1,720,975 research outputs found
Once again about the Steinmetz transformer model and the ongoing subdivision of its leakage inductance
No full textPurpose: Despite its well-founded criticism and lack of proper justification under core saturation conditions, the T-equivalent transformer model (Steinmetz scheme) is obviously championing in the literature. This educational paper aims to explain in a simple manner the limitations of the T-model of a low-frequency transformer and critically analyses some attempts to improve it. Design/methodology/approach: Using a simplified examination of magnetic fluxes in the core and windings and using the modeling in ATPDraw, it is shown that transient transformer models with the indivisible leakage inductance allow circumventing the drawbacks of the T-model. Findings: The authors show the absence of valid grounds for subdividing the leakage inductance of a transformer between its primary and secondary windings. The connection between the use of individual leakage inductances and inaccurate prediction of inrush current peaks is outlined as an important example. Practical implications: The presented models can be used either as independent tools or serve as a reference for subsequent developments. Social implications: Over generations, the habitual transformer T-equivalent is widely used by engineers and Electromagnetic Transients Program experts with no attention to its inadequacy under core saturation conditions. Having studied typical winding configurations, the authors have shown that neither of them has any relation to the T-equivalent. Originality/value: This educational paper will contribute to the correct understanding of the transients occurring in a transformer under abnormal conditions such as inrush current or ferroresonance events, as well as during an out-of-phase synchronization of step-up generator transformers
Structure and properties of the hybrid and topological transformer models
No full textThree topological transformer models for low-frequency transient simulations are considered in increasing order of their topology. It is first shown that the well-known hybrid model is not a strictly topological one, but despite this, it can be used in many applications. The next is the simplified model, which is a strict electric equivalent of the popular magnetic model, and the last is a detailed electric model, which takes into account thicknesses of transformer windings. This model improvement allows one to reproduce experimentally observed behaviors of the magnetic fluxes in the legs and yokes under short circuit conditions
TOPOLOGICAL REVERSIBLE MODEL OF THREE-PHASE FIVE-LIMB TRANSFORMER
Get PDFPurpose. The paper continues the authors’ studies devoted to transients in three-phase five-limb transformers. The main purpose of the work is to propose a method of evaluating model parameters, which cover transformer operations in saturation. This purpose is achieved by using the concept of the model reversibility.
Methodology. The method of obtaining model parameters employs magnetic transformer model and is based on the idea of the model reversibility. The solution is found by equating input reluctances seen from the terminals of the innermost and outermost windings to the reluctances of the corresponding windings on air.
Findings. The modeling of GIC events represented in the paper is the most accurate ever obtained for three-phase, five-leg transformers. The model is validated by close agreement of the predicted values and waveforms of the phase currents and reactive power with those measured in tests performed on two 400 MVA transformers connected back-to-back and to a 400 kV power network. The validity of the model was verified at 75 and 200 A dc currents in the transformer neutral. It is shown that the model is a reliable tool in evaluating inrush currents.
Originality. The originality and advantage of the method proposed is its ability to determine the model parameters without fitting to experimental data obtained in regimes with highly saturated core. The method ensures the reversibility of the three-winding transformer model that is its correct behavior regardless of which winding is energized.
Practical value. The practical value and significance of the paper is caused by the fact that the model proposed is a simple and reliable tool for power system studies. As a practically important example, time domain response of transformer subjected to geomagnetically induced currents (GIC) is analyzed and compared with results of a comprehensive field experiment
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
TOPOLOGICAL TRANSIENT MODELS OF THREE-PHASE FIVE-LIMB TRANSFORMER
No full textPurpose. To show capabilities of topological models of three-phase, five-limb transformer to correctly represent transformer operation in regimes with high flux densities in the core. As a practically important example, time domain response of transformer subjected to geomagnetically induced currents (GIC) is analyzed and compared with results of a comprehensive field experiment.Methodology. Transformer magnetic model, which takes into account geometry of the core and windings, is transformed in a dual electric equivalent scheme, whose transient is calculated by using EMTP-ATP. The results obtained demonstrate the importance of incorporating the positive and zero sequence impedances of power network.Findings. A simple and reliable model of five-limb transformer is proposed. It was found that the presence of the transformer tank can be effectively accounted for by linear inductances representing the paths of the off-core fluxes from yoke to yoke. The modeling of GIC events represented in the paper is the most accurate ever obtained for three-phase, five-leg transformers. The model is validated by close agreement of the predicted values and waveforms of the phase currents and reactive power with those measured in tests performed on two 400 MVA transformers connected back-to-back and to a 400 kV power network.
Originality. It is shown that a simplified non-hysteresis model developed portrays the behavior of the of five-limb transformer under GIC condition with the same good accuracy as its hysteresis model. Both the transformer models are well grounded. So, the paper dispels some misconceptions about the influence of the hysteresis properties of the core material and tank in transient modeling of five-leg transformers.Practical value. The practical value and significance of the paper is caused by the fact that the model proposed is a simple and reliable tool for power system studies. The paper warns of using unnecessary complicated models whose parameters are difficult to be estimated by experiments or calculations
Topological transient models of three-phase, three-legged transformer
Get PDFThe paper further develops a modeling concept of three-phase, three-legged transformer with tank walls represented as a distributed parameter structure. The circuital models proposed replicate accurately all possible zero-sequence impedances and losses of a three-winding transformer. Several model versions are compared to each other and to a conventional topological model with respect to their transient behavior during inrush and short circuit events, and in the presence of geomagnetically induced current (GIC). It was found that in all the cases considered, except that with a large GIC, all the models yield similar results. The reliability of the models is due to the representation of the tank walls behavior in a physical way, allowing one to observe the field distribution over the wall thickness as a function of transformer excitation. The modeled results are in a close agreement with positive and zero sequence data measured on a 25 MVA transformer as well as with inrush current test on a 300 kVA unit
Variations on the Author
Get PDF“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
Get PDFWe 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
Get PDFWe 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
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