The concept of multilevel voltage source inverters and their modulation topologies are described. The concept of the Optimized Harmonic Stepped-Waveform (OHSW) technique for a multilevel inverter is presented. By applying this concept, specific harmonics can be eliminated, and the output voltage THD can be improved. A procedure to achieve the appropriate switching angles of the OHSW is proposed. Experimental results are presented to verify the concept. The proposed OHSW technique is implemented on a multilevel inverter using cascaded-inverter with separated dc sources. Comparison between the Selective Harmonic Eliminated PWM and the OHSW is also presented. / Master of Science

Voltage source inverters have become widely used in the last decade primarily due to the fact that the dangers and limitations of relying on fossil fuel based power generation have been seen and the long term effects felt especially with regards to climate change. Policies and targets have been implemented such as from the United Nations climate change conference (COPxx) concerning human activities that contribute to global warming from individual countries. The most effective way of reducing these greenhouse gases is to turn to renewable energy sources such as the solar, wind etc instead of coal. Converters play the crucial role of converting the renewable source dc power to ac single phase or multiphase. The advancement in research in renewable energy sources and energy storage has made it possible to do things more efficiently than ever before. Regular or 2 level inverters are adequate for low power low voltage applications but have drawbacks when being used in high power high voltage applications as switching components have to be rated upwards and also switch between very high potential differences. To lessen the constraints on the switching components and to reduce the filtering requirements, multilevel inverters (MLI's) are preferred over two level voltage source inverters (VSI's). This thesis discusses the implementation of various types of MLI's and compares four different pulse width modulation (pwm) techniques that are often used in MLI under consideration: three, five, seven and nine level inverters. Harmonic content of the output voltage is recorded across a range of modulation indices for each of the three popular topologies in literature. Output from the inverter is filtered using an L only and an LC filter whose design techniques are presented. A generalized prediction algorithm using machine learning techniques to give the value of the expected THD as the modulation index is varied for a specific topology and PWM switching method is proposed in this study. Simulation and experimental results are produced in five level form to verify and validate the proposed algorithm.

No description available.

The global requirement for sustainable energy provision will become increasingly important over the next fifty years as the environmental effects of fossil fuel use become apparent. Therefore, the issues surrounding integration of renewable energy supplies need to be considered carefully. The focus of this work was the development of an innovative computer aided design of a 3 Dimensional renewable energy platform for Togo’s smart grid power system infrastructure. It demonstrates its validation for industrial, commercial and domestic applications. The Wind, Hydro, and PV system forming our 3 Dimensional renewable energy power generation systems introduces a new path for hybrid systems which extends the system capacities to include, a stable and constant clean energy supply, a reduced harmonic distortion, and an improved power system efficiency. Issues requiring consideration in high percentage renewable energy systems therefore includes the reliability of the supply when intermittent sources of electricity are being used, and the subsequent necessity for storage and back-up generation The adoption of Genetic algorithms in this case was much suited in minimizing the THD as the adoption of the CHB-MLI was ideal for connecting renewable energy sources with an AC grid. Cascaded inverters have also been proposed for use as the main traction drive in electric vehicles, where several batteries or ultra-capacitors are well suited to serve as separate DC sources. The simulation done in various non-linear load conditions showed the proportionality of an integral control based compensating cascaded passive filter thereby balancing the system even in non-linear load conditions. The measured total harmonic distortion of the source currents was found to be 2.36% thereby in compliance with IEEE 519-1992 and IEC 61000-3 standards for harmonics This work has succeeded in developing a more complete tool for analysing the feasibility of integrated renewable energy systems. This will allow informed decisions to be made about the technical feasibility of supply mix and control strategies, plant type, sizing and storage sizing, for any given area and range of supply options. The developed 3D renewable energy platform was examined and evaluated using CAD software analysis and a laboratory base mini test. The initial results showed improvements compared to other hybrid systems and their existing control systems. There was a notable improvement in the dynamic load demand and response, stability of the system with a reduced harmonic distortion. The derivatives of this research therefore proposes an innovative solution and a path for Togo and its intention of switching to renewable energy especially for its smart grid power system infrastructure. It demonstrates its validation for industrial, commercial and domestic applications.

A multilevel Space Vector PWM (SVPWM) technique is developed for a 3-level 3-phase PWM Voltage Source Inverter using a 3-phase coupled inductor to ensure high performance operation. The selection of a suitable PWM switching scheme for the Coupled Inductor Inverter (CII) topology should be based on the dual requirements for a high-quality multilevel PWM output voltage together with the need to minimize high frequency currents and associated losses in the coupled inductor and the inverter switches. Compared to carrier-based multilevel PWM schemes, the space vector techniques provide a wider variety of choices of the available switching states and sequences. The precise identification of pulse placements in the SVPWM method is used to improve the CII performance. The successful operation of the CII topology over the full modulation range relies on selecting switching states where the coupled inductor presents a low winding current ripple and a high effective inductance between the upper and lower switches in each inverter leg. In addition to these requirements, the CII operation is affected by the imbalance inductor common mode dc current. When used efficiently, SVPWM allows for an appropriate balance between the need to properly manage the inductor winding currents and to achieve harmonic performance gains. A number of SVPWM strategies are developed, and suitable switching states are selected for these methods. Employing the interleaved PWM technique by using overlapping switching states, the interleaved Discontinuous SVPWM (DSVPWM) method, compared to other proposed SVPWM methods, doubles the effective switching frequency of the inverter outputs and, as a result, offers superior performance for the CII topology by reducing the inductor losses and switching losses. The inverter operation is examined by means of simulation and experimental testing. The experimental performance comparison is obtained for different PWM switching patterns. The inverter performance is affected by high-frequency inductor current ripple; the excessive inductor losses are reduced by the DSVPWM method. Additional experimental test results are carried out to obtain the inverter performance as a variable frequency drive when operated in steady-state and during transient conditions. The CII topology is shown to have great potential for variable speed drives. / Power Engineering and Power Electronics

The most important advantages of Multilevel Inverters are the absence of a coupling transformer for medium voltage applications and low harmonic current content. In this way, relocatable and economical STATCOM systems can be realized. Complex control algorithms and the isolation problems of measurement devices and power supplies are the main challenging parts of this type of application. In this study, the design, realization, and the performance of a Voltage Source Type Cascaded Multilevel Converter Based STATCOM will be investigated in terms of digital computation, control hardware and the semiconductors devices commercially available in the market. This research work is fully supported by the Public Research Grant Committee (KAMAG) of TUBiTAK within the scope of National Power Quality Project of Turkey with the project No: 105G129.

Multilevel inverters offer many well-known advantages for use in high-voltage and high-power applications, but they are also well suited for low-power applications. A single phase inverter is developed in this paper to deliver power from a residential-scale system of Photovoltaic panels to the utility grid. The single-stage inverter implements a novel control technique for the reversing voltage topology to produce a stepped output waveform. This approach increases the granularity of control over the PV systems, modularizing key components of the inverter and allowing the inverter to extract the maximum power from the systems. The adaptive controller minimizes harmonic distortion in its output and controls the level of reactive power injected to the grid. A computer model of the controller is designed and tested in the MATLAB program Simulink to assess the performance of the controller. To validate the results, the performance of the proposed inverter is compared to that of a comparable voltage-sourced inverter.

No description available.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This Master Thesis presents a study considering multilevel inverters formed from two and three level cascaded inverters. The literature presents several papers on asymmetric multilevel inverters and most studies are related to control and/or modulation techniques considering a binary or trinary topology. Also, there are studies that consider a small variation of the inverters input voltage and analyses the system behavior. However, any of these studies performs analysis to obtain the best relation between input voltage of the multilevel system, while this together with switching angles are capable of generating an output voltage with minimum THD. This Master Thesis was developed aiming to get this voltage relation for multilevel systems formed by two and three level inverters. Furthermore, it seeks to find a pattern for the voltage ratio for several cascaded inverters. To obtain the desired results, techniques of selective harmonic elimination was applied to reduce low order harmonics, reducing the need for output filter. To reduce the computational efforts for obtaining the switching angles, the author used considerations of genetic algorithms. This is necessary due to the significant increase of the variables to be determined as the number of cascaded inverters increase. / Esta dissertação de mestrado apresenta um estudo considerando inversores multiníveis formados a partir de inversores dois e três níveis conectados em cascata. Na literatura são apresentados vários trabalhos envolvendo inversores multiníveis assimétricos. As pesquisas, na sua maioria, tratam sobre técnicas de modulação considerando uma topologia binária ou trinaria. Ainda, são apresentadas pesquisas que consideram uma pequena variação nas tensões de entrada dos inversores e analisam o comportamento do sistema. Contudo, ainda não existe um estudo que realize uma análise para obtenção da melhor relação entre as tensões de entrada do sistema multinível. Sendo que estas relações de tensão, juntamente com os ângulos de comutação, são capazes de gerar uma tensão de saída com uma THD mínima. Esta dissertação foi realizada buscando obter esta relação de tensões para sistemas multiníveis formados a partir de inversores dois e três níveis. Além disso, é realizada a busca de um padrão para esta relação de tensão para n inversores conectados em cascata. Para a obtenção dos resultados desejados foram aplicadas técnicas de eliminação seletiva de harmônicos. Com esta técnica, as harmônicas de baixa ordem são eliminadas, reduzindo a necessidade do filtro de saída. Além disso, considerações sobre algoritmos genéticos são realizadas. Este algoritmo é utilizado para reduzir o esforço computacional para a obtenção dos ângulos de comutação. Isto é necessário devido ao aumento significativo das variáveis a serem determinadas à medida que se eleva o número de inversores conectados em cascata.

A DC to AC inverter can be classified in different topologies; some of these topologies are three level and multilevel inverter. Both types have some advantages and disadvantages. Three level inverters can be applied for low power applications because it is cheaper and has less semiconductor losses at high switching frequencies with poor total harmonic distortion (THD). Multilevel inverters (MLI) can be applied for higher power applications with less THD. However, the MLI has more cost and conductive power losses in comparison with three level inverters. In order to overcome the limitations of three and multilevel topologies, this thesis presents a new controlling topology of multilevel DC/AC inverters. The proposed multilevel inverter topology is based on a current source inverter, which consists of a buck/boost, boost and flyback converters, and an H-bridge inverter. The output voltage of the inverter is shaped through the control of just one main semiconductor switch. This new topology offers almost step-less output voltage without the need for multi DC source or several capacitor banks as in the case of other multilevel inverter topologies. The efficiency of the proposed topology is higher than other inverter topologies for medium power applications (2-10 kW). The proposed topology also generates smaller Total Harmonic Distortion (THD) compared to other inverter topologies. The two main key aspects of the proposed circuit is to keep the switching losses as low as possible and this is achieved through the control of a single switch at relatively low frequency and also to generate an improved AC Voltage waveform without the need for any filtering devices. The output frequency and voltage of the proposed circuit can be easily controlled according to the load requirements. The proposed inverter topology is ideal for the connection of renewable energy; this is due to its flexibility in varying its output voltage without the need of fixed turns-ratio transformers used in existing DC/AC inverter topologies. The harmonic contents of the output of this proposed topology can be controlled without the need of any filter. The simulation and practical implementation of the proposed circuits are presented. The practical and simulation results show excellent correlation.