RCAAP Repository

Alternative energy sources have for some time attracted great interest in the area of static converter development. This fact is related in greater part to issues such as sustainability and detrimental effects on the natural environment, which all contribute to the viability of this type of energy source. In this context, power electronics performs important tasks making viable the connection of all these kind of clean power sources to the conventional grid and also to the load. From this perspective, a new challenge must be faced which is the development of energy management systems capable of providing intelligent planning and control of appliances in low- and high-power applications. That being so, this Ph.D thesis intends to contribute presenting a novel dual transformerless single-stage current source inverter fed by a proton exchange membrane fuel cell (PEMFC) and a photovoltaic (PV) array. High voltage gain and totally controlled output voltage can be achieved without using dedicated step-up dc dc converters or transformers either. The main feature of this inverter structure is the intelligent power management technique which focuses on the extraction of maximum power from the PV array keeping the PEMFC as an energy storage system. Theoretical analysis is presented and corroborated by experimental results of a laboratory prototype.

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This work presents the analysis and experimental development of a topological structure of hybrid rectifier able to maintain a regulated output DC link for the occurrence of balanced and unbalanced voltage sags in the AC mains supply. Moreover, under normal conditions of the supply voltage, the structure operates draining currents with low harmonic content and high power factor. It is noteworthy that these operational characteristics are achieved with the switched converters that comprises the structure processing only a portion of total power output, making the most efficient and robust set. The new topology is denominated Three Phase Hybrid Rectifier with Series Compensation DC Bus and has as main characteristic an innovative voltage regulation technique based on insertion of a DC-DC converter with output capacitor connected in series with the capacitor available in the DC bus of the main converter. This additional converter is responsible for regulating the DC link voltage according to a specific reference value, operating only in the voltage sag condition and processing only a portion of the total output power drained by the load in this operating condition. To analyze the behavior of the proposed topology has been implemented in a computer model PSIM® platform, it was possible to verify the efficiency of the control strategy for sinusoidal current imposition based on the real-time calculation of DHTI during normal conditions of supply voltage. In addition, It was evaluated the behavior of the structure regulating the DC link voltage on the occurrence of voltage sags. In order to corroborate the exposed theory a 5 kW prototype was constructed and evaluated in the laboratory. This was tested in the regular voltage supply condition and during the occurrence of sag conditions types A, B, C, D, E, F and G. It was observed that in normal conditions the proposed solution is effective in mitigating harmonic content of the AC input current. During the occurrence of voltage sags the output DC link was set at the preset value in the control strategy. In both cases the switched converters processed only a fraction of the total power output, which makes the structure very attractive for applications with high power levels.

Fundação de Amparo a Pesquisa do Estado de Minas Gerais

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

Power transformers are quite costly and essential to provide reliable electrical power system operation. Besides their maintenance or substitution costs, transformer failures must be taken into account, since its will have a large impact on the utility financial health due to the temporary loss of power delivery capability. Concerning transformer failure statistics, investigations carried out in many utilities in the world reveal that the effect of electromechanical stress caused by short-circuit currents is a relevant cause of failure in such equipment and they cause onerous financial damage. Failures caused by mechanical stress due to external short-circuit and due to inrush currents are an important aspect to be considered. The excessive strength caused in transformer conductors/windings due to electromagnetic forces can reduce the transformer lifetime or even cause irreversible damages of them. Therefore, the investigation of the harmful effects caused by transient phenomena becomes imperative. With this in mind, this work aims at investigating the electromagnetic forces and mechanical stresses due to external short-circuit and inrush currents inside the transformer. The studies are carried out using a time domain transformer model based on magnetomotive forces and magnetic reluctances, which allows simulating the transformer transient and steady state behavior regarding the electric, magnetic and mechanical aspects. The methodology is applied in two transformer models operating under rated and short-circuit conditions. Due to the lack of mechanical stress experimental values, a comparative performance analysis is obtained by comparing the simulated results and the well accepted results from finite element program. The results obtained from simulations are evaluated through of the impacts provoked in the variables used to analyze the mechanical stresses which occur in the transformers due to short-circuit and inrush currents. From the mechanical stress calculated it is presented a methodology that establishes a correlation between the phenomena here investigated and the impact in the transformer lifetime. This can assist, previously, in the reduction of the number of unexpected failures and, consequently, in financial damages.

This paper presents the study of Parallel Multiobjective Evolutionary Algorithms to enable the automation of exterior lighting designs by computers and results in an optimized version of the algorithm. The resulting algorithm basically works with variable length chromosomes and for which intrinsic operators of crossover and mutation were created. The fitness function was determined through a statistical evaluation method (difference of means), thus enabling the comparison of how different options of fitness functions could impact the performance of the proposed parallel multi-objective evolutionary algorithm. The chosen fitness function enables to develop more efficiently automated designs for exterior lighting. Moreover, adding to the proposed evolutionary algorithm, an application was developed in which the user chooses which the heights of the poles, lamps and fixtures to use and also the layout of the area to be illuminated (allowed to be irregular). Within this area, can be defined sub-areas where there are restrictions on the placement of lighting poles. The user must be set average illumination with a respective tolerance range, though. As a case study, the area of an airport parking lot in the city of Uberlândia-MG (Brazil) is presented. Evolved designs present a low coefficient of variation evaluated for 30 runs. This demonstrates that the system is converging on designs for similar metrics. By identifying the worst and the best of designs achieved by the system for those executions, one could note that there are savings regarding installed capacity when compared to the design of reference: 37.5 % for the worst evolved design and 50.0 % for the best evolved design. Also, evolved designs have better lighting uniformity and energy efficiency, as well as their respective quantities of lighting poles have decreased.

The changes brought by the constant technological evolution, improvement of production processes with high competitiveness among the companies form a scenario where people are subjected to stressful situations. Occupational stress affects people regardless of age, gender and role within the company. Governments and companies expend a lot of resources because of sick leave, benefit payments, troubled professional relationships and low levels of productivity due to the effects of stress at work. In addition, prolonged exposure to stress and the lack of mechanisms to manage it can cause serious health problems of people. On the other hand, current research shows that digital games have positive potential for coping with stress. However, systems or stress management programs that suggest the action of playing as a way to lower stress levels, do not specify which games should be used. This choice is sometimes in charge of the patient himself. Playing at the workplace is different from playing at home. It is important that the choice of games for coping with occupational stress is made based on workplace environmental characteristics, employee, game and supervised by a Psychologist to avoid the unwanted consequences of using these games. Thus, there is a need to provide psychologic tools that assist them in such task. Researches in the context of digital games and stress have focused on proving that digital games can be used when coping with stress. However, no studies were found to address this goal. This study therefore aims to propose a strategy for the development of search engines and digital entertainment games for coping with occupational stress. The strategy proposed here is based on the relationship among the information coping strategies, personality traits, games preferences, stressors and restrictions at work environment. This information characterizes the company and the employee in the context of occupational stress. The strategy can be used as a guide in the development of tools to assist the Psychologist in the task of suggesting game.

The computational and empirical modelling of geophysical phenomena, such as F region zonal plasma drifts, is a basic tool for the understanding and forecasting of effects that have great social-economic impact on the human activities. This work proposes, develops and implements a model, based on Computer Aided Geometric Design techniques, to F region zonal plasma drifts over Jicamarca, Peru. Amongst the developed models, that which shows itself as having the greatest success uses Bézier curve fitting with least-square methods. Other models used, such as wavelet theory, minimax and Kalman filters, had also been attempted, but without the same success. The main data source was the incoherent scatter radar measurements installed in the Jicamarca Radio Observatory. This data was preprocessed in order to carry out error handling and to improve its statistical quality. The preprocessed data formed the basis to the quiet and disturbed time zonal dritfs models construction. The models had been implemented in the functional programming language Objective Caml, an efficient, concise and well tailored programming language in numerical calculus.

Conselho Nacional de Desenvolvimento Científico e Tecnológico

Conselho Nacional de Desenvolvimento Científico e Tecnológico

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

Conselho Nacional de Desenvolvimento Científico e Tecnológico

Conselho Nacional de Desenvolvimento Científico e Tecnológico

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

The next generation of high-speed mobile/wireless networks will support multimedia applications. The support of such applications imposes Quality of Service (QoS) requirements that have been considered very restrictive in this network environment due to the difficulty of managing the scarce bandwidth, the droppings because of handoff and blocking calls. Thus, in this work, we propose alternative schemes of CAC (Call Admission Control) combined with adaptive bandwidth reservation schemes of adjacent cells, in an end-to-end approach. The results obtained from performance studies based on modeling and simulation, demonstrated that the proposed schemes called Directed Reserve (RDir) and Total Reserve (RT), have presented the lowest CDP (Call Dropping Probability), when compared with similar schemes described in the literature. Thus, these schemes are capable to promote the maintenance of the accepted calls within the network and improve the quality of service of the cellular mobile networks with support to multimedia traffic

Many computer algebra systems have already been proposed and implemented. Most of them are implemented in or even implement languages without the referential transparency property, making it difficult, if not impractical, to reason about algebra programs. This thesis presents a computer algebra system implemented as a library in the Haskell programming language, a modern functional language with the desired referential transparency property. The author presents the foundations and basic algorithms for manipulation of algebraic expressions in a declarative context, compatible with Mathematics. The adequacy of the constructs provided by the Haskell programming language is considered when implementing such a library in a modular way, so that it can be easily extended with the inclusion of new algebraic formulas and manipulations. Such extensions should keep compatibility with prior versions of the library. This work also contributes for showing that modern functional languages like Haskell are viable for day to day programming, even beating conventional languages in some aspects, like level of abstraction.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior