1,720,975 research outputs found
Robust Grid Synchronisation in Three-Phase Distributed Power Generation Systems by Synchronous Reference Frame Pre-Filtering
No full textIn this paper a synchronous reference frame three-phase phase-locked-loop (PLL) is considered, suitable for distributed power generation systems, e.g. photovoltaic or wind power converters. Digital pre-filtering of measured voltages within synchronous frame and based on a bank of multiple notch filters is proposed in order to enhance immunity to non-ideal condition of the grid and raise the dynamic performance of the loop with respect to classical filtering solution in the three-phase reference frame. Detailed analytical derivation of the different harmonic components generated by grid non-ideal components is fully analysed in order to define specifications on the notch filters. More general results are obtained as compared to literature. Extensive simulation results is presented accounting for actual digital implementation and considering limitations on voltage harmonics and unbalance imposed by standard regulations, demonstrating better overall performance as compared to state-of-art proposals
A Novel Proposal to Increase the Power Factor of Photovoltaic Grid-Connected Converters at Light Loads
No full textIn this paper the current control performance of photovoltaic inverters is taken into account. The analytical contribution to the output current due to the grid voltage is analysed and a novel current reference feed-forward is considered in order to compensate for that effect and guarantee a proper current control when feedback current to current controller is measured on the inverter-side inductor of an LCL filter, i.e. as frequent in commercial converters. Dynamical performance of the system, especially at low currents, is also raised by means of a feed-forward action on voltage reference as a function of measured grid voltage, which is normally not considered in literature when resonant controllers are adopted. A novel filtering of the needed voltage feed-forward adopting resonant term is introduced and analysed, leading to an increase in the performance of the current control. Extensive analytical and simulation analysis taking into account all the actual implementation characteristics and limitations is provided to demonstrate the validity of the proposals
System and method for offsetting the input voltage unbalance in multilevel inverters or the like
No full textThe system for offsetting the input voltage unbalance in multilevel inverters or the like comprises a control unit operatively associated with a multilevel inverter for converting direct current into alternate current, the control unit being suitable for piloting the multilevel inverter for generating an output current depending on a reference current, and an equalisation unit for equalising the input voltages of the multilevel inverter having first generation means of a harmonic component of order equal to the reference current, out of phase with respect to the fundamental component of the reference current, detection means of the unbalance of the input voltages to the multilevel inverter, regulation means of the amplitude of the harmonic component depending on the detected unbalance, for offsetting the unbalance. The method for offsetting the unbalance of the input voltages in multilevel inverters or the like comprises a control phase of a multilevel inverter for converting direct current into alternate current, in which the multilevel inverter is piloted for generating an output current depending on a reference current, a generation phase of a harmonic component of order equal to the reference current, out of phase with respect to the fundamental component of the reference current, a detection phase of the unbalance of the input voltages to the multilevel inverter and a regulation phase of the amplitude of the harmonic component depending on the detected unbalance, for offsetting the unbalance
Advances on Inter-Harmonic Variable-Frequency Injection-Based Grid-Impedance Estimation Methods Suitable for PV Inverters
No full textThe aim of this paper is to provide some original and recent advances regarding the implementation of grid impedance estimation techniques based on additional inter-harmonic injection. The effect of the type of injection (i.e. current or voltage) is analysed first and the most convenient approach justified by means of simple qualitative considerations and simulations. A convenient injection pattern and a novel processing algorithm based on a moving window Discrete Fourier Transform (DFT) and averaging filter is introduced and analysed. The method provides a reduction of the number of injected bursts needed to obtain a certain impedance estimation accuracy with respect to state-of-art proposals, therefore leading to a reduction of both current THD and harmonic power losses. Finally the effect of the spectral leakage due to the non-integer ratio between processing sampling and the inter-harmonic injection frequencies, especially in case of natural variations of grid frequency, are fully analysed and a simple but efficient and original solution based on samples interpolation presented
Device and method for converting direct current into alternate current
Full text linkThe device for converting direct current into alternate current comprises a multilevel converter associated with at least a source of direct current and a modulation unit having piloting means for piloting the converter for the conversion of the direct current into an alternate output current, in which the modulation unit comprises comparison means for comparing the output current value with a preset positive threshold value and a preset negative threshold value, the piloting means being suitable for piloting the converter with a pulse modulation of the unipolar type in the event of the output current value being above the positive threshold value or below the negative threshold value and with a pulse modulation of the complementary type in the event of the output current value being below the positive threshold value and above the negative threshold value. The method for converting direct current into alternate current comprises a piloting phase of a multilevel converter for the conversion of a direct voltage into an alternate output voltage, a comparison phase of the output current value with a preset positive threshold value and a preset negative threshold value, the piloting phase being suitable for piloting the converter with a pulse modulation of the unipolar type in the event of the output current value being above the positive threshold value or below the negative threshold value and with a pulse modulation of the complementary type in the event of the output current value being below the positive threshold value and above the negative threshold value
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
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