Projecto e implementa����o de uma fonte de alta tens��o para produ����o de descargas el��ctricas

O objetivo deste trabalho consistiu em projetar, construir e testar um prot��tipo em laborat��rio de uma fonte de alimenta����o de alta tens��o que permita descargas el��tricas {est��veis e de dimens��es reduzidas}, de modo a que possa ser utilizada, dada a sua essencialidade, na fabrica����o de redes de per��odo longo (LPG) em fibra ��tica nos chamados turning points. Estes s��o pontos de elevada sensibilidade, fundamentais no desenvolvimento tecnol��gico de sensores em fibra ��tica, em particular, de sensores refractom��tricos. O prot��tipo da fonte de alimenta����o �� composto por um regulador do tipo BUCK, um inversor para alimenta����o do transformador de alta tens��o, o circuito de realimenta����o e controlo PWM e um microcontrolador para o comando da fonte. Posteriormente procedeu-se �� otimiza����o dos par��metros de descarga, o que conduziu a fabrica����o de redes de per��odo longo com per��odos inferiores a 150 micr��metros. Este �� um resultado sem paralelo a n��vel internacional no que concerne ao uso da t��cnica do arco el��trico.

Modelamento e simula����o de um sistema inversor trif��sico para inje����o de energia na rede de distribui����o a partir de fontes renov��veis do tipo FV

Este trabalho baseia se na necessidade de aumentar as fontes renov��veis de energia, reduzindo assim a depend��ncia de fontes n��o renov��veis, principalmente as poluentes como as de provenientes de combust��veis fosseis. A fonte de energia renov��vel explorada neste trabalho �� a advinda de energia solar, com a utiliza����o de pain��is solares e m��todos de extra����o para converter esta energia em energia el��trica e assim poder utilizar esta energia de forma eficiente. A energia produzida por pain��is fotovoltaicos se apresenta em forma de corrente continua, tendo assim a necessidade do uso de conversores CC-CA, ou ditos inversores de tens��o, para utiliza����o da mesma, j�� que a maioria do equipamentos que utilizam energia el��trica s��o constru��dos em forma a serem abastecidos com energia el��trica em corrente alternada. Como este trabalho foca na inje����o da energia produzida pelos pain��is FV na rede de distribui����o de baixa tens��o, faz se necess��rio o uso de um PLL para garantir que o sistema inversor esteja em sincronismo com a rede de distribui����o e possa garantir a entrega de energia ativa. Por fim mas n��o menos importante, �� utilizado neste projeto t��cnicas de MPPT para garantir um maior aproveitamento da energia proveniente dos pain��is FV, ajudando assim a melhorar a efic��cia deste tipo de energia, sendo mais fi��vel e vi��vel.

Study of vanadium doped ZnO films prepared by dc reactive magnetron sputtering at different substrate temperatures

ZnO films doped with vanadium (ZnO:V) have been prepared by dc reactive magnetron sputtering technique at different substrate temperatures (RT���500 C). The effects of the substrate temperature on ZnO:V films properties have been studied. XRD measurements show that only ZnO polycrystalline structure has been obtained, no V2O5 or VO2 crystal phase can be observed. It has been found that the film prepared at low substrate temperature has a preferred orientation along the (002) direction. As the substrate temperature is increased, the (002) peak intensity decreases. When the substrate temperature reaches the 500 C, the film shows a random orientation. SEM measurements show a clear formation of the nano-grains in the sample surface when the substrate temperature is higher than 400 C. The optical properties of the films have been studied by measuring the specular transmittance. The refractive index has been calculated by fitting the transmittance spectra using OJL model combined with harmonic oscillator.

Effects of sulphurization time on Cu2ZnSnS4 absorbers and thin films solar cells obtained from metallic precursors

We report the results of a study of the sulphurization time effects on Cu2ZnSnS4 absorbers and thin film solar cells prepared from dc-sputtered tackedmetallic precursors. Three different time intervals, 10 min, 30min and 60 min, at maximum sulphurization temperature were considered. The effects of this parameter' change were studied both on the absorber layer properties and on the final solar cell performance. The composition, structure, morphology and thicknesses of the CZTS layers were analyzed. The electrical characterization of the absorber layer was carried out by measuring the transversal electrical resistance of the samples as a function of temperature. This study shows an increase of the conductivity activation energy from 10 meV to 54meV for increasing sulphurization time from 10min to 60min. The solar cells were built with the following structure: SLG/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Ni:Al grid. Several ac response equivalent circuit models were tested to fit impedance measurements. The best results were used to extract the device series and shunt resistances and capacitances. Absorber layer's electronic properties were also determined using the Mott���Schottky method. The results show a decrease of the average acceptor doping density and built-in voltage, from 2.0 1017 cm���3 to 6.5 1015 cm���3 and from 0.71 V to 0.51 V, respectively, with increasing sulphurization time. These results also show an increase of the depletion region width from approximately 90 nm���250 nm.

BabeLO : an extensible converter of programming exercises formats

In the last two decades, there was a proliferation of programming exercise formats that hinders interoperability in automatic assessment. In the lack of a widely accepted standard, a pragmatic solution is to convert content among the existing formats. BabeLO is a programming exercise converter providing services to a network of heterogeneous e-learning systems such as contest management systems, programming exercise authoring tools, evaluation engines and repositories of learning objects. Its main feature is the use of a pivotal format to achieve greater extensibility. This approach simplifies the extension to other formats, just requiring the conversion to and from the pivotal format. This paper starts with an analysis of programming exercise formats representative of the existing diversity. This analysis sets the context for the proposed approach to exercise conversion and to the description of the pivotal data format. The abstract service definition is the basis for the design of BabeLO, its components and web service interface. This paper includes a report on the use of BabeLO in two concrete scenarios: to relocate exercises to a different repository, and to use an evaluation engine in a network of heterogeneous systems.

CuxSnSx+1 (x = 2, 3) thin films grown by sulfurization of metallic precursors deposited by dc magnetron sputtering

We report the results of the growth of Cu-Sn-S ternary chalcogenide compounds by sulfurization of dc magnetron sputtered metallic precursors. Tetragonal Cu2SnS3 forms for a maximum sulfurization temperature of 350 ��C. Cubic Cu2SnS3 is obtained at sulfurization temperatures above 400 ��C. These results are supported by XRD analysis and Raman spectroscopy measurements. The latter analysis shows peaks at 336 cm-1, 351 cm-1 for tetragonal Cu2SnS3, and 303 cm-1, 355 cm-1 for cubic Cu2SnS3. Optical analysis shows that this phase change lowers the band gap from 1.35 eV to 0.98 eV. At higher sulfurization temperatures increased loss of Sn is expected in the sulphide form. As a consequence, higher Cu content ternary compounds like Cu3SnS4 grow. In these conditions, XRD and Raman analysis only detected orthorhombic (Pmn21) phase (petrukite). This compound has Raman peaks at 318 cm-1, 348 cm-1 and 295 cm-1. For a sulfurization temperature of 450 ��C the samples present a multi-phase structure mainly composed by cubic Cu2SnS3 and orthorhombic (Pmn21) Cu3SnS4. For higher temperatures, the samples are single phase and constituted by orthorhombic (Pmn21) Cu3SnS4. Transmittance and reflectance measurements were used to estimate a band gap of 1.60 eV. For comparison we also include the results for Cu2ZnSnS4 obtained using similar growth conditions.

Cu2ZnSnS4 solar cells prepared with sulphurized dc-sputtered stacked metallic precursors

In the present work we report the details of the preparation and characterization results of Cu2ZnSnS4 (CZTS) based solar cells. The CZTS absorber was obtained by sulphurization of dc magnetron sputtered Zn/Sn/Cu precursor layers. The morphology, composition and structure of the absorber layer were studied by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and Raman scattering. The majority carrier type was identified via a hot point probe analysis. The hole density, space charge region width and band gap energy were estimated from the external quantum efficiency measurements. A MoS2 layer that formed during the sulphurization process was also identified and analyzed in this work. The solar cells had the following structure: soda lime glass/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Al grid. The best solar cell showed an opencircuit voltage of 345 mV, a short-circuit current density of 4.42 mA/cm2, a fill factor of 44.29% and an efficiency of 0.68% under illumination in simulated standard test conditions: AM 1.5 and 100 mW/cm2.

Adapting remote labs to learning scenarios: case studies using VISIR and RemotElectLab

Remote laboratories are an emergent technological and pedagogical tool at all education levels, and their widespread use is an important part of their own improvement and evolution. This paper describes several issues encountered on laboratorial classes, on higher education courses, when using remote laboratories based on PXI systems, either using the VISIR system or an alternate in-house solution. Three main issues are presented and explained, all reported by teachers, that gave support to students' use of remote laboratories. The first issue deals with the need to allow students to select the actual place where an ammeter is to be inserted on electric circuits, even incorrectly, therefore emulating real-world difficulties. The second one deals with problems with timing when several measurements are required at short intervals, as in the discharge cycle of a capacitor. In addition, the last issue deals with the use of a multimeter in dc mode when reading ac values, a use that collides with the lab settings. All scenarios are presented and discussed, including the solution found for each case. The conclusion derived from the described work is that the remote laboratories area is an expanding field, where practical use leads to improvement and evolution of the available solutions, requiring a strict cooperation and information-sharing between all actors, i.e., developers, teachers, and students.

Using remote labs to serve different teacher's needs a case study with VISIR and RemotElectLab

Remote Laboratories are an emergent technological and pedagogical tool at all education levels, and their widespread use is an important part of their own improvement and evolution. This paper describes several issues encountered on laboratorial classes, on higher education courses, when using remote laboratories based on PXI systems, either using the VISIR system or an alternate in-house solution. Three main issues are presented and explained, all reported by teachers that gave support to students use of remote laboratories. The first issue deals with the need to allow students to select the actual place where an ammeter is to be inserted on electric circuits, even incorrectly, therefore emulating real world difficulties. The second one deals with problems with timing when several measurements are required at short intervals, as in the discharge cycle of a capacitor. And the last issue deals with the use of a multimeter in DC mode when reading AC values, a use that collides with the lab settings. All scenarios are presented and discussed including the solution found for each case. The conclusion derived from the described work is that the remote laboratories area is an expanding field, where practical use leads to improvement and evolution of the available solutions, requiring a strict cooperation and information sharing between all actors, i.e. developers, teachers and students.

Constru����o de um variador de frequ��ncia

A dificuldade de controlo de um motor de indu����o, bem como o armazenamento de energia CC e posterior utiliza����o como energia alternada promoveram o desenvolvimento de variadores de frequ��ncia e inversores. Assim, como projeto de tese de mestrado em Automa����o e Sistemas surge o desenvolvimento de um variador de frequ��ncia. Para elabora����o do variador de frequ��ncia efetuou-se um estudo sobre as t��cnicas de modula����o utilizadas nos inversores. A t��cnica escolhida e utilizada �� a Sinusoidal Pulse Width Modulation (SPWM). Esta t��cnica baseia-se na modela����o por largura de impulso (PWM), o qual �� formado por compara����o de um sinal de refer��ncia com um sinal de portadora de elevada frequ��ncia. Por sua vez, a topologia escolhida para o inversor corresponde a um Voltage Source Inverter (VSI) de ponte trif��sica completa a tr��s terminais. O desenvolvimento da t��cnica de modula����o SPWM levou ao desenvolvimento de um modelo de simula����o em SIMULINK, o qual permitiu retirar conclus��es sobre os resultados obtidos. Na fase de implementa����o, foram desenvolvidas placas para o funcionamento do variador de frequ��ncia. Assim, numa fase inicial foi desenvolvida a placa de controlo, a qual cont��m a unidade de processamento e que �� respons��vel pela atua����o de Insulated Gate Bipolar Transistors (IGBTs). Para al��m disso, foi desenvolvida uma placa para prote����o dos IGBTs (evitando condu����o simult��nea no mesmo terminal) e uma placa de fontes isoladas para alimenta����o dos circuitos e para atua����o dos IGBTs. Ainda, foi desenvolvida a t��cnica de SPWM em software para a unidade de controlo e finalmente foi desenvolvida uma interface gr��fica para intera����o com o utilizador. A valida����o do projeto foi conseguida atrav��s da varia����o da velocidade do motor de indu����o trif��sico. Para isso, este foi colocado a funcionar a diversas frequ��ncias de funcionamento e a diferentes amplitudes. Para al��m disso, o seu funcionamento foi tamb��m validado utilizando uma carga trif��sica equilibrada de 3 l��mpadas de forma a ser visualizada a varia����o de frequ��ncia e varia����o de amplitude.

DC Fuzzy Multicriteria Approach to Increase the Probability of Delivering Power in Distribution Networks

This paper proposes a methodology to increase the probability of delivering power to any load point through the identification of new investments. The methodology uses a fuzzy set approach to model the uncertainty of outage parameters, load and generation. A DC fuzzy multicriteria optimization model considering the Pareto front and based on mixed integer non-linear optimization programming is developed in order to identify the adequate investments in distribution networks components which allow increasing the probability of delivering power to all customers in the distribution network at the minimum possible cost for the system operator, while minimizing the non supplied energy cost. To illustrate the application of the proposed methodology, the paper includes a case study which considers an 33 bus distribution network.

Increase of the Delivered Power Probability in Distribution Networks using Pareto DC Programming

A methodology to increase the probability of delivering power to any load point through the identification of new investments in distribution network components is proposed in this paper. The method minimizes the investment cost as well as the cost of energy not supplied in the network. A DC optimization model based on mixed integer non-linear programming is developed considering the Pareto front technique in order to identify the adequate investments in distribution networks components which allow increasing the probability of delivering power for any customer in the distribution system at the minimum possible cost for the system operator, while minimizing the energy not supplied cost. Thus, a multi-objective problem is formulated. To illustrate the application of the proposed methodology, the paper includes a case study which considers a 180 bus distribution network

Modela����o e identifica����o de motor DC

O modelo matem��tico de um sistema real permite o conhecimento do seu comportamento din��mico e �� geralmente utilizado em problemas de engenharia. Por vezes os par��metros utilizados pelo modelo s��o desconhecidos ou imprecisos. O envelhecimento e o desgaste do material s��o fatores a ter em conta pois podem causar altera����es no comportamento do sistema real, podendo ser necess��rio efetuar uma nova estima����o dos seus par��metros. Para resolver este problema �� utilizado o software desenvolvido pela empresa MathWorks, nomeadamente, o Matlab e o Simulink, em conjunto com a plataforma Ardu��no cujo Hardware �� open-source. A partir de dados obtidos do sistema real ser�� aplicado um Ajuste de curvas (Curve Fitting) pelo M��todo dos M��nimos Quadrados de forma a aproximar o modelo simulado ao modelo do sistema real. O sistema desenvolvido permite a obten����o de novos valores dos par��metros, de uma forma simples e eficaz, com vista a uma melhor aproxima����o do sistema real em estudo. A solu����o encontrada �� validada com recurso a diferentes sinais de entrada aplicados ao sistema e os seus resultados comparados com os resultados do novo modelo obtido. O desempenho da solu����o encontrada �� avaliado atrav��s do m��todo das somas quadr��ticas dos erros entre resultados obtidos atrav��s de simula����o e resultados obtidos experimentalmente do sistema real.

Planeamento de redes de distribui����o de energia

O planeamento de redes de distribui����o tem como objetivo assegurar a exist��ncia de capacidade nas redes para a fornecimento de energia el��trica com bons n��veis de qualidade de servi��o tendo em conta os fatores econ��micos associados. No ��mbito do trabalho apresentado na presente disserta����o, foi elaborado um modelo de planeamento que determina a configura����o de rede resultante da minimiza����o de custos associados a: 1) perdas por efeito de joule; 2) investimento em novos componentes; 3) energia n��o entregue. A incerteza associada ao valor do consumo de cada carga �� modelada atrav��s de l��gica difusa. O problema de otimiza����o definido �� resolvido pelo m��todo de decomposi����o de benders que contempla dois tr��nsitos de pot��ncias ��timos (modelo DC e modelo AC) no problema mestre e escravo respectivamente para valida����o de restri����es. Foram tamb��m definidos crit��rios de paragem do m��todo de decomposi����o de benders. O modelo proposto classifica-se como programa����o n��o linear inteira mista e foi implementado na ferramenta de otimiza����o General Algebraic Modeling System (GAMS). O modelo desenvolvido tem em conta todos componentes das redes para a otimiza����o do planeamento, conforme podemos analisar nos casos de estudo implementados. Cada caso de estudo �� definido pela varia����o da import��ncia que cada uma das vari��veis do problema toma, tendo em vista cobrir de alguma todos os cen��rios de opera����o expet��veis. Atrav��s destes casos de estudo verifica-se as v��rias configura����es que a rede pode tomar, tendo em conta as import��ncias atribu��das a cada uma das vari��veis, bem como os respetivos custos associados a cada solu����o. Este trabalho oferece um consider��vel contributo no ��mbito do planeamento de redes de distribui����o, pois comporta diferentes vari��veis para a execu����o do mesmo. �� tamb��m um modelo bastante robusto n��o perdendo o ���norte��� no encontro de solu����o para redes de grande dimens��o, com maior n��mero de componentes.

Controlo vetorial (foc) de um motor de indu����o trif��sico aplicado a um ve��culo el��trico

Os motores de indu����o (MI) com rotor em gaiola de esquilo est��o muito disseminados nos atuais sistemas de varia����o de velocidade (���drives���). A sua simplicidade e robustez, aliadas a baixos pre��os (comparativamente com outras m��quinas) e ampla gama de pot��ncias dispon��veis, s��o as principais raz��es. A evolu����o verificada nos dom��nios da eletr��nica de pot��ncia e nos sistemas de controlo (em particular, o controlo digital), permitiram aplicar os MI em sistemas de elevado desempenho din��mico (e.g., controlo de bin��rio e/ou posi����o), substituindo os motores DC, cujas caracter��sticas din��micas e simplicidade de controlo os tornavam a primeira escolha em tais aplica����es. Com efeito, as ���drives��� baseadas em MI apresentam caracter��sticas din��micas em tudo semelhantes aos sistemas DC, incluindo a possibilidade de funcionamento nos quatro quadrantes do plano (T, nr). No entanto, a complexidade do conversores e, sobretudo, dos sistemas de controlo �� muito mais elevada nos sistemas AC. Os sistemas baseados no controlo vectorial s��o os mais usuais nas ���drives��� baseadas nas m��quinas AC convencionais (ass��ncronas e s��ncronas). Existem outras metodologias tamb��m usadas na ind��stria (e.g., controlo direto do bin��rio ��� DTC), mas neste trabalho somente o controlo vetorial ser�� abordado.