1,720,995 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
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
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
Optimal System Control of a Back-to-Back Power Converter for Wind Grid-Connected Converter
No full textThe most common topology for micro wind turbine systems is
composed of the wind turbine directly connected to a Permanent
Magnet Synchronous Generator (PMSG) followed by a AC/DC/AC
converter. The converter is realized with a AC/DC passive rectifier, a DC/DC boost converter and a DC/AC full-bridge inverter which injects the electric power into the grid.
Considering the cost reduction of power switches and Digital
Signal Processors (DSP), back-to-back inverter topologies have to be considered for new projects, since they allow to obtain better energy productivity of the wind systems.
In this paper a complete solution based on back-to-back topology
is presented.
This solution includes the sensorless vector control for the
PMSG, the Maximum Power Point Tracker (MPPT) algorithm, the
d-q vector control of the single-phase grid connected inverter and the power flow control strategies from the PMSG to the grid. It is worth noticing that the two full-bridge power converters are controlled by two different DSP which can exchange information only through the value of the DC Link voltage.
Experimental results confirm the effectiveness of the adopted solutions
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
A DC Offset Current Compensation Strategy in Transformerless Grid-Connected Power Converters
No full textA line frequency transformer is usually employed in grid-connected power converters, from both renewable and traditional energy sources, in order to suppress the DC current component and the ground leakage current. Solutions employing a high frequency transformer or employing no transformer at all have recently been investigated in order to reduce size, weight and cost. As a consequence, unless a suitable remedy is adopted, a DC current component exceeding the limits enforced by international standards may be injected into the grid.This paper proposes a simple and cheap solution to reduce the DC current component injected into the grid in the case of a full-bridge, single-phase, transformerless converter. The proposed strategy is intrinsically insensitive to offset measurement errors and can be utilized as a robust and dynamic offset compensator for the current transducer.The simulation results have confirmed the theoretical behavior of the proposed solution, while the experimental ones, performed for different values of output power and for different current control architectures, have shown its effectiveness
Digital Control of Actual Grid-Connected Converters for Ground Leakage Current Reduction in PV Transformerless Systems
No full textThe design of a PV grid-connected converter usuallycomprehends a galvanic isolation between the grid and thephotovoltaic panels. Recently, in low power systems, thegalvanic isolation has been removed with the aim to increaseefficiency and reduce the cost of the converter. Due to thepresence of a parasitic capacitance between the photovoltaiccells and the metal frame of the PV panel, usually connected toearth, a high value of common mode current (i.e. groundleakage current) can arise. In order to limit the ground leakagecurrent (which deteriorates the power quality and generatesEMI), new converter topologies have been proposed. Theireffectiveness is based on the symmetrical (ideal) commutationsof the power switches and some of them adopt a further voltagelevel derived from a capacitive divider of the DC bus voltage.Unfortunately, in actual implementations, asymmetrical powerswitches transients and variations of this added voltage lead tohigher ground leakage current with respect to the ideal case.After a review of the state of the art this paper investigates thesetwo issues and presents a particular solution (based on digitalcontrol and PWM strategy) that, in conjunction with acompensation strategy of power switches actual commutations,guarantees low ground leakage current regardless theparameters tolerance of the power circuit. Simulation andexperimental results confirm the effectiveness of the proposedsolution
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