1,720,990 research outputs found
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
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
Saturation Compensation Strategy for Grid Connected Converters Based on Line Frequency Transformers
No full textThis paper deals with the compensation of magneticsaturation in line frequency transformer coupled, grid-connectedpower converters. The developed strategy originates from thechoice of controlling the actual current injected in the grid (outputof the line frequency transformer) instead of the input current ofthe transformer, as usually proposed. As a matter of fact, betterpower quality cannot neglect the direct control of the injected gridcurrent. An effective solution providing zero steady-state error atgrid frequency and superior disturbance rejection involves the useof a d–q current controller synchronous with the grid voltage. Forthis reason, the proposed saturation compensation strategy hasbeen implemented inside the d–q reference frame. The downsideis that closing the current feedback at the transformer output rendersimpossible to control the dc current at transformer input,leading to core saturation. A core saturation compensation strategy,based only on the monitoring of the output current distortionaround voltage zero crossings, is here presented. Moreover, sincethe proposed solution does not rely on dc measures, offset problemsare avoided. Experimental results confirm the effectiveness of theproposed control architecture and of the strategy adopted to avoidtransformer core saturation
Active Common-Mode Filter for Ground Leakage Current Reduction in Grid-Connected PV Converters Operating With Arbitrary Power Factor
No full textThis paper proposes a solution for reducing the
ground leakage current in transformerless single-phase gridconnected
photovoltaic converters. This is obtained with the
introduction of an active common-mode filter able to compensate
for variations of the output common-mode voltage of the
power converter. The active common-mode filter is applied to a
widespread and efficient full-bridge driven by a three-level pulse
width modulation, allowing the power converter to operate with
low ground leakage current and with an arbitrary power factor.
After showing the desired voltage waveform for common-mode
voltage compensation, this paper presents the design guidelines
for the needed additional magnetic component together with the
power loss considerations for all the devices added for the proposed
solution. Experimental results show the performance of the
proposed solution in terms of ground leakage current reduction,
effectiveness of dead-time compensation, total harmonic distortion
of the injected grid current, and power losses
Active Common-Mode Filter for Photovoltaic Transformerless Inverters
No full textIn the recent years the research on photovoltaic converters has been focused on topologies that do not feature a line frequency transformer, in order to reduce costs and successfully increase the overall efficiency. The main problem in removing the galvanic isolation is that the parasitic capacitance present between the cell and the metal frame enclosing the panel
(usually earth-connected) enables a common-mode current to be
injected into the grid by the converter.
This paper proposes an active common-mode filter able to
compensate the high-frequency common-mode voltage variations
at the output of the power converters, that represent the principal
cause of ground leakage current. The advantage of this solution
concerns the capability to operate with an arbitrary power factor.
In this work it is applied in cascade to a converter with standard full-bridge topology driven by a three-level PWM strategy. Simulation and experimental results shows the feasibility 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
Review of Design Solutions for Internal Permanent Magnet Machines Cogging Torque Reduction
No full textInternal permanent magnet synchronous machinesare spreading in industrial production.They feature high torque density and extended speed rangethat are key issues in many fields of applications, however theircogging torque is typically quite high.Many methods and design guidelines for cogging torquereduction exist in literature and this paper compares them. Forthis purpose, the different design guidelines are applied to acommon reference machine in order to assess their effectiveness.Computer Finite Element Analysis (FEA) are carried out for eachcase in order to compare the cogging torque reduction capabilityof the different techniques. The side effects of these techniques,such as back-emf and rated torque profile distortions, willbe taken into account. The paper contribution is to comparethe various cogging torque reduction techniques and magneticgeometries on a common reference machine to identify the mosteffective ones
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
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