Teses em Engenharia Elétrica (Doutorado) - PPGEE/ITEC

URI Permanente para esta coleçãohttps://repositorio.ufpa.br/handle/2011/2317

O Doutorado Acadêmico inicio-se em 1998 e pertence ao Programa de Pós-Graduação em Engenharia Elétrica (PPGEE) do Instituto de Tecnologia (ITEC) da Universidade Federal do Pará (UFPA).

Navegar

Agora exibindo 1 - 20 de 199

Acesso aberto (Open Access)
Abordagem Inteligente com Combinação de Características Estruturais para Detecção de Novas Famílias de Ransomware
(Universidade Federal do Pará, 2024-03-22) MOREIRA, Caio Carvalho; SALES JUNIOR, Claudomiro de Souza de; País de Nacionalidade Brasi
Ransomware is a malicious software that aims to encrypt user files and demand a ransom to unlock them. It is a cyber threat that can cause significant financial damage, as well as compromise privacy and data integrity. Although signature-based detection scanners commonly combat this threat, they fail to identify unknown ransomware families (variants). One method to detect new threats without the need to execute them is static analysis, which inspects the code and structure of the software, along with classification through intelligent approaches. The Detection of New Ransomware Families (DNFR) can be evaluated in a realistic and challenging scenario by categorizing and isolating families for training and testing. Hence, this thesis aims to develop an effective static analysis model for DNFR, which can be applied in Windows systems as an additional security layer to check executable files upon receipt or before execution. Early ransomware detection is essential to reduce the likelihood of a successful attack. The proposed approach comprehensively analyzes executable binaries, extracting and combining various structural features, and distinguishes them between ransomware or benign software employing a soft voting model comprising three machine learning techniques: Logistic Regression (LR), Random Forest (RF), and eXtreme Gradient Boosting (XGB). Results for DNFR demonstrated an average accuracy of 97.53%, precision of 96.36%, recall of 97.52%, and F-measure of 96.41%. Additionally, scanning and predicting individual samples took an average of 0.37 seconds. This performance indicates success in quickly identifying unknown ransomware variants and adapting the model to the constantly evolving landscape, suggesting its applicability in antivirus protection systems, even on resource-limited devices. Therefore, the method offers significant advantages and can assist developers of ransomware detection systems in creating more resilient, reliable, and fast-response solutions.
Acesso aberto (Open Access)
Abordagem para o desenvolvimento de um etiquetador de alta acurácia para o Português do Brasil
(Universidade Federal do Pará, 2011-10-21) DOMINGUES, Miriam Lúcia Campos Serra; FAVERO, Eloi Luiz; http://lattes.cnpq.br/1497269209026542
Part-of-speech tagging is a basic task required by many applications of natural language processing, such as parsing and machine translation, and by applications of speech processing, for example, speech synthesis. This task consists of tagging words in a sentence with their grammatical categories. Although these applications require taggers with greater precision, the state of the art taggers still achieved accuracy of 96 to 97%. In this thesis, corpus and software resources are investigated for the development of a tagger with accuracy above of that of the state of the art for the Brazilian Portuguese language. Based on a hybrid solution that combines probabilistic tagging with rule-based tagging, the proposed thesis focuses on an exploratory study on the tagging method, size, quality, tag set, and the textual genre of the corpora available for training and testing, and evaluates the disambiguation of new or out-of-vocabulary words found in texts to be tagged. Four corpora were used in experiments: CETENFolha, Bosque CF 7.4, Mac-Morpho, and Selva Científica. The proposed tagging model was based on the use of the method of transformation-based learning (TBL) to which were added three strategies combined in a architecture that integrates the outputs (tagged texts) of two free tools, Treetagger and -TBL, with the modules that were added to the model. In the tagger model trained with Mac-Morpho corpus of journalistic genre, tagging accuracy rates of 98.05% on Mac-Morpho test set and 98.27% on Bosque CF 7.4 were achieved, both of journalistic genres. The performance of the proposed hybrid model tagger was also evaluated in the texts of Selva Científica Corpus, of the scientific genre. Needs of adjustments in the tagger and in corpora were identified and, as result, accuracy rates of 98.07% in Selva Científica, 98.06% in the text set of Mac-Morpho, and 98.30% in the texts of the Bosque CF 7.4 have been achieved. These results are significant because the accuracy rates achieved are higher than those of the state of the art, thus validating the proposed model to obtain a more reliable part-of-speech tagger.
Acesso aberto (Open Access)
Uma abordagem SDN para virtualização de redes
(Universidade Federal do Pará, 2016-11-25) PINHEIRO, Billy Anderson; ABELÉM, Antônio Jorge Gomes; http://lattes.cnpq.br/5376253015721742
The virtual SDN networks (vSDNs) have emerged from the association of virtualization and Software-Dened Networking (SDN), providing greater control and better use of network resources. Several studies have already shown the feasibility and benets of this approach. However, the issue still lacks solutions that can virtualize a network in a scalable, intuitive and simplied manner. Thus, this thesis proposes an SDN approach to network virtualization with the aim of reducing the limitations on vSDNs. Therefore, it is suggested to adopt in network virtualization the separation between data plans (distributed) and control (centralized), the global network view and use of ow abstraction to manage the communication between the dierent points. To support our proposition, three solutions were developed: the CIM-SDN (Common Information Model for Software-Dened Networking), to enable the use of formal representation of the new elements of vSDNs; The NVP (Network Virtualization Proxy), to provide greater scalability by separating the control plane into centralized and decentralized parts; And Graph Virtualization Layer (GVL), to provide greater use of abstractions between the hypervisor and controllers, thereby simplifying the understanding and use of the network. Proof of concept tests was carried out for the three proposed solutions, demonstrating the feasibility of the approach.
Acesso aberto (Open Access)
Algoritmo genético com interação social nebulosa
(Universidade Federal do Pará, 2012-04-19) TEIXEIRA, Otávio Noura; OLIVEIRA, Roberto Célio Limão de; http://lattes.cnpq.br/4497607460894318
This work presents a new new hybrid metaheuristic and bioinspired in nature, based on three main pillars, namely: Genetic Algorithms; Game Theory; and Fuzzy Logic. Thus, the Fuzzy Social Interaction Genetic Algorithm , or F-SIGA, is based and characterized by allowing individuals in the population the possibility to participate in the Social Interaction process. This step is prior to the selection process for the generation of offspring, and in it they can make gains through disputes with other individuals. For this, each individual is characterized by two chromosomes, one related to solving the problem in question; and, the other, with the gene encoding a behavioral strategy. As disputes environment is used the Prisoner's Dilemma game, in 2-person and N-person versions, including the fuzzy approach. In addition, individuals are evaluated by a fitness function that includes: a representation of the problem´ solution, the gains made in disputes and also the experience factor, using the experience acquired by the individual as well as a component to assist in the evolving process of the population. This characteristic gave rise to the ESIA algorithm - not originally planned - where only the information obtained from social interactions are considered in the selection of individuals for reproduction stage. Methodologically, the work evolved into the emergence of the ESIA algorithm, which is a new class of Evolutionary Algorithms based on social interaction. Thus, this work presents four algorithms: SIGA, NpSIGA, F-SIGA and ESIA, with its theoretical foundations and also practical results of applying them to global optimization problems with and without constraints; and the instances of the Travelling Salesman Problem.
Acesso aberto (Open Access)
Algoritmo memético cultural para otimização de problemas de variáveis reais
(Universidade Federal do Pará, 2019-03-29) FREITAS, Carlos Alberto Oliveira de; SILVA, Deam James Azevedo da; http://lattes.cnpq.br/8540875293894747; OLIVEIRA, Roberto Célio Limão de; http://lattes.cnpq.br/4497607460894318
Technology has made great strides in recent years, but computing resources for certain applications need optimization so that the costs involved in solving some problems are not high. There is a very broad area of research for the development of efficient algorithms for multimodal optimization problems. In the last two decades the use of evolutionary algorithms in multimodal optimization has been shown to be a success. Among these evolutionary algorithms, which are global search algorithms, one can cite the use of Cultural Algorithms. A natural enhancement of the Cultural Algorithm is its hybridization with some other local search algorithm, so as to have the advantages of global search combined with local search. However, the local search Cultural Algorithms used for multimodal optimization are not always evaluated by efficient statistical tests. The objective of this work is to analyze the behavior of the Cultural Algorithm, with populations evolved by the Genetic Algorithm, when the local search heuristics are used: Tabu Search, Beam Search, Climbing and Simulated Annealing. One of the contributions of this work was the updating of the topographic knowledge of the cultural algorithm by the use of the triangular area defined by the best results found in the local search. To perform the analysis, a memetic algorithm was developed by hybridizing the cultural algorithm with the local search heuristics mentioned, being selected one at a time. Real world problems usually have multimodal characteristics, so the evaluations were performed using multimodal benchmark functions, which had their results evaluated by non-parametric tests. In addition, the memetic algorithm was tested on real optimization problems with constraints in the engineering areas. In the evaluations carried out, the developed Cultural Algorithm presented better results when compared to the available results of the researched scientific literature.
Acesso aberto (Open Access)
Algoritmos culturais com abordagem memética e multipopulacional aplicados a problemas de otimização
(Universidade Federal do Pará, 2012-04-20) SILVA, Deam James Azevedo da; OLIVEIRA, Roberto Célio Limão de; http://lattes.cnpq.br/4497607460894318
In many optimization problems is hard to reach a good result or a result close to the optimum value in a feasible time, especially when working on large scale. So, many of these problems are addressed by heuristics or metaheuristics running search for better solutions within the defined search space. Within the natural computing algorithms there are the cultural and genetic algorithms. These are evolutionary metaheuristics complement each other due to the dual mechanism of cultural heritage/genetic. The purpose of this paper is to study and use such mechanisms adding local search heuristics and multipopulation applied to combinatorial optimization problems (knapsack and travel salesman problems), constrained problems and multimodal functions. Some experiments have been conducted to assess the performance of the proposed combination of meta-heuristisc and heuristics mechanisms against approaches found in literature as applied to problem addressed here.
Acesso aberto (Open Access)
Alocação ótima de geração distribuída em redes de distribuição utilizando algoritmo híbrido baseado em cuckoo search e algoritmo genético
(Universidade Federal do Pará, 2018-09-02) OLIVEIRA, Victoria Yukie Matsunaga de; AFFONSO, Carolina de Mattos; http://lattes.cnpq.br/2228901515752720
This thesis presents a novel Cuckoo Search (CS) algorithm called Cuckoo-GRN (Cuckoo Search with Genetically Replaced Nests), which incorporates the benefits of genetic algorithm (GA) into the CS algorithm. The proposed method handles the abandoned nests from CS more efficiently by genetically replacing them, significantly improving the performance of the algorithm by establishing optimal balance between diversification and intensification. The algorithm is used for the optimal location and size of distributed generation units in a distribution system, in order to minimise active power losses while improving system voltage stability and voltage profile. The allocation of single and multiple distribution generation units is considered. The proposed algorithm is extensively tested in mathematical benchmark functions as well as in the 33-bus and 119-bus distribution systems. Simulation results show that Cuckoo-GRN can lead to a substantial performance improvement over the original CS algorithm and others techniques currently known in literature, regarding not only the convergence but also the solution accuracy.
Acesso aberto (Open Access)
Análise comparativa das propriedades ressonantes de nanopartículas e de nanoantenas bowtie de ouro de diferentes geometrias
(Universidade Federal do Pará, 2012-12-14) SANTOS, Thaís Lira Tavares dos; COSTA, Karlo Queiroz da; http://lattes.cnpq.br/7932708321834647; DMITRIEV, Victor Alexandrovich; http://lattes.cnpq.br/3139536479960191
This work presents a comparative analysis of the resonant properties of gold nanoparticles and gold bowtie nanoantennas with new triangular geometries. The proposed geometries are as follows: the curved side triangular ones and the triangular geometries with one corner formed by three tips. The investigated properties are the resonant responses, the spatial distributions of the electric near-field and the resonant wavelengths. The current density inside the nanostructures is also analysed for better understanding of the electric near-field enhancement. For the case of bowtie nanoantennas, the research is also focused on the study of the influence of a silicon dioxide substrate on their resonant properties and on the study of their characteristics in the far-field region (scattering cross section and radiation pattern). The numerical results are obtained in the visible and near infrared regions of the electromagnetic spectrum, simulated by the finite integration technique. For isolated nanoparticles, these results show that the suggested geometries have electric near-field intensity around 160% higher and resonant wavelength redshifted by 15%, as compared to the equilateral triangular geometry. In the case of bowtie nanoantennas, the new geometries have electric near-field intensity 90% higher and resonant wavelength blueshifted by 15%, as compared to the triangular equilateral bowtie nanoantenna. The results of this work can serve for modeling, fabrication and designing of gold nanoparticles and gold bowtie nanoantennas for different applications, for example, microscopy and optical fibre sensors.
Acesso aberto (Open Access)
Análise de dados e definição de indicadores para a regulação de usinas termelétricas dos sistemas isolados
(Universidade Federal do Pará, 2006-11-28) CONDE, Cláudio Luciano da Rocha; PINHO, João Tavares; http://lattes.cnpq.br/0847897516772421; BRANCO, Tadeu da Mata Medeiros; http://lattes.cnpq.br/8911039344594817
This thesis proposes an efficient and low cost monitoring system of electrical parameters and fuel consumption of thermal plants of the Brazilian isolated electrical systems, almost all located in the Amazon Region. These systems have many operational problems, usually related to the great distances and lack of data necessary to provide power quality and fuel consumption control. The analysis of the acquired data after the implantation of the considered system in four thermal plants in the State of Pará became possible, among other results, the development of an indicator (IMU) that can be used in the short and average planning studies. The viability of the proposed solution for the operative and regulatory control stimulated the development of a methodology to install monitoring systems all over the isolated systems of Brazil, included in a Brazilian Electricity Regulatory Agency (ANEEL) regulatory act.
Acesso aberto (Open Access)
Análise de desempenho de algoritmos para classificação de sequências representando faltas do tipo curto-circuito em linhas de transmissão de energia elétrica
(Universidade Federal do Pará, 2019-12-05) FREIRE, Jean Carlos Arouche; MORAIS, Jefferson Magalhães de; http://lattes.cnpq.br/5219735119295290; CASTRO, Adriana Rosa Garcez; http://lattes.cnpq.br/5273686389382860
Maintaining power quality in electrical power systems depends on addressing the major disturbances that may arise in their generation, transmission and distribution. Within this context, many studies have been developed aiming to detect and classify short circuit faults in electrical systems through the analysis of the electrical signal behavior. Transmission line fault classification systems can be divided into two types: online and post fault classification systems. In the post-missing scenario the signal sequences to be evaluated for classification have variable length (duration). In sequence classification it is possible to use conventional classifiers such as Artificial Neural Networks, Support Vector Machine, K-nearest neighboors and Random forest. In these cases, the classification process usually requires a sequence preprocessing or a front end stage that converts the raw data into sensitive parameters to feed the classifier, which may increase the computational cost of the classification system. An alternative to this problem is the FBSC-FrameBased-Sequence Classification (FBSC) architecture. The problem with FBSC architecture is that it has many degrees of freedom in designing the model (front end plus classifier) and it should be evaluated using a complete dataset and rigorous methodology to avoid biased conclusions. Considering the importance of using efficient short-circuit fault classification methodologies and mainly with low computational cost, this paper presents the results of the KNN-DTW (K-Nearest Neighbor) algorithm analysis study associated with Dynamic similarity measurement. Time Warping (DTW) and HMM (Hidden Markov Model) algorithm for fault classification task. These two techniques allow the direct use of data without the need for front ends for signal pre-processing, as well as being able to handle multivariate and variable time series, such as signal sequences for the post-miss case. To develop the two proposed systems for classification, simulated data of short-circuit faults from the UFPAFaults public database were used. To compare results with methodologies already presented in the literature for the problem, the FBSC architecture was also evaluated for the same database. In the case of FBSC architecture, different front ends and classifiers were used. The comparative assessment was performed from the measurement of error rate, computational cost and statistical tests. The results showed that the HMM-based classifier was more suitable for the problem of classification of short circuits on transmission lines.
Acesso aberto (Open Access)
Análise de esforços eletromagneto-mecânicos nos enrolamentos de um transformador sob condições de correntes de inrush
(Universidade Federal do Pará, 2016-04-27) FONSECA, Wellington da Silva; SOEIRO, Newton Sure; http://lattes.cnpq.br/7303174583088137; NUNES, Marcus Vinícius Alves; http://lattes.cnpq.br/9533143193581447
This thesis presents an analysis of the effect of electromagnetic origin forces on structural deformation, stress and safety factor in power transformers when they are subjected to inrush currents. The methodology adopted is based on a magneto-mechanical assessment of the transformer using the finite element method (FEM). In this sense, the operating condition of its transformer energization is considered to accurately estimate the magnetic field density in the ferromagnetic structure and to find the values of mechanical stresses or forces in the axial and radial directions on the windings. These components are distributed over energized the high-voltage windings for observing the mechanical loading, which are more susceptible to such current actions. With this goal, it is implemented two- dimensional (2D) and threedimensional (3D) modeling of the discretized transformer structures in consolidated software, which are based on the Finite Elements Method, intended so to obtain more precise results in calculating the variables described. Comparisons between the results from the numerical method with those obtained by analytical methods are made, demonstrating that the FEM provides more precise distribution of stresses along the windings of the transformers when subjected to operating limits conditions as under currents inrush. It is also presented the magnetic flux distribution in the transformer, with high currents circulating in concentric winding of this. It modifies the flux path, causing in a significant increase in the leakage field and consequently on radial and axial forces. The results obtained for the structural behavior of the windings during inrush currents were also seen that the ends of the windings, where the spacers are located, producing high structural weakness due to the axial electromagnetic forces. This also implies a reduction in the safety factor at the ends windings ends and should be considered on the design of power transformers.
Acesso aberto (Open Access)
Análise de estruturas planares em THz baseadas em grafeno
(Universidade Federal do Pará, 2016-01-28) NASCIMENTO, Clerisson Monte do; DMITRIEV, Victor Alexandrovich; http://lattes.cnpq.br/0684541646225359
In this work we analyse the properties of scattering of electromagnetic waves in graphene surfaces and the planar plasmonic based devices made of the same material, both in THz frequency region. The work is presented in form of four scientific papers. In the first one a numerical analysis of the plasmonic waves propagation in graphene elements is performed. The influence of geometrical configuration, chemical potential variation, angle of incidence and polarizations is analysed. That results give us the information to project two devices (the second and third) based on frequency selective surfaces (FSS) on THz range and that are composed only by graphene elements and dielectric substrates without the insertions of different metals. The first device consists in a THz electromagnetic filter made by an planar array of graphene ring-shaped elements placed in both sides of a dielectric substrate. The second device presents new multifunctional graphene device that can operate either as an electromagnetic, dynamically controlled, filter or as an eletromagnectic switch. Both devices operates based on Fano resonance effect. The fourth paper presents a new method of analysis of periodic planar structure, based on group theory approach. This method takes account the transversal and longitudinal components of induced current in the structure. By using this, one can obtain more information about the device properties than by using exitenting methods, which uses only longitudinal components of the induced currents. As application, we suggest an analyse a periodic array of graphene elements.
Acesso aberto (Open Access)
Análise de risco de geração de eletricidade com sistemas fotovoltaicos conectados à rede usando o método de Monte Carlo
(Universidade Federal do Pará, 2015-01-15) PEREIRA, Edinaldo José da Silva; PINHO, João Tavares; http://lattes.cnpq.br/0847897516772421
This work presents a risk analysis of the economic feasibility for energy generation systems, based on the Monte Carlo Method. Initially is presented a vision of the state of the art concerned to the participation of the main sources of renewable energy in the world, in particular in Brazil, in order to highlight the importance of them in the world. The efficient and rational use of energy is evidenced when the design of any energy efficient building, being the GEDAE/UFPA’s building presented as an example of construction that seeks to apply the characteristics of energy efficiency, bioclimatic architecture and environmental comfort. Based in the GEDAE/UFPA’s building, one of its SFCRs is presented and its economic analysis is performed using initially a deterministic approach and later a probabilistic analysis, via Monte Carlo Method. The cost of electric energy produced and the net present value, in a first moment, are used as dependent variables in the last analysis, due their easy interpretation. Modified internal rate of return and the discounted payback are dependent variables used later to better qualify the investor's decision against the risks assumed, since the use of only the first two may lead, as occurred in the case study analyzed, to results with comparable risks between the various alternatives proposed. In Brazil, in front of lack of an effective national policy for development of SFCRs, a national proposal to leverage the use of these systems is presented, quantifying the risk assumed by the investor and by the Federal Government adopting such proposal. The results show that SFCRs in Brazil nearly achieved the parity tariff for three-phase residential consumers. In addition, each year maintenance of the proposed adoption of incentive presented, here called green bonus, over 800 MWp could be added to the brazilian energy matrix. And if in this proposal, were applied a reduction of import taxes or an incentive for the establishment of a national industry that could reach a decrease in acquisition costs of the PV generator/inverter set up to 20%, an annual insertion of almost 1 GWp in the national energy matrix could be achieved, with a payback time for producer lower than six years.
Acesso aberto (Open Access)
Análise de sensores plasmônicos baseados em Nano estruturas estratificadas utilizando o Método das imagens complexas discretas
(Universidade Federal do Pará, 2018-05-11) SOUZA, Nadson Welkson Pereira de; COSTA, Karlo Queiroz da; http://lattes.cnpq.br/7932708321834647
In this thesis, the theoretical analysis of a surface plasmon resonance (SPR) sensor coupled to metallic nanoparticles is presented. The sensor is considered in the Surface Plasmon Coupled Emission (SPCE) configuration, where the metallic nanoparticles (or analytes) are modeled by equivalent dipoles located above the sensor. A complete set of Green’s dyadic functions for the electric field is estimated for planar, multilayer, and uniaxial media in a computationally efficient and convenient manner for applications using the Discrete Complex Imaging Method (DCIM) via the Generalized Pencil of Function Method (GPOF). The principle of complex path deformation in a two-level contour is employed to express an original integral as a sum of complex images. The results related to the surface plasmon resonance, reflection, near and far field of planar sensors are presented and it is described how these nanoparticles influence the ability to increase the near field strength and alter the sensitivity response of these optical sensors. In addition, for comparison purposes, some results are calculated by numerical and experimental methods. The results show suitable agreement, which proves that DCIM is an efficient and powerful theoretical method for analyzing SPR sensors in the SPCE configuration.
Acesso aberto (Open Access)
Análise de sistemas não-lineares e síntese de operadores inversos por séries de volterra diagonais
(Universidade Federal do Pará, 2019-08-22) TEIXEIRA, Raphael Barros; BAYMA, Rafael Suzuki; http://lattes.cnpq.br/6240525080111166; COSTA JÚNIOR, Carlos Tavares da; http://lattes.cnpq.br/6328549183075122
This work proposes innovative strategies for the analysis of nonlinear systems and the synthesis of inverse operators using the Volterra diagonal series. By expressing the output explicitly from the input, the Volterra series enable nonlinear analysis in the frequency domain. However, the multidimensional nature of the model confers several difficulties to its systematic use. This work takes a new look at the so-called Volterra series in diagonal coordinates, in which Volterra operators are expressed as a set of linear and one-dimensional filters that process nonlinear polynomial terms of the input. The proposition of the rational form for these filters leads to exact and compact Volterra models, which exhibit a direct connection with modern nonlinear formalisms, notably the Wiener and Hammerstein block structured models, and the non-linear, autoregressive polynomial models with exogenous input (NARX). In particular, it is proposed a strategy to obtain diagonal Volterra models from the polynomial NARX. The strategy is called derivative method, because it depends only on the established results of the differential calculus. This is important because a NARX model can fit relatively well to experimental data to describe a wide variety of practical systems. A subsequent study through the Volterra series comes as an additional natural step of analysis. This result also opens up possibilities for non-linear synthesis. A problem that has received increasing attention in systems engineering is that of the synthesis of inverse nonlinear operators, through which it tries to reverse distortions generated by the underlying system, preserving the integrity of the information of interest. In this case we propose a strategy of synthesis of Volterra inverse diagonal operators for particular classes of nonlinear polynomial models. It is a numerical approach where the synthesis is driven by an optimization problem that is inspired by the classic inverse p-order operator. Keywords: Non-linear systems, Volterra series diagonals, systems identification, nonlinear analysis, dynamic inversion
Acesso aberto (Open Access)
Análise do enxofre corrosivo em óleo mineral isolante e remoção do dibenzil dissulfeto com nanotubos de carbono baseados em matriz metálica
(Universidade Federal do Pará, 2012-05-07) SARAIVA, Augusto Cesar Fonseca; DEL NERO, Jordan; http://lattes.cnpq.br/5168545718455899
Unexpected failures in transformers led to the identification of copper sulphide formation, deposited on the conductors. The qualitative presence evaluation of corrosive sulfur in insulating mineral oil, led to a change in the Brazilian standards ABNT NBR 10505, which assesses the presence of corrosive sulfur in mineral oil, change in the time and test temperature to 150 °C / 48 hours. Performing the chemical speciation of organosulfur compounds (by gas chromatography coupled to mass spectrometry), were found 13 compounds in the Nynas oil and 9 compounds in the Petrobras oil. DBDS, which was found only in the Nynas oil, and it was the compound with the highest concentration. To perform the determination of the presence of DBDS in insulating mineral oil, a method by gas chromatography coupled to mass spectrometry was developed, and it was possible to quantify the amount of DBDS in insulating mineral oil samples derived from the power transformer. The passivation process by tolutriazol and benzotriazol, used as anticorrosive additives in the core of electrical equipament, was evaluated; after a while the effect of the addition of passivating agent becomes ineffective with time. The solution for the remotionl of DBDS from mineral oil was realized by using carbon nanotube reinforced by metallic matrix as agent to remove the DBDS. It was found that the adsorption agent was able to realize the complete DBDS remotion from insulating mineral oil up to a volume of 4000 mL (oil contaminated with DBD).
Acesso aberto (Open Access)
Análise e modelagem em larga escala para as frequências 8, 9, 10 e 11 ghz em ambientes indoor
(Universidade Federal do Pará, 2019-12-06) BATALHA, Iury da Silva; BARROS, Fabrício José Brito; http://lattes.cnpq.br/9758585938727609; CAVALCANTE, Gervásio Protásio dos Santos; http://lattes.cnpq.br/2265948982068382
Within the context of studies related to radiopropagation, this work presents a proposal for large-scale modeling of propagation loss for 8, 9, 10 and 11 GHz bands in relation to the number of walls, distance and polarization. Measurement campaigns were conducted in the Annex II corridor and in a teaching laboratory located at the Federal University of Pará. The measurement campaign was performed using VV and HH co-polarized directional horn antennas and VH cross polarization nantennas in Line of Sight (LoS) and Line of Sight (NLoS) conditions, the transmitter was fixed within the environment with 0 dBm transmission with VV and HH antenna array and 15 dBm for VH. Directional antennas for transmitter and receiver with 29.3∘ elevation and 29∘ azimuth were used for frequencies 8, 9, 10 and 11 GHz. The Minium Mean Square Error (MMSE) technique was applied to determine the values of the equation parameters as: PLE , XPD, HHPD, and OPLE. The proposed propagation loss model presented satisfactory results compared to the measured data presenting a low standard deviation. A point-to-point standard deviation analysis is also presented within the two environments for the studied frequencies. For the corridor the standard deviation values using polarized V-V antennas were 7, 7.5, 5.6 and 5 dB, and for cross-polarized V-H antennas were 5, 6.2, 2.3 and 3.5 dB for studies frequencies. For the laboratory the values of standard deviation for polarized V-V antennas were 7, 7, 6.5 and 7.3 dB and for polarized H-H antennas were 9.3, 6.1, 6.1 and 6 dB. The polarization loss factor (XPD) presented in the extension of the CIX model for the corridor present values of 19.3, 28.7, 21.3 and 14.3 for the frequencies of 8, 9, 10 and 11 GHz, respectively.
Acesso aberto (Open Access)
Análise e otimização de coberturas de invisibilidade esféricas estratificadas em camadas homogêneas e isotrópicas
(Universidade Federal do Pará, 2012-06-29) MARTINS, Tiago Carvalho; DMITRIEV, Victor Alexandrovich; http://lattes.cnpq.br/3139536479960191
In this work, we analyze and optimize invisibility cloaks stratified in concentric spherical homogeneous and isotropic layers, in which both the total scattering cross section and the number of layers have been minimized. In order to increase the range of frequencies in which there is invisibility, dispersive effects are taken into account. In microwaves, We obtained discretized invisibility cloaks (obtained from anisotropic cloaks) with significant reductions (greater than 20 dB) of the total scattering cross section, for only 20 layers (which is achieved in the literature with at least 80 layers). We obtained a reduction of 32 dB in the total scattering cross section for a cloak stratified in only 13 layers. This result was obtained in microwaves. In microwaves, we optimized dispersive invisibility cloaks which present a bandwidth 5.4 times larger than would be obtained by a optimized cloak without dispersive effects. Cloaks are designed to operate in optical frequencies, for a wide range of frequencies.
Acesso aberto (Open Access)
Análise magnética e mecânica em transformadores sob correntes de energização e energização solidária
(Universidade Federal do Pará, 2019-10-01) LIMA, Diorge de Souza; BEZERRA, Ubiratan Holanda; http://lattes.cnpq.br/6542769654042813
The power transformer is one of the most important equipment in the electric power system, allowing the feasibility of connecting the generating centers to the consumer centers, even over long distances. Reliable and continuous operation is of fundamental importance for service maintenance and is subject to various types of disturbances that can lead to failures. In this perspective, studies of the dynamic behavior of transformer windings through computer simulations have been widely used to safely and accurately evaluate their operation. Therefore, this paper presents the methodology for research on a 50 MVA power transformer using the finite element method for static and time domain analysis. Thus, the study was performed by means of magnetic-mechanical couplings. In the first analysis (circuit study), the ATPDraw software was used to obtain the behavior of the inrush current and solidarity energization during the transformer bank energization. Therefore, in the ANSYS MAXWELL software magnetic studies were performed. For this, a real 3D model was used (taking into account the characteristics of the lamination core and windings, being in disc). Thus, the results of the behavior of magnetic induction and magnetic forces in the windings of the equipment are presented. Finally, in the ANSYS STRUCTURAL software, structural (mechanical) studies were performed. Also, as before, a close-to-real 3D model was used, presenting as results the behavior of the total deformation in the winding, the mechanical stress suffered and the degree of safety during the occurrence of energization. The static studies were considered three operating conditions: nominal condition, sympathetic inrush and inrush current. For the nominal condition, the equipment's plate data was used, for the energizing condition (sympathetic inrush and inrush current) the largest amplitude obtained during the simulation was used. It is noteworthy that for the time domain analysis, only the condition of the inrush current was analyzed, both for the high computational cost required and for being the worst condition in the static analysis.
Acesso aberto (Open Access)
Análise multifísica e experimentos em sensores ópticos usados na medição de temperatura em rotores de hidrogeradores
(Universidade Federal do Pará, 2017-04-12) LEITE, Reinaldo Corrêa; DMITRIEV, Victor Alexandrovich; http://lattes.cnpq.br/0684541646225359
Fiber Bragg gratings (FBGs) offer new possibilities to monitor accurately the rotor temperature. Dozens of sensors can be mounted in series in a single fiber and used to measure the temperature in several points of the rotor winding. Such sensors installed directly on the rotor winding surface are thermally isolated from the cooling air by a silicone layer. Because of the temperature gradient in this structure, the sensor is exposed to thermo-mechanical stresses and therefore can be deformed. Since the FBG probes are sensitive to both temperature and strain, the knowledge of each effect separately is necessary to ensure that the temperature readings are not affected by strain. Experimental results obtained in rotor winding mockup tests with thermistors and FBG sensors show that the temperature readings by the FBG are 4.5°C above the temperature defined by the thermistors which were used as references. Multi-physics simulations were carried out to calculate the strain and temperature in the FBG assembly. The theoretical and experimental results are in a good agreement and show that the temperature gradient across the FBG sensor assembly is sufficient to cause a strain that induces an axial stress in the FBG sensor causing an error in the temperature reading.