1,720,994 research outputs found
Advanced Current-limiting Control of Inverter-interfaced Distributed Energy Resources to Develop Self-Protected Microgrids
Get PDFIn the upcoming “smart grid” era, advanced control schemes are required for inverter- interfaced DERs to guarantee stability of inverter-dominated feeders and microgrids. Nevertheless, in many of the recently proposed methods, the safe and stable operation of inverters can not be analytically guaranteed under normal and abnormal grid conditions. In this thesis, single-phase grid-connected inverters are initially considered and an enhanced Current-Limiting Droop (CLD) controller is proposed. In contrast to the original CLD, which limits the inverter current under a lower value than its maximum during faults, the proposed controller fully utilizes the inverter capacity. An inherent current limitation is proven through nonlinear ultimate boundedness theory and is shown to facilitate the operation of Fault-Ride-Through (FRT) schemes. Furthermore, conditions for asymptotic stability of the closed-loop system are derived. Additionally, a new CLD scheme is proposed, which operates without the need of a PLL and introduces a virtual inertia property to DERs. In the sequel, three-phase grid-connected inverters are investigated and a new controller in the dq-frame is proposed to deal with FRT in three-phase systems. Initially, a novel method to divide the current into its symmetrical components during unbalanced faults is proposed. Hence, based on an adaptive bounded integral controller, the proposed scheme provides voltage support to both positive and negative sequences, while ensuring the current boundedness and asymptotic stability of the closed-loop system. In the final part of this thesis, the safe and stable operation of three-phase inverter-based microgrids is investigated. Particularly, an advanced controller is proposed to deal with extreme load conditions. Through the proposed scheme, the limitation of the inverter current during transients is guaranteed, without the need of online adaptation techniques. Furthermore, the proposed approach significantly simplifies the stability analysis of microgrids, since it can be investigated through a Jacobian matrix of reduced size
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
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