1,721,006 research outputs found
A Five-Level Single-Phase Grid-Connected Converter for Renewable Distributed Systems
No full textIn low-power renewable systems, a single-phase grid-connected converter is usually adopted. This paper deals with a novel five-level converter topology that follows this trend. A review of the state of the art of the five-level topologies and a theoretical power loss comparison with the proposed solution is realized. The proposed converter architecture is based on a full-bridge topology with two additional power switches and two diodes connected to the midpoint of the dc link. Since the two added levels are obtained by the discharge of the two capacitors of the dc link, the balancing of the midpoint voltage is obtained with a specific pulse width modulation (PWM) strategy. Simulation and experimental results show the effectiveness of the proposed solution
Detection Method of the DC bias in Distribution Power Transformers
No full textNon-linear loads such as AC drives switch-mode
power supplies and grid-connected converters can cause, besides
the generation of unwanted current harmonics, a DC current
component injection into the grid. This DC current component
can lead to magnetic saturation of the distribution power transformers, leading to higher current distortion and overheating.
This paper presents a method to detect the DC current
component flowing into the distribution power transformer with
a precise measurement of the DC voltage component at the
transformer winding. A magnetic sensor has been developed and
implemented in a closed-loop control system to achieve a high
sensitivity and guarantees a good linearity with a high rejection
ratio to grid voltage variations. Simulation and experimental
results confirm the effectiveness of the proposed approach
Compensation strategy of actual commutations for PV transformerless grid-connected converters
No full textPhotovoltaic grid-connected converters usually embed a high-frequency or a line transformer, which guarantees galvanic isolation between the photovoltaic field and the mains. In order to increase efficiency and reduce the total cost of the system, the transformer has been removed, and special topologies of converter have been studied in order to limit the ground leakage current that arises with the galvanic connection. In fact, the parasitic capacitance between the photovoltaic cells and the metallic frame of the panel allows high ground leakage current (i.e. common mode current) to be injected into the grid. Actual solutions rely heavily on the symmetry of the system to address the problem. This paper presents a novel strategy to compensate for non-ideal switching behavior of power devices, which is immune to layout asymmetries and tolerance of parameters. Simulation results show the feasibility of the proposed solution. ©2010 IEEE
Distributed cogeneration plants
No full textThis article proposes a new control scheme for a distributed cogeneration (microcogeneration) plant using a three-phase induction machine as an electric generator. This control scheme assures a better energy-conversion quality. The plant can be operated as a grid-connected generator with load tracking or as an isolated generator. In isolated operation, the induction generator is excited with a current-controlled voltage source inverter (CCVSI) having a suitable capacitive bank in the dc side. During grid-connected operations, the same converter can be used for power factor correction. Energy generation and its optimal use are the key issues in actual social conditions. In this framework, the European Union (EU) anticipates that the amount of energy produced by cogeneration plants will double within five years, and this might be an underestimated growth, as its potential is still unexploited. Cogeneration or combined heat and power (CHP) means the combined production ofthermal, electrical, and mechanical energy.Recently, distributed cogeneration or micro-CHP was constantly in the spotlight of the industrial and scientific community.This article proposes a new control scheme for a distributed cogeneration (microcogeneration) plant using a three-phase induction machine as an electric generator. This control scheme assures a better energy-conversion quality. The plant can be operated as a grid-connected generator with load tracking or as an isolated generator. In isolated operation, the induction generator is excited with a current-controlled voltage source inverter (CCVSI) having a suitable capacitive bank in the dc side. During grid-connected operations, the same converter can be used for power factor correction. © 2009 IEEE
A Sensor to Detect the DC Bias of Distribution Power Transformers
No full textThe widespread use of power converters in low-voltage distribution grids has given rise to issues regarding the power quality. In fact, non-linear loads such as AC Drives, switching-mode power supplies and grid-connected converters can cause, besides the generation of several current harmonics, also a DC current component injection into the grid. This DC current component can lead to magnetic saturation of the distribution power transformers; in this condition, the transformers present distorted current waveforms, increased power absorption and overheating, that can damage the transformer insulations. This paper presents a way to diagnose the magnetic saturation by a non-direct measurement of the DC current component flowing in the power transformer. In other words, the proposed solution provides an information about the total DC injection produced by the sum of all the electric devices connected to the distribution power transformer. The DC current component causes a DC voltage drop across the parasitic resistance of the transformer's winding: sensing this DC voltage drop allows to evaluate the DC current component. A magnetic sensor was developed in order to obtain a great sensitivity, and the implemented closed loop control allowed to guarantee a good linearity with a high rejection ratio of the grid voltage variations. Simulation and experimental results confirm the effectiveness of the proposed approach
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
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
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