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Ribeiro H.S.,Telecommunications Institute of Portugal | Ribeiro H.S.,University of Lisbon | Ribeiro H.S.,Polytechnic Institute of Tomar | Borges B.V.,Telecommunications Institute of Portugal | Borges B.V.,University of Lisbon
IEEE Transactions on Industrial Electronics | Year: 2011

The inclusion of a few additional diodes and passive elements in the high-frequency full-bridge acdc converter with galvanic isolation permits one to achieve sinusoidal input-current wave shaping and output-voltage regulation simultaneously without adding any auxiliary transistors. Recently, this procedure, together with an appropriate control process, has been used to obtain low-cost high-efficiency single-stage converters. In an attempt to improve the performance of such converters, this paper introduces three new single-stage full-bridge acdc topologies with some optimized characteristics and compares them with the ones of the existing full-bridge single-stage topologies. The approach used consists in the definition of the operating principles identifying the boost function for each topology, their operating limits, and the dependence between the two involved conversion processes. Experimental results for each topology were obtained in a 500-W modular laboratory prototype that was built with the necessary flexibility to allow the realization of each different topology. Comparison was evaluated in terms of input-current distortion, efficiency, and output-voltage disturbances that result from the input-current wave-shaping process. © 2010 IEEE. Source


Ribeiro H.S.,University of Lisbon | Ribeiro H.S.,Polytechnic Institute of Tomar | Borges B.V.,University of Lisbon
IEEE Transactions on Industrial Electronics | Year: 2014

AC-DC conversion with power factor correction using full-bridge single-stage (FBSS) converters may introduce various complex operating limits and control problems that are not usually described in literature. This is mainly due to the fact that they devaluate and restrict the number of applications of those topologies. These problems are identified as primary transformer saturation and output voltage perturbations. The operation at light loads and with wide load variation is also a complex issue that leads to uncontrollable input current and dc bus voltage. This paper clarifies such technical problems referring their impact in the FBSS topologies operation, and presents design criteria and control solutions to minimize their effects. A low cost free wheeling circuit (FWC) that allows null input current to solve the light load operation problem is also presented. © 2013 IEEE. Source


Barros C.,University of Lisbon | Antunes O.S.,Polytechnic Institute of Tomar
Energy Sources, Part B: Economics, Planning and Policy | Year: 2014

This artical analyzes the efficiency and productivity growth of a representative sample of Angolan oil blocks from 2002 to 2008, using the directional distance function and the Luenberger productivity indicator. The primary advantage of this approach is that both input contractions and output expansions are considered. The model generates a productivity indicator that is decomposed into the two usual constituents of productivity growth: technological change and efficiency change. For comparative purpose a Malmquist productivity index is presented. The results show that, on average, Angolan oil blocks did experience some productivity growth during the period analyzed. In addition, the incidence of technological change was positive. Copyright © Taylor & Francis Group, LLC. Source


Gomes M.H.,Polytechnic Institute of Tomar | Saraiva J.T.,INESC Porto
Electric Power Systems Research | Year: 2010

MicroGrids represent a new paradigm for the operation of distribution systems and there are several advantages as well as challenges regarding their development. One of the advantages is related with the participation of MicroGrid agents in electricity markets and in the provision of ancillary services. This paper describes two optimization models to allocate three ancillary services among MicroGrid agents - reactive power/voltage control, active loss balancing and demand interruption. These models assume that MicroGrid agents participate in the day-ahead market sending their bids to the MicroGrid Central Controller, MGCC, that acts as an interface with the Market Operator. Once the Market Operator returns the economic dispatch of the MicroGrid agents, the MGCC checks its technical feasibility (namely voltage magnitude and branch flow limits) and activates an adjustment market to change the initial schedule and to allocate these three ancillary services. One of the models has crisp nature considering that voltage and branch flow limits are rigid while the second one admits that voltage and branch flow limits are modeled in a soft way using Fuzzy Set concepts. Finally, the paper illustrates the application of these models with a Case Study using a 55 node MV/LV network. © 2010 Elsevier B.V. All rights reserved. Source


Campos R.,Polytechnic Institute of Tomar | Campos R.,University of Beira Interior | Dias G.,University of Caen Lower Normandy | Jorge A.M.,INESC Porto | And 2 more authors.
ACM Computing Surveys | Year: 2014

Temporal information retrieval has been a topic of great interest in recent years. Its purpose is to improve the effectiveness of information retrieval methods by exploiting temporal information in documents and queries. In this article, we present a survey of the existing literature on temporal information retrieval. In addition to giving an overview of the field, we categorize the relevant research, describe the main contributions, and compare different approaches.We organize existing research to provide a coherent view, discuss several open issues, and point out some possible future research directions in this area. Despite significant advances, the area lacks a systematic arrangement of prior efforts and an overview of state-of-The-Art approaches.Moreover, an effective end-To-end temporal retrieval system that exploits temporal information to improve the quality of the presented results remains undeveloped. © 2014 ACM. Source

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