Symmetrical multilevel converters with two quadrant DC-DC feeding

In the technology sector of power electronics and control, the multilevel converter technology is still a rather new research area, but the application possibilities in the field of power drives and energy will demand more solutions with this promising technology. In the future, more converter systems will be realized with the multilevel topology. Up to now, multilevel converters have only been used in very particular applications, mainly due to the high costs and complexity of the multilevel converter system. The high costs are due to the fact that the latest technology on semiconductors, magnetic material for inductor and transformer cores and control system technology had to be used. But nowadays new developments in the fields of power semiconductors such as the IGBT, IGCT and perhaps in the future SiC switches as well as improvements of the performance of magnetic cores used in medium frequency transformers will favor the multilevel converters for many other application fields. It can be noted that the industrial trend is moving away from heavy and bulky passive components towards power converter systems using more and more semiconductor elements controlled by powerful processor systems integrating intelligent multi-task control algorithms. The presented work is a contribution to the large field of multilevel converters. It shows a certain kind of multilevel converter in a single phase and a three-phase configuration, called the series-connected four-quadrant converters (SCFQ). The two specialties of the presented converter type are a) that all the multilevel converter steps are fed by an identical DC voltage and b) that every multilevel converter step is realized with an individual AC-DC converter or four-quadrant converter. This type of multilevel converter is called multilevel converter with symmetrical feeding. In this work, a general theoretical development has been done for the use of this multilevel converter type. A special type of DC-DC converter is presented, in order to feed the individual four-quadrant converters of the multilevel converter with a constant DC voltage. All the developments and methods used are based on mathematical expressions. Various simulations using the latest software simulation tools are accomplished and are used to study different cases. The feasibility of the developments is underlined with a series of experimental results with all types of the used converters, which have been realized in the framework of this thesis. The main application for the multilevel converter presented in this work is the frontend power converter in locomotives. Instead of using a heavy low-frequency transformer to reduce the high-voltage from the catenary to a supportable voltage for the semiconductors, a multilevel converter concept is used. The multilevel converter is directly coupled to the catenary. There are many advantages compared to the existing solutions. In the same context, a novel solution of a multilevel converter has been developed for a locomotive usable on different power lines. The converter allows not only the operation on the high AC voltage power line (15kV), but also can be coupled to a medium-voltage DC power line (3kV). Three different configuration types of the locomotive converter have been developed and tested in a complex simulation environment. Besides the locomotive application, there are many more interesting applications for the symmetrical multilevel converter, e.g. in the fields of energy transmission (FACTS, static VAR compensators, electronic high-voltage transformers, etc.) and industrial drives. But certainly in the future with the availability of cheap semiconductors adapted to the needs of the multilevel converter, even more applications in lower power fields will be realized.

Multiphysical Characterization of Medium-Frequency Power Electronic Transformers

Irma Villar

Europe is currently making a great effort in order to improve the sustainability and reduce the environmental impact of its energy and transportation systems. A key role on these initiatives is played by efficient generation systems, like cogeneration, and clean or renewable energies, like wind or solar energy, as well as, by efficient and improved transportation technologies. In the evolution of these energy and transport systems, the development of Power Electronic Converters with greater functionality, higher reliability, higher efficiency, lower cost, and more sophisticated control will be essential. The main goal of future Power Electronic Converters will be to increase power density, reduce cost and improve reliability. This way, volume, weight and material reduction as well as reliability will gain the future market. A great contribution of these goals will be made by new high-power semiconductor devices, which permit the extension of the frequency range of power converters, and consequently the reduction of magnetic components. A good example of one of these systems are medium-frequency power conversion systems, also known as Power Electronic Transformers, which are able to convert electric power as convectional transformers but with increased features: volume and weight reduction, power transfer and quality control etc. The present work introduces a complete characterization of a medium-frequency power transformer, suitable for efficient Power Electronic conversion systems. The motivation of the present work stems out from the need to evaluate the constraints of conventional transformer characterization and design methodologies. The proposed expressions are able to successfully address the problematic related to non-sinusoidal waveforms, typical of medium-frequency power transformers. Moreover, a design methodology for the optimal design of medium-frequency power transformers is introduced. The characterization, as well as the design methodology, are verified by means of finite element simulations and measurement results.

EPFL2010

Modular DC/DC Converter for DC Distribution and Collection Networks

Stephan Kenzelmann

A major change in the electrical transmission and distribution system is taking place in Europe at the moment. The shift from a centralised energy production to a distributed generation profoundly changes the behaviour of the grid. Environmental or social issues associated with the construction of new power lines to relieve bottlenecks, together with aged equipment dating from the 1960s, pose some serious challenges to government, the research community and the economy. Concepts of reactive compensation, harmonic cancellation, voltage stability, power quality and bulky low-frequency transformers need to be redefined for power exchange and transmission in the future. Photovoltaics, wind turbines, fuel cells, storage systems and uninterruptible power supplies use many power electronic interface circuits, where DC intermediate levels already exist. Large photovoltaic- or wind- powered installations, which are connected to a cable network, are characterised by non-negligible distances due to their low power-by-surface density. On the side of the consumer, current trends show an increasing use of DC in end-user equipment. In such a context, the numerous advantages of power electronics and DC cables may sometimes out-weigh their higher cost. In the future, high-power semiconductor devices that allow higher switching frequencies of the converters may make it possible to down-size even more the passive components. This would significantly reduce raw material consumption and therefore cost, something that is crucial for the market to accept the technology. In the first part of this PhD thesis, the advantages of DC distribution in terms of transmission losses are illustrated with the help of three case studies. The second part and the main contribution of this thesis is the analysis of a promising candidate for a power electronic transformer, the key component of any DC based grid. It is a bidirectional isolated DC/DC converter based on modular multilevel converters, which are well suited for medium or even high voltage range. The motivation was to investigate a converter operation with important voltage elevation ratios, capable of adapting the voltage level between low, medium and high voltage. A medium-frequency isolation stage provides the possibility of downsizing the passive components. Two modulation methods, a multilevel and a two-level operation, were analysed and compared in terms of losses. The modular DC/DC converter is an attractive solution for the sensitive aspect of the short-circuit behaviour of classical DC links and power lines. The converter can also handle short circuits without the need for additional protection devices, such as circuit breakers. Given the many advantages of DC systems (reduced environmental impact, reduced space requirements, reduced raw material use, high power quality, power flow control, low transmission losses), this new technology must, at least, be considered when assessing the extension or the renovation of conventional AC grids.

EPFL2012

Convertisseurs multiniveaux asymétriques alimentés par transformateurs multi-secondaires basse-fréquence

Joseph Song Manguelle

In the last years, static power converters have become widely used in various applications. They can be found in domestic applications, railways, urban and ship transport, and even in several industrial systems. Some of these applications require a high or medium voltage power supply that is easily adjustable while providing good spectral performances. To overcome the maximum blocking voltage limits of the main power switches, multilevel techniques and other new power conversion topologies have been developed. They are series/parallel associations of existing power semiconductors, and allow generating an output voltage with many levels. The number of power semiconductors needed in these topologies increases as the number of levels increases. The power converter circuit becomes more complex and its reliability decreases. This thesis focuses on three-phase multilevel converters based on a series connection of single phase inverters (partial cells) in each phase. It's shown that, feeding partial cells with unequal DC-voltages (asymmetric feeding), increases the number of levels of the generated output voltage without any supplemental complexity to the existing topology. Voltage resolution is increased through interpolating the generated output voltage phasor (three phase voltage in α-β frame). This is achieved by seeking the non redundant switching states of the power switches. The resultant converter can generate a very high resolution voltage phasor up to the possible maximum resolution. This approach is generalized for any number of partial cells. Each partial cell is fed through a three-phase diode rectifier, fed itself through the windings of a multi-secondary low frequency power transformer. From analytical expressions in continuous and discontinuous current conix duction modes, it is shown that, from a supply network point of view, a symmetrical multilevel converter has a smaller total harmonic distortion than an asymmetrical multilevel converter with the same number of partial cells per phase. An asymmetrical multilevel converter is not more interesting than a classical three-phase converter, but its total harmonic distortion is compatible to the recommended IEEE std 519-1992. It is also shown that the advantages of an asymmetric multilevel converter from a load point of view (generation of a high resolution voltage phasor, possibility to choose the number of redundant switching states, reduction of the number of power semiconductors for the same voltage resolution, flexibility for the DC-voltage feeding choice) and the advantages of a symmetrical multilevel converter from a supply network point of view (smaller total harmonic distorsion) can be combined. Simulation results and the experimental test setup showed the reliability of the suggested approach.

EPFL2004