Multiport Energy Gateway

The global problem of energy supply and the reduction of the carbon footprint can only be solved by a massive replacement of fossil fuels or atomic energy by renewable resources. As the latter are mostly weather-dependent, they are uncontrollable and unpredictable in the long term. Their relative volatility can be mitigated by integrating a large number of these resources, by their geographically decentralized distribution, and by the use of storage elements. These ones being mostly DC, their integration would be eased considerably by the use of DC grids which allow the synchronization of resources to be avoided and eliminate line impedance effects, thus allowing energy to be transfered over longer distances. In the past, voltage step-up and step-down transformations were only possible with the help of low-frequency (50-60Hz) and bulky transformers, which are still used nowadays in most AC grids. But the progress of the last two decades in the field of power semiconductors now allows the realization of high-performance DC-DC converters with increasingly high voltages. A promising concept is the solid-state-transformer (SST), which uses a multi-modular structure integrating multiple medium-frequency transformers and offers an efficient and compact solution for voltage adaptation. While several researches on this particular subject have been done or are in progress for bi-port converters, the multiport version with the objective of integrating storage elements on additional ports has been studied only very few. Therefore, this thesis proposes a new converter structure combining both the SST principle for medium voltage and the multiport resonant converter principle to limit switching losses and achieve higher switching frequencies, necessary for an increased power density. After a brief overview of existing storage technologies and multiport converters topologies, a description of the proposed structure and its operating principle are presented. It results in a relatively simple operation based on operating modes dependent on the direction of the energy flow and allowing the use of one or more storage elements independently. Then, the mathematical models required to describe the converter are developed for each of these operating modes and make it possible to highlight the operating areas benefiting from soft switching and having a higher efficiency. They are also used to define sizing criteria that are applied to the design of a prototype converter for the medium-voltage grid and which parameters are used as the basis for the simulation. Finally, a description of the converter dynamics is presented using simplified models from which the control structure is derived and the controller parameters are calculated. In order to illustrate each of the elements developed in this work, a low-voltage prototype is designed to support certain points with experimental results.

Dujic, Drazen
Lausanne, EPFL

 Record created 2019-09-23, last modified 2019-10-04

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