Electric power conversion

Résumé

In all fields of electrical engineering, power conversion is the process of converting electric energy from one form to another. A power converter is an electrical or electro-mechanical device for converting electrical energy. A power converter can convert alternating current (AC) into direct current (DC) and vice versa; change the voltage or frequency of the current or do some combination of these. The power converter can be as simple as a transformer or it can be a far more complex system, such as a resonant converter. The term can also refer to a class of electrical machinery that is used to convert one frequency of alternating current into another. Power conversion systems often incorporate redundancy and voltage regulation. Power converters are classified based on the type of power conversion they do. One way of classifying power conversion systems is according to whether the input and output are alternating current or direct current. Finally, the task of all power converters is

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https://en.wikipedia.org/wiki/Electric_power_conversion

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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

Chargement

Analysis of Empirical Core Loss Evaluation Methods For Non-Sinusoidally Fed Medium Frequency Power Transformers

Alfred Rufer, Irma Villar, Frédéric Zurkinden

Nowadays medium voltage conversion systems, like traction and energy distribution systems, aim to increase power density. Volume, weight and material reduction are gaining the market today. Due to the development of new high power semiconductor devices a new working frequency range is conceived, where magnetic component reduction is feasible. The core element of these new conversion systems is the medium frequency transformer. For an optimized design of the conversion system, the correct determination of transformer core losses is essential. Sinusoidal approaches, accurate enough for line frequency transformers, do not cope with the non-sinusoidal waveforms of future medium voltage conversion systems. In the past few years several empirical methods derived from the original Steinmetz equation have been introduced to determine magnetic core losses for non-sinusoidal waveforms. This paper proposes extended expressions for core loss determination in case of rectangular three level voltage waveforms, typical of high power converters. These expressions are compared in simulation and with experimental results obtained in the same conditions using a reduced scale dc-ac conversion system prototype that includes a 61.6 kVA/2 kHz transformer. It is shown that some of these techniques allow a considerable loss estimation accuracy (error below 12% for any ratio).

2008

Chargement

Nouveaux mécanismes de commutation pour des structures dédiées aux convertisseurs de puissance de haute efficacité et interrupteurs du futur

Daniel Siemaszko

Nowadays, the scarcity of cheap energy sources requires a particular effort in optimizing the performance of all conversions. The electric power vector is particulary expected to grow in importance, in conjunction with the development of renewable energy sources. Research in the field of power electronics considers several aspects, including conversion topologies, command strategy as well as the structure and performance of power switches. This work focusses on the power component and the study of dedicated power converting structures. One goal is the use of active switches to replace elements with spontaneous behaviors presenting dissipative phenomena, in order to attempt to reduce switching and conduction losses. Indeed, several works introduce bidirectional monolithic switches adopting new structures, and predict high efficiency components in the near future. On the other hand, the emergence of components without spontaneous properties leads to a redefinition of switching mechanisms in power converters in order to enable the replacement of diodes by active devices. One aspect of this work is the investigation of a close control system including the detection of zero-crossings based on the measurement of both voltage and current in bidirectional switches. A voltage-current representation for the exploitation of experimental results is provided in order to allow for comparison with theoretical behaviors. Several material and internal structure types of components are compared in terms of switching performances. The concept of automatic switching was recently introduced to describe mechanisms based on the detection of non-compliant situations in the switching cell, namely the short-circuit of a voltage source and the opening of a current source. The proposal of applying a new control strategy allows to compensate for the absence of spontaneous extinction. Where sources of a classic switching cell are in a situation of energy exchange or isolation, a third state appears in which source values (converter input/output currents and voltages) are subject to fast changes. The use of passive elements becomes compulsory in order to allow a limitation in the theoretical infinite variations of source values. An intelligent close control system allowing a bidirectional component to switch its state on the basis of measurements of both current and voltage was designed and implemented. An automatic diode with synthesized spontaneous switching functions was studied and experimented for the validation of the proposed self-switching mechanisms model including passive circuits. The use of bidirectional switches allows the extension of conventional converter topologies to new modes of operation. This work presents the modification of a current converter topology traditionally implemented with thyristors, to allow for compensation of reactive power. The spontaneous extinction of thyristors is replaced by a controlled extinction which becomes independent of the polarity of source values. However, the forced opening of the input line inductors current path requires to be absorbed by an input filter whose study is proposed in this work.

EPFL2009

Chargement

EPFL2004

Nouveaux mécanismes de commutation pour des structures dédiées aux convertisseurs de puissance de haute efficacité et interrupteurs du futur

Daniel Siemaszko

EPFL2009