Contribution to the development of cryo-converters for power electronics

Overview & Synopsis

Rail transport is one of the most energy-efficient modes of transportation and one of the lowest in terms of CO₂ emissions. In urban areas, where civil engineering works are difficult to carry out, reinforcing the electric traction grid is a challenge. Superconducting traction substations, which have the potential to reduce losses and offer a smaller footprint, may offer a promising solution. Within the framework of the SuperRail project, this thesis explores the feasibility of a cryogenic diode rectifier for such substations. Four research directions were pursued. First, a repeatable experimental protocol for the measurement of the I-V characteristic of power diodes was established, allowing measurements at 293 K and 77 K up to 4800 A while minimizing self-heating. More than 60 commercial diodes were characterized, and the results were shared in an open database. Second, studies were conducted to better understand certain peculiar I-V characteristics, assess measurement repeatability and evaluate the influence of temperature between 77 K and 293 K. Third, the diode rectifier currently used in 1500 V DC traction substations was analyzed, with a focus on identifying the modifications necessary for cryogenic operation. Finally, three cryogenic rectifiers prototypes were developed and tested iteratively, with progressively increasing power and complexity. The final prototype, a 100 kW three-phase diode rectifier with RC snubbers and cooled by liquid nitrogen cooling plates, achieved efficiencies greater than 99% at cryogenic temperatures. The main contributions of this thesis include: (i) the establishment of a repeatable experimental protocol for the I-V characterization of kA-level diodes at 77 K, (ii) the study of diode behaviors at cryogenic temperatures, and (iii) the development and experimental characterization of a 100 kW cryogenic rectifier, representative of a traction substation, demonstrating the feasibility of cryogenic power electronics for railway applications.