1,720,976 research outputs found
A novel assessment of unintentional islanding operations in distribution networks
Get PDFThis thesis aims to investigate an unexpected islanding operation (IO) which has been identified in a real distribution network. The process of recording and processing the data obtained from the field measurements in the distribution network (DN) has been the starting point of this research. It has to be underlined that this IO raised a problem and became a major challenge for the distribution operator. Therefore, the aim of this thesis is twofold; solving a real problem as well as further enhance the current research studies about IOs in DNs. IOs have been object of study during the recent years due to the rapid proliferation of the distributed generation (DG) within the so-called smart grids (SGs). Commonly, the power of these DG resources ranges between hundreds of kW and few MW and are allocated at either low voltage or medium voltage levels. One of the significant issues that these resources are raising is, undoubtedly, the IOs. These situations occur when a portion of the grid operates in parallel with the main grid following a disconnection. Thereby IOs, where the DG is energising the grid after a CB opening, must be identified and tripped in the minimum time possible. Failure to do so, the list of hurdles may include; power quality (PQ) disturbances (e.g., frequency and voltage out of range), a safety hazard for the network personnel or out-of-phase reclosings. That is the reason why the research towards the anti-islanding protection methods has elicited great interest.
Fundamentally, the substantial improvement of this thesis lies in the fact that, in this IO, there are no DG resources, but large induction motors. In fact, the grid remains energised after the CB disconnection due to the induction motors (IMs) which transiently, act as generators. The island begins with the CB operation and ends when the CB recloses the circuit to restore the electrical supply. This rapid reclosing operation is widely adopted in DNs to avoid manual operations in self-extinguished faults and typically ranges between 0.5 and 1s. Given the fact that usually IOs are originated in the presence of DG, indeed, this IO is utterly unexpectedly for the DSO.
Due to the phenomenon mentioned above, the specific goals of this thesis are described down below:
1. The first goal of this thesis focuses on developing a model suitable for validation purposes. To make a proper model validation, the simulations results obtained with this model will be compared with those obtained from field measurements. Thus, once the model has been validated, a thorough investigation regarding the most influential factors will be carried out.
2. The second goal of this thesis falls within the scope of the PQ. During the IO mentioned above, a new voltage sag topology is observed. Consequently, the efforts will be focused on modelling this new type of sag.
3. The third goal of this thesis emerges from the protective point of view. Once the IO has been defined and characterised, the need for identifying and preventing it becomes the main concern. In such a way, the third pillar of the thesis is targeted at implementing a suitable tool to prevent this particular IO. Besides, this new tool will be compared with the currently available methods for ID developed for scenarios with DG.Aquesta tesi te com a objectiu investigar una operació en illa no intencional, que ha set identificada en una xarxa de distribució real. El procés de registre i processament de les dades obtingudes a partir de les mesures de camp en la xarxa de distribució, ha estat el punt de partida d’aquesta investigació. Cal subratllar que aquesta operació en illa va plantejar un problema i es va convertir en un repte important per l’operador de distribució. Per tant, l'objectiu d'aquesta tesi és doble; resoldre un problema real, així com millorar els estudis de recerca actuals sobre les illes no intencionals en xarxes de distribució elèctrica. El fenomen de les illes dins una xarxa elèctrica, han estat objecte d’estudi durant els darrers anys a causa de la ràpida proliferació de la generació distribuïda. Habitualment, la potència d’aquests recursos distribuïts oscil·la entre centenars de kW i pocs MW i s’assignen a nivells de baixa tensió o mitja tensió. Una de les qüestions importants que plantegen aquests recursos és, sens dubte, les illes. Aquestes situacions es produeixen quan una part de la xarxa elèctrica funciona en paral·lel amb la xarxa principal després d’una desconnexió. Per això, les illes no intencionals es donen quan la generació distribuïda energitza la xarxa després de la obertura d’un interruptor. Principalment, l’objectiu es identificar aquesta situació i desconnectar dites fonts en el mínim temps possible. En el cas de que això no succeeixi, els següents disturbis poden produir-se; pertorbacions de la qualitat de potència (PQ) (per exemple, freqüència i tensió fora del rang), un perill per a la seguretat del personal de la xarxa o bé reconnexions fora de fase. Aquesta és la raó per la qual la investigació vers els mètodes de protecció “anti-islanding” han despertat un gran interès. Essencialment, la millora substancial d’aquesta tesi rau en el fet que, en aquesta illa, no hi ha recursos energètics distribuïts, sinó grans motors d’inducció. Així, la xarxa elèctrica continua energitzada després de la desconnexió del interruptor a causa dels motors d’inducció, que actuen de forma transitòria com a generadors. L’illa comença amb l’obertura del interruptor i finalitza quan aquest tanca el circuit per restablir el subministrament elèctric. Aquesta operació de reconnexió ràpid es freqüent en xarxes de distribució per evitar operacions manuals en faltes temporals i generalment oscil·la entre 0,5 i 1s. Tenint en compte que generalment les illes tenen l'origen en presència de generació distribuïda , realment, la illa elèctrica objecte d’aquesta tesi és inesperada per l’operador de distribució. A causa del fenomen esmentat anteriorment, els objectius específics d'aquesta tesi es descriuen a continuació: 1. El primer objectiu d'aquesta tesi se centra a desenvolupar un model adequat per la validació. Per fer una validació adequada del model, es compararan els resultats de les simulacions obtinguts amb aquest model amb els obtinguts de les mesures de camp. Així, un cop validat el model, es durà a terme una investigació completa sobre els factors més influents. 2. El segon objectiu d'aquesta tesi entra dins de l'àmbit d'aplicació del PQ. Durant l’esmentada illa, s’observa una nova topologia de forat de tensió. En conseqüència, els esforços se centraran en modelar aquest nou tipus de forat. 3. El tercer objectiu d'aquesta tesi s’emmarca en el punt de vista de proteccions. Un cop definida i caracteritzada l’illa, la necessitat d’identificar-la i prevenir-la esdevé la principal preocupació. D’aquesta manera, el tercer pilar de la tesi té com a objectiu la implementació d’una eina adequada per prevenir aquesta particular illa. A més, es compararà aquesta nova eina amb els actuals mètodes utilitzats per a identificar les illes en escenaris amb generació distribuïda
Development and performance analysis of a multi-functional algorithm for AC microgrids: Simultaneous power sharing, voltage support and islanding detection
No full textA large part of the current microgrids (MGs) operate in grid-connected mode and act as slaves following the voltage and frequency dictated by the main grid. Therefore, these MGs are not expected to be operated in parallel as islands. Hence, according to the IEEE 1457-2018 standard, the power generation units in these grid-connected MGs (GCMGs) have to be disconnected in less than 2 s if islanding occurs. On the contrary, if the islanding operation (IO) does not occur, these MGs exchange power with the electrical grid. This paper explores the feasibility of a multi-functional algorithm (MA) for these GCMGs in an attempt to address three tasks simultaneously; (i) power sharing, (ii) voltage support and, (iii) islanding detection (ID). The MG object of study comprises an electronically interfaced photovoltaic (PV) unit supplemented by a battery energy storage system (BESS) equipped in combination with a bidirectional charger. The model of the GCMG has been implemented in MATLAB, where an extensive set of simulations has been performed. The results demonstrate the effectiveness and benefits of the proposed MA, which fulfils several functions all at once
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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Optimal tuning of fractional order controllers for dual active bridge-based DC microgrid including voltage stability assessment
Get PDFIn this article, three evolutionary search algorithms: particle swarm optimization (PSO), simulated annealing (SA) and genetic algorithms (GA), have been employed to determine the optimal parameter values of the fractional-order (FO)-PI controllers implemented in the dual active bridge-based (DAB) DC microgrid. The optimum strategy to obtain the parameters of these FO-PI controllers is still a major challenge for many power systems applications. The FO-PI controllers implemented in the DAB are used to control the DC link voltage to the desired value and limit the current flowing through the converter. Accordingly, the investigated control system has six parameters to be tuned simultaneously; Kp1, Ki1, ¿1 for FO-PI voltage controller and Kp2, Ki2, ¿2 for FO-PI current controller. Crucially, this tuning optimization process has been developed to enhance the voltage stability of a DC microgrid. By observing the frequency-domain analysis of the closed-loop and the results of the subsequent time-domain simulations, it has been demonstrated that the evolutionary algorithms have provided optimal controller gains, which ensures the voltage stability of the DC microgrid. The main contribution of the article can be considered in the successful application of evolutionary search algorithms to tune the parameters of FO-based dual loop controllers of a DC microgrid scheme whose power conditioner is a DAB topology.Peer ReviewedPostprint (published version
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
- …
