement towards a clean technology for energy production and the constraints in reducing the CO2 emissions are some factors facilitating the growth of distributed power generation systems based on renewable energy resources. Consequently, large penetration of distributed generators has been reported in some countries creating concerns about power system stability. This leads to a continuous evolution of grid interconnection requirements towards a better controllability of generated power and an enhanced contribution of distributed power generationsystems to power system stability. As an example, the latest published grid codes stress the ability of distributed generators, especially wind turbines, to stay connected during short grid disturbances and in addition to provide active/reactive power control at the point of common coupling.Based on the above facts, the need for improving and adding more features to the control of distributed power generation systems (DPGS) arises. As a consequence, this thesis focuses on grid monitoring methods and possible approaches in control in order to obtain a more reliable and exible power generation system during normal and faulty grid conditions.First part of the thesis investigates possible algorithms for fast and accurate identi cation of utility network variables such as voltage amplitude, frequency, phase angle and line impedance. Special attention has been paid to grid synchronization algorithm in terms of accurate estimation of grid voltage phase angle. Has been found that identi cation of positive and negative sequence components and in addition, the capability of the algorithm to follow only the positive sequence component, plays a crucial role in providing a clean synchronization angle even during severevoltage unbalance caused by grid faults. Several algorithms for phase angle detection have been investigated and two novel algorithms that prove to be very robust against grid voltage disturbances have been developed.The proposed synchronization algorithms have been further developed in order to estimate also the amplitude of grid voltage and the frequency of utility network. As a result, fast and accurate identi cation of both variables has been achieved. In addition, positive and negative sequence components of grid voltage can also be calculated. Simple, yet powerful ltering techniques, based on second order generalized integrator (SOGI) and delay signal cancellation(DSC) have been used to separate the sequence components. Simulation and experimental results attest the accuracy and e ectiveness of the developed algorithms in identifying the frequency, phase angle and magnitude of grid voltages during severe distortions of utility network. Methods for identi cation of grid impedance have also been investigated in this thesis. The state of the art methodologies for assessing the value of line impedance have been studied, leadingto the development of two new methods for identi cation of line impedance. First method uses variations in both active and reactive power at the point of common coupling (PCC) to obtain current and voltage uctuation in two operating points, based on which the algorithm is extracting the values for both resistive and inductive part of line impedance. Because this methodology can create voltage fluctuation at the connection point, it can be subject for flicker emission. As a consequence, a second method, which uses a grid voltage control loop to set the reference for reactive power has been developed. If in the rst method the variations in power are made consecutively ( rst variation in active power is made then variation in reactive power follows), in the second method the variation of reactive power occurs at the same time when variation in active power happens, in order to cancel out the voltage uctuation at the point of common coupling. The results presented con rm the accuracy and e ectiveness of both methods developed.A considerable part of this thesis is dealing with control of grid side converter of a distribution system during normal and faulty grid conditions. The state of the art control structures for grid tied power converters have been initially identi ed and di erent types of controllers have been studied and compared. The possibility of using the information about grid variables into the control structure in order to improve the control of DPGS has also been investigated. Asa consequence, improved behavior of resonant controller has been noticed if grid frequency information is forwarded to its internal model. Additionally, controllers such as dead beat and hysteresis controller improve their robustness to parameter mismatch if the identi ed value of grid impedance is passed to the controller. Moreover, several control strategies to provide exible active and reactive power control during grid faults have been developed using the information of positive and negative sequence components. Simulation and experimental results are presentedin order to validate the above studies.The research done in this thesis makes it possible to assess the behavior and control abilities of a grid connected power converter when running on faulty grid conditions. Furthermore, this study can serve as a good basis for deeper investigation in some particular areas and also for further research on control of DPGS during grid disturbances.The movement towards a clean technology for energy production and the constraints in reducing the CO2 emissions are some factors facilitating the growth of distributed power generation systems based on renewable energy resources. Consequently, large penetration of distributed generators has been reported in some countries creating concerns about power system stability. This leads to a continuous evolution of grid interconnection requirements towards a better controllability of generated power and an enhanced contribution of distributed power generationsystems to power system stability. As an example, the latest published grid codes stress the ability of distributed generators, especially wind turbines, to stay connected during short grid disturbances and in addition to provide active/reactive power control at the point of common coupling.Based on the above facts, the need for improving and adding more features to the control of distributed power generation systems (DPGS) arises. As a consequence, this thesis focuses on grid monitoring methods and possible approaches in control in order to obtain a more reliable and exible power generation system during normal and faulty grid conditions.First part of the thesis investigates possible algorithms for fast and accurate identi cation of utility network variables such as voltage amplitude, frequency, phase angle and line impedance. Special attention has been paid to grid synchronization algorithm in terms of accurate estimation of grid voltage phase angle. Has been found that identi cation of positive and negative sequence components and in addition, the capability of the algorithm to follow only the positive sequence component, plays a crucial role in providing a clean synchronization angle even during severevoltage unbalance caused by grid faults. Several algorithms for phase angle detection have been investigated and two novel algorithms that prove to be very robust against grid voltage disturbances have been developed.The proposed synchronization algorithms have been further developed in order to estimate also the amplitude of grid voltage and the frequency of utility network. As a result, fast and accurate identi cation of both variables has been achieved. In addition, positive and negative sequence components of grid voltage can also be calculated. Simple, yet powerful ltering techniques, based on second order generalized integrator (SOGI) and delay signal cancellation(DSC) have been used to separate the sequence components. Simulation and experimental results attest the accuracy and e ectiveness of the developed algorithms in identifying the frequency, phase angle and magnitude of grid voltages during severe distortions of utility network. Methods for identi cation of grid impedance have also been investigated in this thesis. The state of the art methodologies for assessing the value of line impedance have been studied, leadingto the development of two new methods for identi cation of line impedance. First method uses variations in both active and reactive power at the point of common coupling (PCC) to obtain current and voltage uctuation in two operating points, based on which the algorithm is extracting the values for both resistive and inductive part of line impedance. Because this methodology can create voltage fluctuation at the connection point, it can be subject for flicker emission. As a consequence, a second method, which uses a grid voltage control loop to set the reference for reactive power has been developed. If in the rst method the variations in power are made consecutively ( rst variation in active power is made then variation in reactive power follows), in the second method the variation of reactive power occurs at the same time when variation in active power happens, in order to cancel out the voltage uctuation at the point of common coupling. The results presented con rm the accuracy and e ectiveness of both methods developed.A considerable part of this thesis is dealing with control of grid side converter of a distribution system during normal and faulty grid conditions. The state of the art control structures for grid tied power converters have been initially identi ed and di erent types of controllers have been studied and compared. The possibility of using the information about grid variables into the control structure in order to improve the control of DPGS has also been investigated. Asa consequence, improved behavior of resonant controller has been noticed if grid frequency information is forwarded to its internal model. Additionally, controllers such as dead beat and hysteresis controller improve their robustness to parameter mismatch if the identi ed value of grid impedance is passed to the controller. Moreover, several control strategies to provide exible active and reactive power control during grid faults have been developed using the information of positive and negative sequence components. Simulation and experimental results are presentedin order to validate the above studies.The research done in this thesis makes it possible to assess the behavior and control abilities of a grid connected power converter when running on faulty grid conditions. Furthermore, this study can serve as a good basis for deeper investigation in some particular areas and also for further research on control of DPGS during grid disturbances
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Adjustable Speed Drives with Square-wave Input Current:A Cost Effective Step in Development to Improve Their Performance
No full textkoamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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