Photovoltaic (PV) system is one of the most prominent energy sources, producing electricity directly from sunlight. In additionally, it is easy to install and is supported financially by many governments as part of their strategy to reduce CO2 gas emissions, and to achieve their agreed set of reduction targets by 2020. In the meantime, researchers have been working on the PV system to make it more efficient, easy to maintain, reliable to use and cost effective. In the stand-alone PV system, a battery is required. This is due to the fluctuating nature of the output energy delivered by the PV arrays owing to the weather conditions and the unpredictable behaviour of uses with regard to the consumption of energy. During the hours of sunshine, the PV system is directly feeding the load and any surplus electrical energy is stored in the battery at a constant current. During the night, or during a period of low solar irradiation, the energy is supplied to the load from the battery. However, the stand-alone PV system is designed to provide an acceptable balance between reliability and cost, which is a major challenge to the designer owing to the approaches used to size the PV arrays and the battery bank. As a result, the unpredictable, quick daily changes on the PV output is not dependable. Moreover, battery performance, length of life and energy efficiency depends on the rate at which it is discharged. Therefore, it is essential to use other methods to deal with any quick variation in energy. In this thesis, a super capacitor is used to solve this problem, as it can deal with the fast-changing weather, or a rapid variation in the energy requirements of the customer. A critical evaluation with in-depth analysis of the placement and the implementation for the super-capacitor in the PV standalone system has been carried out. The results show, super-capacitor capacitance and the converter efficiency affect the delivered load energy. However, the bi-directional topology performs better than uni-directional under the same conditions. Finally, a further improvement of the system at component level, has been developed through an energy recovery snubber for the switching transition and achieved a recovery of energy for the resistive load, 94.44% for the turn on transition and 92.86% for the turn off transition. Moreover, for the inductive load, 78.33% and 97.33% of energy has been recovered for the turn on and for the turn off transition respectively.

Pulse-width modulation methods are widely used for the synthesis of ac voltages at the terminals of a voltage-sourced converter (VSC). Traditionally sinusoidal pulsewidth modulation (SPWM) has been used. A powerful alternative for this purpose is space-vector modulation (SVM), in which the converter is placed in a finite number of states in order to best approximate the reference voltage. This method offers better utilization of the dc bus voltage and provides several degrees of freedom for enhancement of the harmonic spectrum as well as switching losses. This thesis studies the SVM method for two- and three-level VSCs. A model for implementation of SVM in the electromagnetic transients simulation program PSCAD/EMTDC is developed. The model is able to generate firing pulses in linear as well as overmodulation range and is used to study the performance of different SVM strategies in terms of their harmonic spectra and associated converter and harmonic losses. The model is also used to demonstrate the suitability of the method for network applications. The thesis also employs genetic algorithms to find an optimized SVM sequence for improved harmonic performance. An objective function is defined that seeks to minimize the most significant harmonic components of the generated waveform, while keeping the other harmonic components within the acceptable range outlined in the available standards. The obtained sequence shows great improvement over the conventionally-used SVM sequence. / October 2007

DC to DC step-down applications with high input voltage and high voltage conversion ratio operational requirements, such as photovoltaic battery chargers, are subject to high conduction losses, high switching losses and substantial reverse-recovery losses when minority carrier principle diodes are used. The recent introduction of silicon carbide diodes with high breakdown voltages has made possible the elimination of reverse-recovery losses at high voltage levels and as such has sparked interest in their use due to the potential efficiency improvements. This report presents the results of a comprehensive analysis on the use of silicon carbide diodes and their counterparts, ultrafast silicon diodes, in conventional buck converters and isolated current-fed buck converters in high input voltage and high voltage conversion ratio step-down applications. The analysis illustrates both theoretically, with the use of steady-state average models, and experimentally the substantial efficiency benefits of the use of reverse-recovery free silicon carbide diodes in the conventional buck converter and the small but significant improvement in the efficiency of the isolated current-fed buck converter. The improvements of the conventional buck converter paired with silicon carbide diodes are shown to be significant enough to grant the variant the most efficient position for power levels below 1 kW. In addition, the four variants are categorized based on their cost and performance; therefore, providing engineers with a convenient guide to aid their selection of the appropriate converter depending on the operational requirements.

Pulse-width modulation methods are widely used for the synthesis of ac voltages at the terminals of a voltage-sourced converter (VSC). Traditionally sinusoidal pulsewidth modulation (SPWM) has been used. A powerful alternative for this purpose is space-vector modulation (SVM), in which the converter is placed in a finite number of states in order to best approximate the reference voltage. This method offers better utilization of the dc bus voltage and provides several degrees of freedom for enhancement of the harmonic spectrum as well as switching losses. This thesis studies the SVM method for two- and three-level VSCs. A model for implementation of SVM in the electromagnetic transients simulation program PSCAD/EMTDC is developed. The model is able to generate firing pulses in linear as well as overmodulation range and is used to study the performance of different SVM strategies in terms of their harmonic spectra and associated converter and harmonic losses. The model is also used to demonstrate the suitability of the method for network applications. The thesis also employs genetic algorithms to find an optimized SVM sequence for improved harmonic performance. An objective function is defined that seeks to minimize the most significant harmonic components of the generated waveform, while keeping the other harmonic components within the acceptable range outlined in the available standards. The obtained sequence shows great improvement over the conventionally-used SVM sequence.

Power inverters, used to convert DC power to AC, are often used in e.g. solar power applications. However, they tend to be impractically large and expensive; as such, power miniaturization is an active research area. In this thesis, several classes of modern power inverters are evaluated and compared with regards to size, efficiency and output quality in order to identify areas of potential improvement. Methods for estimation of THD, power losses and input ripple are created and verified against a simulation of a five-level neutral-point-clamped inverter with SPWM control. Finally, this design is implemented physically and is found to achieve 94.5% efficiency and 7% THD under low voltage laboratory conditions, while remaining smaller than an average textbook.

Pulse-width modulation methods are widely used for the synthesis of ac voltages at the terminals of a voltage-sourced converter (VSC). Traditionally sinusoidal pulsewidth modulation (SPWM) has been used. A powerful alternative for this purpose is space-vector modulation (SVM), in which the converter is placed in a finite number of states in order to best approximate the reference voltage. This method offers better utilization of the dc bus voltage and provides several degrees of freedom for enhancement of the harmonic spectrum as well as switching losses. This thesis studies the SVM method for two- and three-level VSCs. A model for implementation of SVM in the electromagnetic transients simulation program PSCAD/EMTDC is developed. The model is able to generate firing pulses in linear as well as overmodulation range and is used to study the performance of different SVM strategies in terms of their harmonic spectra and associated converter and harmonic losses. The model is also used to demonstrate the suitability of the method for network applications. The thesis also employs genetic algorithms to find an optimized SVM sequence for improved harmonic performance. An objective function is defined that seeks to minimize the most significant harmonic components of the generated waveform, while keeping the other harmonic components within the acceptable range outlined in the available standards. The obtained sequence shows great improvement over the conventionally-used SVM sequence.

DC to DC step-down applications with high input voltage and high voltage conversion ratio operational requirements, such as photovoltaic battery chargers, are subject to high conduction losses, high switching losses and substantial reverse-recovery losses when minority carrier principle diodes are used. The recent introduction of silicon carbide diodes with high breakdown voltages has made possible the elimination of reverse-recovery losses at high voltage levels and as such has sparked interest in their use due to the potential efficiency improvements. This report presents the results of a comprehensive analysis on the use of silicon carbide diodes and their counterparts, ultrafast silicon diodes, in conventional buck converters and isolated current-fed buck converters in high input voltage and high voltage conversion ratio step-down applications. The analysis illustrates both theoretically, with the use of steady-state average models, and experimentally the substantial efficiency benefits of the use of reverse-recovery free silicon carbide diodes in the conventional buck converter and the small but significant improvement in the efficiency of the isolated current-fed buck converter. The improvements of the conventional buck converter paired with silicon carbide diodes are shown to be significant enough to grant the variant the most efficient position for power levels below 1 kW. In addition, the four variants are categorized based on their cost and performance; therefore, providing engineers with a convenient guide to aid their selection of the appropriate converter depending on the operational requirements.

DC to DC step-down applications with high input voltage and high voltage conversion ratio operational requirements, such as photovoltaic battery chargers, are subject to high conduction losses, high switching losses and substantial reverse-recovery losses when minority carrier principle diodes are used. The recent introduction of silicon carbide diodes with high breakdown voltages has made possible the elimination of reverse-recovery losses at high voltage levels and as such has sparked interest in their use due to the potential efficiency improvements. This report presents the results of a comprehensive analysis on the use of silicon carbide diodes and their counterparts, ultrafast silicon diodes, in conventional buck converters and isolated current-fed buck converters in high input voltage and high voltage conversion ratio step-down applications. The analysis illustrates both theoretically, with the use of steady-state average models, and experimentally the substantial efficiency benefits of the use of reverse-recovery free silicon carbide diodes in the conventional buck converter and the small but significant improvement in the efficiency of the isolated current-fed buck converter. The improvements of the conventional buck converter paired with silicon carbide diodes are shown to be significant enough to grant the variant the most efficient position for power levels below 1 kW. In addition, the four variants are categorized based on their cost and performance; therefore, providing engineers with a convenient guide to aid their selection of the appropriate converter depending on the operational requirements. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

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Submitted by Irene Nascimento (irene.kessia@ufpe.br) on 2016-11-08T17:29:28Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertacao_Felipe_Correa_CD.pdf: 5991890 bytes, checksum: bf897f6d56367daee94655b05dc520e3 (MD5) / Made available in DSpace on 2016-11-08T17:29:28Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertacao_Felipe_Correa_CD.pdf: 5991890 bytes, checksum: bf897f6d56367daee94655b05dc520e3 (MD5) Previous issue date: 2015-07-31 / Capes / O conversor nove chaves (Nine-Switch Inverter - NSI) foi proposto recentemente e, desde então, um grande número de aplicações foi explorado para esta nova topologia, especialmente como um possível substituto do tradicional conversor back-to-back. A principal vantagem do NSI é o menor número de chaves (nove em vez de doze do conversor back-to-back), que, como consequência, acarreta em restrições nas amplitudes de saída do inversor. Na literatura, foram propostas diferentes técnicas de modulação por largura de pulso (Pulse Width Modulation - PWM) que respeitam estas restrições. Diante deste cenário, o presente trabalho propõe duas abordagens para a generalização da modulação no NSI: a modulação escalar quasi generalizada e a generalizada, que são baseadas no conceito de generalização para o inversor trifásico fonte de tensão (VSI) convencional e que podem ser facilmente implementadas com um reduzido esforço computacional. Este conceito propicia, através de uma abordagem sistemática e direta, a geração de qualquer técnica de PWM, seja ela contínua ou descontínua, através do ajuste de graus de liberdade específicos. Objetivando a redução de perdas, são apresentados os graus de liberdade específicos que, quando aplicados nas modulações propostas, aumentam o rendimento do inversor. Resultados de simulação confirmam a funcionalidade das modulações. / The nine-switch inverter (NSI) has been proposed recently and, since then, a large number of applications were explored for this new topology, specially as a possible substitute to the traditional back-to-back converter. The main advantage of the NSI is its lesser number of switches (nine instead of twelve of the back-to-back converter), which, as a consequence, leads to some restrictions in the total attainable amplitude at its outputs. In literature, many pulse width modulation (PWM) techniques were proposed respecting those restrictions. In this scenario, this paper proposes two generalization approaches for the modulation of the NSI: the quasi generalized scalar modulation and the generalized scalar modulation, which are based on the generalization concept for the voltage source inverter (VSI), easily implemented with a reduced computational effort. Such concept leads to a systematic and straight approach to the generation of any continuous or discontinuous PWM technique for the NSI, simply adjusting a few degrees of freedom of the modulation strategy. Aiming the reduction of the power losses, specific degrees of freedom are presented, for the proposed modulations. Simulation results confirm the validity of the proposed methods.