Modular Multilevel GaN Based Ultra-High Power Density Electric Power Conversion and Transmission on the Lunar Surface

NASA’s Watts on the Moon Challenge is seeking solutions to transfer at least 1.065 kW power from a 120 V dc source to a 24-32 V dc load over a 3-km distance under the same environmental conditions as the Lunar surface (i.e., 77 K temperature and 1 mTorr pressure). The selected solution from the author’s team proposed utilizing two modular multilevel Gallium Nitride (GaN) based isolated dc-dc converters to connect the 120 V dc source with the 24-32 V dc load bank via 1.5 kV rated dc transmission lines. The modular multilevel converters feature frequency multiplication, high step-down voltage ratio and low device voltage stress. In the converters, GaN gate injection transistor (GaN GIT) and GaN High-Electron-Mobility Transistor (GaN HEMT) devices are chosen as switching devices, due to the merits of lower power loss, radiation hardness and ability to work under cryogenic and vacuum conditions. In addition, LiFePO4 battery based energy storage with a power condition system is added in parallel with the load to provide uninterrupted power. Active heating units and passive multi-layer insulation are designed to manage the proper operation of the battery under cryogenic and vacuum conditions. Furthermore, the hardware design for the power converters has been validated at 1.5 kV/1 kW condition. The operation of submodules in the developed converter has been demonstrated at 77 K temperature and less than 10^-6 Torr pressure. The efficiency of the designed converter can achieve 97.42%. In total, the proposed power conversion and transmission system has a total system mass of 103.2 kg, which is significantly lower than the 150 kg limit specified by NASA's requirement.