Design and implementation of a DCX converter for off-grid photovoltaic based electrolysis process

Abstract

T HE focus of this master thesis is about an LLC resonant converter, behaving as a DCX converter (DC-DC transformer), for off-grid photovoltaic-based electrolysis industrial process. Due to the similar operation with DC currents at low voltages, both photovoltaic (PV) and electrolysis (EL) systems can be directly interfaced by through DC-DC converters. Different industrial electrolysis processes such as copper electro-refining or the emerging green hydrogen production are suitable for DC applications. Among the DC-DC converters possibilities, the DCX-LLC converter appears to be a very attractive solution that will be studied in this master thesis. LLC resonant converter consists in an input inverter, a resonant tank, a high-frequency transformer, and an output rectifier. The resonant tank includes two inductors and one capacitor, naming this topology LLC resonant converter. Thanks to the resonant parameters, the DCX-LLC can operates with soft-switching capacity, resulting in high efficiency. Also, thanks to the use of wide-bandgap semiconductors technology, the converter can operate at high-switching frequency. Main advantages of the DCX-LLC converter are high efficiency, high power density, electrical isolation and a maximum performance when the switching frequency is equal or close to the resonant frequency. With the main objective of obtaining for maximum power efficiency, a comparison between of all combinations of full/half bridge (FB/HB) for input inverter and center tapped/full bridge (CT/FB) for output rectifier in terms of power losses and efficiency is performed. Moreover, single DCX module or input-series output-parallel configurations are also considered, giving a total of eight possible DC-DC topologies: FB-FB, FB-CT, HB-FB, HB-CT and the respective ISOP configuration. The analysis concludes that the one module half bridge-center tapped (HB-CT) converter is the most efficient topology for this purpose. Finally, a 1.1 kW half bridge-center tapped DCX-LLC prototype is built and tested to verify the theoretical analysis.