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Auxiliary power supply (APS) plays a key role in ensuring the safe operation of the main circuit elements including gate drivers, sensors, controllers, etc. in medium voltage (MV) silicon carbide (SiC)-based converter systems. Such a converter requires APS to have high insulation capability, low common-mode coupling capacitance (Ccm ), and high-power density. Furthermore, considering the lifetime and simplicity of the auxiliary power supply system design in the MV converter, partial discharge (PD) free and multi-load driving ability are the additional two factors that need to be addressed in the design. However, today’s state-of-the-art products have either low power rating or bulky designs, which does not satisfy the demands. To improve the current designs, this thesis presents a 1 MHz isolated APS design using gallium nitride (GaN) devices with MV insulation reinforcement. By adopting LCCL-LC resonant topology, the proposed APS is able to supply multiple loads simultaneously and realize zero voltage switching (ZVS) at any load conditions. Since high reliability under faulty load conditions is also an important feature for APS in MV converter, the secondary side circuit of APS is designed as a regulated stage. To achieve MV insulation (> 20 kV) as well as low Ccm value (< 5 pF), a current-based transformer with a single turn structure using MV insulation wire is designed. Furthermore, by introducing different insulated materials and shielding structures, the APS is capable to achieve different partial discharge inception voltages (PDIV). In this thesis, the transformer design, resonant converter design, and insulation strategies will be detailly explained and verified by experiment results. Overall, this proposed APS is capable to supply multiple loads simultaneously with a maximum power of 120 W for the sending side and 20 W for each receiving side in a compact form factor. ZVS can be realized regardless of load conditions. Based on different insulation materials, two different receiving sides were built. Both of them can achieve a breakdown voltage of over 20 kV. The air-insulated solution can achieve a PDIV of 6 kV with Ccm of 1.2 pF. The silicone-insulated solution can achieve a PDIV of 17 kV with Ccm of 3.9 pF.

Abstract: Solid oxide fuel cells (SOFC) can be fuelled with external-reformed natural gas or directly with raw natural gas which usually contains significant amount of H2S to reduce the system complexity. The anode catalyst as well as the current collector employed in those environments must have excellent coke and sulfur resistances in addition to the general requirements. The coke-sulfur tolerant SOFC serving as a reactor could co-generate power and value-added chemical products, which promises to drastically minimize the energy waste and increase the efficiency. To address the coking issues in fuel cells fed by syngas or pretreated natural gas, Ni-P coated Ni foam is fabricated via electroless plating as the coke resistant current collector. The coated Ni foam with a surface composition of Ni3P + Ni (P > 6.5 wt%) exhibits excellent coking resistance in 24 h treatment in syngas at 750 oC and an essentially identical current collecting capability during the fuel cells application test. The developed Ni-Sx catalyst obtained through in-situ H2S passivation shows superior balance between high activity and good carbon resistance during 200 h test in dry syngas. The discovered H2 selective oxidation effect using this catalyst has been employed in the novel application of SOFC. To deal with the coking and sulfur deactivation issues in SOFC fuelled directly by sour natural gas with H2S concentration up to 0.5%, a Pd-CeO2/La0.3Sr0.7O3-δ composite anode catalyst has been studied and is applied to the fuel cells via infiltration method. Maximum power densities of 642 mW cm-2 and 274 mW cm-2 have been achieved in H2 and in 0.5% H2S + CH4 at 850 oC, respectively. The Pd particles catalyze the electrochemical conversion of the fuels and are proved to be thermodynamically stable, whereas CeO2 promotes their thermal stability on the electrolyte matrix. The electronically conductive titanate can also serve as the current collecting material in sour natural gas. In the application of the coke/sulfur tolerant fuel cells for the co-generation of electricity and CO from syngas, the developed proton-conduction SOFC is able to reach a maximum power density of 812 mW cm-2 at 750 oC and up to 23.4 % H2 is removed in the stream without oxidizing CO. Novel H2 electrocatalytic selective oxidation in syngas using O-2-conducting SOFC is also studied. The Ni-S anode catalyst shows up to 92% H2 selectivity through selective surface diffusion mechanism in 10% H2 + CO.