Aportaciones a las estrategias de control y técnicas de modulación avanzadas de convertidores electrónicos de potencia conectados a la red eléctrica
- Autores: Sergio Vázquez
- Directores de la Tesis: Juan Manuel Carrasco Solís (dir. tes.), Eduardo Galván Díez (codir. tes.)
- Lectura: En la Universidad de Sevilla ( España ) en 2010
- Idioma: español
- Tribunal Calificador de la Tesis: Pedro Rodríguez Cortés (presid.), José Ignacio León Galván (secret.), Mariusz Malinowski (voc.), Patrick Wheeler (voc.), Remus Teodorescu (voc.)
- Materias:
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- Resumen
Multilevel converters are very interesting technologies especially useful for high power and high quality power applications. They provide high voltage operating capability, low common mode voltages, reduced voltage derivatives (dv/dt), voltages with reduced harmonic contents, near sinusoidal currents, small input and output filters, increased efficiency and in some cases, possible fault tolerant operation, [1]. For these reasons, in recent years several multilevel topologies have been introduced. Among them, neutral point clamped (NPC), flying capacitor (FC) and cascaded Hbridge (CHB) converters have become standard for multilevel systems. These topologies are suitable for specific applications due to their own advantages and disadvantages. For instance, CHB converters have been used intensively in motor drives and STATCOMs, because they can easily handle high voltages with a very large number of output levels but using a minimum number of power switches. However, they need isolated dc sources for each dc-link and this complicates the converter design for back to back applications.
Despite the industrial presence and recognizable maturity, [2], there are still several challenges for the further development of these technologies, [3]. The most important issue is to improve the efficiency of multilevel converters because they are conceived to work with high power rating. Three different loss sources can be considered, conduction, switching and harmonic losses.
Conduction losses are related to the converter topology. Once the topology is chosen, the number of power devices connected in series for a given switching state is fixed. Therefore, a good selection of the converter topology is crucial in the system design stage.
Switching and harmonic losses mainly depend on the control algorithm and the modulation technique used to operate the converter. Both can be reduced drastically if these strategies are designed and chosen suitably.
The reduction of the converter losses has several benefits. The first one is the achievement of higher performance converters, allowing the use of simpler cooling systems that are usually cheaper, smaller and lighter. Besides, if the current harmonics are diminished, it is possible to use smaller passive components for the smoothing filters of the converter. Thus lower losses yield converters with reduced size and weight components and this, in general, facilitates the construction of cheaper systems.
This thesis is focused in the study of two-cell single-phase cascaded H-bridge converters (2C-CHB). The work is aimed to cover the three stages that can provide multilevel systems with higher performance. In this way, the system design, a control strategy and several modulation techniques are taken into account. Simulation and practical results are provided to demonstrate the outcomes from the presented theoretical analysis.
The work is organized as follows: Part I provides an introduction about multilevel technologies. It describes the state of the art of these technologies, presenting several topologies, modulation strategies and industry application of multilevel converter.
Part II is devoted to the study of current controls applied to grid connected 2C-CHB. Two different issues are analyzed, the power exchange between the converter and the grid and the controller design process. In Chapter 2 the power exchange process between the cells of a 2C-CHB and the grid is analyzed. From this analysis, the boundaries of the system are addressed, providing a useful tool to the design stage of this kind of systems. Chapter 3 deals with the controller design process. As it is shown in Chapter 3, the whole system controller is composed of three control loops. New controllers are proposed for these loops and all of them are validated through simulations, addressing key issues to the definition of the controller design constants. Besides, the performance achieved with the proposed controller and with conventional ones are compared, showing that the proposed algorithm improves the behavior of the system in all aspects.
Part III is focused in the modulation strategy design for the 2C-CHB. As a result, a new modulation algorithm is proposed. The high performance of the proposed modulation is demonstrated through simulation and practical results and the feasibility of the technique is shown by providing modulation strategies to symmetric and asymmetric 2C-CHB. The proposed method can be compared with PWM and SVM techniques. For this purpose, the use of the redundant state of the converter to achieve the control of the dc-link voltage is also presented and the feedforward operation of the proposed modulation strategy is defined. In both cases it is shown that the algorithm is simple.
Finally, some conclusions are drawn at the end of the work.
In the author's opinion, the following aspects are the main contributions of this thesis:
Analysis of the functional boundaries of the 2C-CHB, [4]. As a result, the limits of this topology have been addressed, providing a useful tool to future systems design.
Design of a repetitive current controller for the 2C-CHB. This controller includes a new scheme to control the dc-link voltage ratio, [5]. The proposed controller provides high current tracking performance, reducing the harmonic contents of the currents and yielding unity power factor systems. Therefore, it allows the power converter losses to be reduced and facilitate the construction of high performance multilevel converters.
Design and implementation of a new family of modulation techniques. These modulation strategies have been proved in several multilevel converter topologies including NPC, FC and symmetric and asymmetric CHB converters. Besides, they can be used with any number of levels and any number of phases, [6]-[13]. It has been demonstrated that 1-dimensional modulation techniques (1DM) provides high performance output voltages with very low harmonic content in the low order frequency. Therefore, it facilitates the design of high performance power electronics systems.
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[12]. J. I. Leon, S. Vazquez, S. Kouro, L. G. Franquelo, J. M. Carrasco, and J. Rodriguez, "Unidimensional modulation technique for cascaded multilevel converters," IEEE Transactions on Industrial Electronics, vol. 56, no. 8, pp. 2981-2986, August 2009.
[13]. J.I. Leon, S. Vazquez, J. Sanchez, R. Portillo, L.G. Franquelo, J.M. Carrasco and E. Dominguez, "Conventional Space Vector Modulation Techniques versus the Single-Phase Modulator for Multilevel Converters," IEEE Transactions on Industrial Electronics, vol.57, no.7, pp.2473-2482, July 2010.
