Modular Power Electronic Converters in the Power Range 1 to 10 kW

Research output: Book/ReportPh.D. thesis

Abstract

Thanks to CO2 emission reduction policies and increasing prices of fossil fuels a significant growth in field of sustainable energy sources (SES) is being observed during last decade. A government support and take-off projects in Europe and US shall ensure an increasing trend in future too. Some of SES based plants , like hydro-, geothermal-, biofuel-plants, use synchronous generators directly connected to the grid. But some other SES technologies, like fuel cell or photovoltaic, require a power electronic converter between the energy source and the load or the grid. Work presented in this thesis concentrates on dc-dc non-isolated converters suitable for high voltage gain applications, like uninterruptible power supply (UPS) and some of sustainable energy sources. A special attention is on reduction of power losses and efficiency improvements in non-isolated dc-dc step-up converters.

During literature study many different non-isolated dc-dc step-up topologies were found, however not all of them are desired for high voltage gain applications. It’s found that converters based on an inductor and a coupled-inductor principle (a boost and a center tapped boost converters) as well as converters derived from isolated converters (a non-isolated flyback-boost, a non-isolated push-pull-boost and a non-isolated two-inductor-boost converters) are good candidates for future investigation. Analysis and comparison of selected, most promising topologies indicated that a non-isolated push-pull-boost and a non-isolated two-inductor-boost converters are the best candidates for applications requiring a high voltage gain.

Design of a high efficiency converter requires a detailed knowledge and accurate
prediction of power losses. For this purpose average steady-state models of selected topologies and component loss models are developed and implemented in MATLAB. Converter models base on analysis of ideal waveforms and are built-up from set of equations describing values essential for power loss calculation, e.g. average or rms current values. These data are used by component models to calculate losses in particular components. It’s important that component models use parameters from datasheets in most cases. It enables performance comparison of different topologies as well as comparison of different components. The proposed modeling approach was verified using a basic boost converter breadboard. With small modification these models may be used for design purposes, like search for optimum output power and optimum switching frequency for given topology and given MOSFETs.

Using developed tools and models the converter breadboard was designed. The breadboard demonstrated a very high efficiency, comparable with present state-of-the-art isolated converters.

A modular converter concept and its influence on a fuel cell converter overall efficiency were investigated too. Based on simulation and measurement results it was demonstrated that the parallel modular converter used in a fuel cell application achieves a high efficiency over wide range of the output power. Moreover, the efficiency increases while the output power decreases, which is opposite to a solid converter solution.
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Thanks to CO2 emission reduction policies and increasing prices of fossil fuels a significant growth in field of sustainable energy sources (SES) is being observed during last decade. A government support and take-off projects in Europe and US shall ensure an increasing trend in future too. Some of SES based plants , like hydro-, geothermal-, biofuel-plants, use synchronous generators directly connected to the grid. But some other SES technologies, like fuel cell or photovoltaic, require a power electronic converter between the energy source and the load or the grid. Work presented in this thesis concentrates on dc-dc non-isolated converters suitable for high voltage gain applications, like uninterruptible power supply (UPS) and some of sustainable energy sources. A special attention is on reduction of power losses and efficiency improvements in non-isolated dc-dc step-up converters.

During literature study many different non-isolated dc-dc step-up topologies were found, however not all of them are desired for high voltage gain applications. It’s found that converters based on an inductor and a coupled-inductor principle (a boost and a center tapped boost converters) as well as converters derived from isolated converters (a non-isolated flyback-boost, a non-isolated push-pull-boost and a non-isolated two-inductor-boost converters) are good candidates for future investigation. Analysis and comparison of selected, most promising topologies indicated that a non-isolated push-pull-boost and a non-isolated two-inductor-boost converters are the best candidates for applications requiring a high voltage gain.

Design of a high efficiency converter requires a detailed knowledge and accurate
prediction of power losses. For this purpose average steady-state models of selected topologies and component loss models are developed and implemented in MATLAB. Converter models base on analysis of ideal waveforms and are built-up from set of equations describing values essential for power loss calculation, e.g. average or rms current values. These data are used by component models to calculate losses in particular components. It’s important that component models use parameters from datasheets in most cases. It enables performance comparison of different topologies as well as comparison of different components. The proposed modeling approach was verified using a basic boost converter breadboard. With small modification these models may be used for design purposes, like search for optimum output power and optimum switching frequency for given topology and given MOSFETs.

Using developed tools and models the converter breadboard was designed. The breadboard demonstrated a very high efficiency, comparable with present state-of-the-art isolated converters.

A modular converter concept and its influence on a fuel cell converter overall efficiency were investigated too. Based on simulation and measurement results it was demonstrated that the parallel modular converter used in a fuel cell application achieves a high efficiency over wide range of the output power. Moreover, the efficiency increases while the output power decreases, which is opposite to a solid converter solution.
Original languageEnglish
Place of PublicationAalborg
PublisherInstitut for Energiteknik, Aalborg Universitet
Number of pages202
StatePublished - 2009
Publication categoryResearch

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