1,721,013 research outputs found
Dynamic Characterization of Three-Phase Inverter in Photovoltaic Applications
Full text linkPower electronic devices are used as an interface between renewable energy sources and the utility grid. A photovoltaic generator that produces dc electricity is interfaced to a three-phase grid with an inverter. A photovoltaic generator is internally a current source that has highly nonlinear terminal characteristics. The properties of the input source has a great impact on the converter dynamics, which is seldom recognized.
Electrical quantities of a power electronic converter are usually regulated to a desired level. This task is laid on the control system which should be tuned to achieve good regulation and disturbance rejection. The knowledge of the converter's small-signal behavior is a great advantage in the control system design.
Conventional small-signal modeling can not be performed in the case of dc-ac converters due to the fact that some of the quantities are sinusoidal by nature and thus have no steady-state solutions. Nevertheless, small-signal modeling of a three-phase inverter can be done if the three-phase variables are transformed into a synchronous reference frame. However, such a model becomes quite complex and e.g. closed-loop transfer functions can not be solved with reasonable effort. In this thesis, a reduced order model is developed based on a dc-dc equivalence of the inverter. The validity of reduced order model was verified by comparing the inverter transfer functions with the proposed model using a simulation model and frequency response analysis. A prototype was constructed and results were verified by comparing measured and predicted frequency responses.
The effect of a photovoltaic generator on the converter dynamics was examined. It was found out that the photovoltaic generator has a profound effect on the small-signal characteristics of the converter especially from the control design point of view. Most important results were the appearance of a right-half-plane zero in control dynamics and negative output impedance with a typical cascaded control scheme. Such a control system has to be implemented in grid-connected photovoltaic systems in order to transfer maximum power to the grid. The negative output impedance can impose stability problems in the converter-grid-interface and reduce damping in the grid. /Kir11Tehoelektroniikkaa käytetään rajapintana liitettäessä uusiutuvia energialähteitä jakeluverkkoon. Aurinkopaneeli tuottaa tasasähköä ja se on liitettävä jakeluverkkoon kolmivaiheisen vaihtosuuntaajan avulla. Se on rakenteeltaan virtalähde, jonka virta-jännite käyttäytyminen on epälineaarista. Aurinkopaneelin ominaisuuksilla on suuri vaikutus vaihtosuuntaajan dynamiikkaan, vaikka tätä ei kirjallisuudessa usein huomioida.
Teholähteet ovat yleensä säädettyjä järjestelmiä. Aurinkosähkösovelluksissa esimerkiksi paneelin jännite pidetään sopivana tehontuoton maksimoimiseksi. Säätöjärjestelmä on viritettävä siten, että se on stabiili ja riittävän nopea. Tämän saavuttamiseksi teholähteen piensignaalikäyttäytyminen on syytä tuntea.
Tavanomaisessa piensignaalimallinnuksessa ratkaistaan teholähteen toimintapiste ja tutkitaan toimintaa tämän pisteen ympäristössä. Vaihtosuuntaajan tapauksessa tämä ei ole mahdollista, sillä osa suureista on sinimuotoisia. Mallinnus voidaan kuitenkin tehdä hyödyntämällä avaruusvektoriteoriaa. Sinimuotoiset suureet muunnetaan verkkotaajuudella pyörivään avaruusvektori-koordinaatistoon, jolloin niistä tulee tasasuureita ja toimintapiste voidaan ratkaista. Edellisen tuloksena saatava malli on kuitenkin hyvin monimutkainen, jonka vuoksi suljetun järjestelmän siirtofunktioita ei pystytä ratkaisemaan. Tässä työssä kehitettiin yksinkertaistettu malli kolmivaiheiselle vaihtosuuntaajalle, josta myös suljetun järjestelmän siirtofunktiot on mahdollista ratkaista. Mallin pätevyys varmennettiin simulointimallien ja taajuusvaste-analyysin avulla. Lopuksi rakennettiin prototyyppi, josta taajuusvasteet voitiin mitata.
Aurinkopaneelin vaikutusta teholähteen piensignaalikäyttäytymiseen tutkittiin prototyypin avulla. Saatujen tulosten perusteella paneelilla on suuri vaikutus vaihtosuuntaajaan erityisesti säätösuunnittelun kannalta. Tärkeimmät tulokset olivat oikean puolitason nollan ilmestyminen teholähteen dynamiikkaan sekä negatiivinen lähtöimpedanssi käytettäessä kaskadisäätöä. Kaskadisäätöä käytetään jakeluverk- koon kytketyissä säätöjärjestelmissä tehontuoton maksimoimiseksi. Negatiivinen lähtö- impedanssi voi aiheuttaa stabiilisuusongelmia ja huonontaa verkon vaimennusta
Factors Affecting Stable Operation of Grid-Connected Three-Phase Photovoltaic Inverters
Full text linkThe amount of grid-connected photovoltaic energy generation has grown enourmously since the beginning of the 21st century. Photovoltaic power plants are interfaced with the utility grid by using three or single-phase inverters which convert the direct current generated by the photovoltaic modules into three or single-phase alternating current. The photovoltaic inverters have been observed to degrade power quality in the grid and to suffer from reliability problems related to their control software. Therefore, the design of these inverters has become a significant research topic in academia and in the power electronic industry.
Control design of a photovoltaic inverter is often based on the small-signal models characterizing its dynamic behavior. In this thesis, the existing small-signal models are upgraded to include the effect of an upstream DC-DC converter and its control mode. In addition, the models are upgraded to include the effect of a phase-locked-loop which is often used as a synchronization method and the effect of the grid-voltage feedforward which is often used to improve the transient performance.
The control mode of the upstream DC-DC converter is shown to have a significant effect on the minimum DC-link capacitance which is required for stable operation due to a RHP-pole in the inverter control dynamics. However, operating the DC-DC converter under input-voltage control is shown to remove the RHP-pole and, consequently, the constraint imposed on the size of the minimum DC-link capacitance.
The phase-locked-loop (PLL) is shown to make the q-component of the inverter’s output impedance resemble a negative resistor. Based on the small-signal models, the negative resistance is shown to appear at the frequencies below the crossover frequency of the PLL. Therefore, a wide-bandwidth PLL causes easily instability due to the negative-resistance behavior when the grid inductance is large.
The grid-voltage feedforward is shown to increase the magnitude of both the d and q-components of the inverter’s output impedance. The PV inverter with grid-voltage feedforward is shown to be more resistant against impedance-based interactions than an inverter without the feedforward
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
Power Electronics in Renewable Energy Systems
No full textThe observed changes in weather conditions have accelerated the installation of renewable energy-based electricity systems around the world. Large-scale utilization of renewable energy sources in electricity production requires the use of power electronic converters to integrate the renewable energy systems into the power grids. This integration brings about certain challenges in terms of stability and robust performance of the power grids, which have to be solved before the wellbeing of the power grids can be guaranteed. This Special Issue of Energies aims to reveal the state-of-art in addressing interfacing problematics. According to the published papers, clear advancements have taken place, but the most critical issues remain unsolved. Direct power control with self-synchronizing synchronverters may be the most promising technique for solving the main stability problem, although many unsolved problems still persist. Another challenge in renewable energy production is the fluctuating nature of the available energy in renewable energy sources, which require utilization of stored energy to smooth the fluctuations. Different storage battery technologies are available, but their production may pose problems in the long term.Peer reviewe
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
Power Electronics in Renewable Energy Systems
No full textThe observed changes in weather conditions have accelerated the installation of renewable energy-based electricity systems around the world. Large-scale utilization of renewable energy sources in electricity production requires the use of power electronic converters to integrate the renewable energy systems into the power grids. This integration brings about certain challenges in terms of stability and robust performance of the power grids, which have to be solved before the wellbeing of the power grids can be guaranteed. This Special Issue of Energies aims to reveal the state-of-art in addressing interfacing problematics. According to the published papers, clear advancements have taken place, but the most critical issues remain unsolved. Direct power control with self-synchronizing synchronverters may be the most promising technique for solving the main stability problem, although many unsolved problems still persist. Another challenge in renewable energy production is the fluctuating nature of the available energy in renewable energy sources, which require utilization of stored energy to smooth the fluctuations. Different storage battery technologies are available, but their production may pose problems in the long term.Peer reviewe
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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