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

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).

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

Cross connected multilevel voltage source inverter topologies for medium voltage applications

Toufann Chaudhuri

Multilevel voltage source inverters where first introduced in the early 1980s. Since then, they have been continuously developed, offering a wide new research area in power electronics. The popularity of multilevel solutions come from the advantages that they offer: improved output quality, voltage sharing in high voltage applications, increased power density or reduction of filtering costs. Two completely new and innovative cross-connected topological families for advanced multilevel voltage source inverters are introduced in this thesis. The motivation for this work stems out from the need to generate multiple output levels while keeping the reliability as high as possible. The offered solutions are able to address the problematic, but of course they do not come without a price: A higher control complexity and more semiconductor blocking voltage capability are necessary in the design of such advanced converters. The Cross Connected Intermediate Level (CCIL) Voltage Source Inverter is the first of the two new topologies presented here. It is built as a cascade of stages using capacitors which are connected to each other by means of cross connected cell structures. The CCIL can be used in several configurations, like redundant or non-redundant switching state configurations for instance. A graphical model based on the physical properties of the inverter is proposed and an original fuzzy logic controller is designed for the balancing of the capacitor voltages and modulation of the inverter. The control algorithm is implemented and verified in simulations. The results are used to benchmark the topology against standard solutions and the conclusions are used to define what applications could benefit from such a converter structure. The Common Cross Connected Stage (CCCS) Voltage Source Inverter is the second original contribution of this work in terms of topology. It is built using the cross connected stage and its capacitor in a common configuration for the three phases of the inverter. Such a design allows to use only one stage per three phases. Because of the intrinsic three phased properties of this topology, a model based on space phasor representation is introduced. With the help of this model, a novel space phasor modulation strategy is derived and proposed. It allows to generate the three phased output voltages while using the available redundancies for balancing of the capacitor voltages. The resulting algorithm is first implemented and tested in simulation, and in a second step a test setup is built and the modulator is coded in VHDL. The simulation and experimental results obtained validate the topology and control concepts. A benchmarking of the CCCS solution is also done to understand what are the benefits and drawbacks of this solution. Analysis and comparison of the new topologies allow to evaluate in an objective way the contributions brought by this work. It is found that the newly proposed solutions cover an area of multilevel inverters where not so many solutions were available prior to this work: Generation of multiple output levels with reduced number of passive and active components (thus increasing the reliability). The drawback is a higher blocking voltage requirement. Conclusions and case study are proposed to help assess the expected performances and choose the most suitable solutions for given applications.

EPFL2008

Cross connected multilevel voltage source inverter topologies for medium voltage applications

Toufann Chaudhuri

EPFL2008

Multiphysical Characterization of Medium-Frequency Power Electronic Transformers

Irma Villar

EPFL2010