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

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.

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.

EPFL2000

Etude formelle pour la synthèse de convertisseurs multiniveaux asymétriques

Jean-Sébastien Mariéthoz

The multilevel converters allow generating a voltage waveform whose magnitude and quality are higher than those of conventional converters. They are devoted to medium and high-voltage, high-switching-frequency applications. Among the series connected cell inverters, the symmetrical multilevel inverters are made of identical cells, which topology is generally the H-bridge inverter. This dissertation deals with asymmetrical multilevel converters, which combine high-voltage cells — made of low conduction loss switches — with low-voltage cells — made of low switching loss switches. The different cells play different roles. The goal is to design a converter that has both better energetic efficiency and higher resolution with the same number of cells. This dissertation develops a set of tools that allows synthesizing an asymmetrical multilevel converter. It develops the cell combination rules, which allow defining several cell association classes. The inverter step uniformity condition determines the finesse that can be reached. The optimized modulation condition allows to reduce the highvoltage cell switching frequency without changing the low-voltage cell switching frequency. This property is essential to derive benefit from combining switches of different characteristics. The power balance condition determine how to control the power sharing between the cells. These association classes are different for the single-phase converters and the three-phase converters — without neutral wire. They allow to select a configuration suitable for a given application. Furthermore, several ways that allow improving the inverter resolution are investigated. The modulation is essential to associate a high-quality signal (which will really be delivered to the load by means of the inverter levels) to the reference signal (which should ideally be applied to the load). The modulator evaluation methods are assessed. An approach—based on the modulated signal decomposition in elementary frames—is proposed. The main modulator drawbacks are highlighted. A modulator—which arrange the frames in order—allows to reduce the modulated signal distortion and number of transitions. The specific control methods that allow to minimize the inverter number of switchings are developed. The active power cell supply implies several conversion stages, which decrease the converter energetic efficiency. Some solutions that allow to reduce these losses are proposed. Finally, this dissertation attempts to determine, which are the more interesting solutions, depending on the application.

EPFL2005

A symmetrical 3 phase - 2 switch PFC-power supply for variable output voltage

Alfred Rufer

Power Factor Corrected rectifiers are a new tendency in development of power supplies. In the last few years, a lot of R&D activities have led to a new category of products for one phase applications. In this comtribuction, a new simplified topology is proposed for three-phase applications, using only two active switches, and able to generate symmetrical currents in the line. Power circuits are presented. Complete simulator results with closed loop operation are given, results of an experimental verification.

1995

Symmetrical multilevel converters with two quadrant DC-DC feeding

EPFL2000

Etude formelle pour la synthèse de convertisseurs multiniveaux asymétriques

Jean-Sébastien Mariéthoz

EPFL2005