Characterisation and design of power electronic converters for superconducting applications

There is a large and growing interest in superconductor applications, as they can offer optimised solutions in different areas, such as all-electric aircraft (superconducting motors), and fusion magnets (superconducting magnet accelerators) and levitation (superconducting Maglev levitation). For these applications, auxiliary circuits (converters, inverters) that utilise power electronics are required. As all superconducting systems require cooling cryostats, it would be better from the engineering point of view to have the entire electrical network inside the cryostat. This, in turn, requires power electronics to operate at lower temperatures. Literature has indicated that characteristics of semiconductor devices at cryogenic temperatures (below 123 K) change; this, in turn, would reflect on a) device selection and b) circuit topologies. Thus, this thesis aims to fill the research gaps in the performance of cryogenic power electronics suitable for superconducting applications mainly a) all-electric aircraft and b) superconducting magnet systems. For the gap in device selection for superconducting applications, different devices including diodes, MOSFETs and IGBTs were tested. The devices are first characterised at room temperature (300 K) and then tested at Liquid Nitrogen (LN2) temperature (77 K). The characterisation tests included 1) static tests, 2) dynamic tests and 3) paralleling tests. For the dynamic tests, this thesis has proposed a bespoke current sensor that can operate at cryogenic temperature with high bandwidth, as commercial current sensors are not suitable for immersion in cryogenic temperature zones. Based on these tests, circuits were built to test cryogenic power electronics suitable for the two superconducting applications mentioned earlier. For the all-electric aircraft, the thesis aimed to fill in two gaps; 1) implementing a cryogenic rectifier integrated with a partial superconducting machine and 2) using different permutations of phase-leg utilising super-junction MOSFETs at cryogenic temperature. At first, the rectifier was immersed in liquid nitrogen and was connected to a superconducting machine where it was shown to have lower losses at that temperature. For the super junction MOSFET, the thesis has tested for the first time the effect of cryogenic temperature on different permutations of diode deactivation circuits in a phase-leg structure. Based on the test results a phase-leg dc/ac using super-junction MOSFET was built and assessed at room and cryogenic temperature to determine its performance. As for the superconducting magnet applications, the thesis has developed a tailored cryogenic dc/dc converter that utilises a nano-crystalline core step up the current at lower temperatures. The converter was able to supply current up to 500 A. In summary, this thesis 1) identifies which devices with a rating of 1200 V are suitable for cryogenic applications 2) develops a bespoke current sensor, 3) tests a rectifier circuit implemented with the superconducting machine, 4) investigates the usage of a cryogenic converter and inverter for superconducting applications.There is a large and growing interest in superconductor applications, as they can offer optimised solutions in different areas, such as all-electric aircraft (superconducting motors), and fusion magnets (superconducting magnet accelerators) and levitation (superconducting Maglev levitation). For these applications, auxiliary circuits (converters, inverters) that utilise power electronics are required. As all superconducting systems require cooling cryostats, it would be better from the engineering point of view to have the entire electrical network inside the cryostat. This, in turn, requires power electronics to operate at lower temperatures. Literature has indicated that characteristics of semiconductor devices at cryogenic temperatures (below 123 K) change; this, in turn, would reflect on a) device selection and b) circuit topologies. Thus, this thesis aims to fill the research gaps in the performance of cryogenic power electronics suitable for superconducting applications mainly a) all-electric aircraft and b) superconducting magnet systems. For the gap in device selection for superconducting applications, different devices including diodes, MOSFETs and IGBTs were tested. The devices are first characterised at room temperature (300 K) and then tested at Liquid Nitrogen (LN2) temperature (77 K). The characterisation tests included 1) static tests, 2) dynamic tests and 3) paralleling tests. For the dynamic tests, this thesis has proposed a bespoke current sensor that can operate at cryogenic temperature with high bandwidth, as commercial current sensors are not suitable for immersion in cryogenic temperature zones. Based on these tests, circuits were built to test cryogenic power electronics suitable for the two superconducting applications mentioned earlier. For the all-electric aircraft, the thesis aimed to fill in two gaps; 1) implementing a cryogenic rectifier integrated with a partial superconducting machine and 2) using different permutations of phase-leg utilising super-junction MOSFETs at cryogenic temperature. At first, the rectifier was immersed in liquid nitrogen and was connected to a superconducting machine where it was shown to have lower losses at that temperature. For the super junction MOSFET, the thesis has tested for the first time the effect of cryogenic temperature on different permutations of diode deactivation circuits in a phase-leg structure. Based on the test results a phase-leg dc/ac using super-junction MOSFET was built and assessed at room and cryogenic temperature to determine its performance. As for the superconducting magnet applications, the thesis has developed a tailored cryogenic dc/dc converter that utilises a nano-crystalline core step up the current at lower temperatures. The converter was able to supply current up to 500 A. In summary, this thesis 1) identifies which devices with a rating of 1200 V are suitable for cryogenic applications 2) develops a bespoke current sensor, 3) tests a rectifier circuit implemented with the superconducting machine, 4) investigates the usage of a cryogenic converter and inverter for superconducting applications