Power Processing Units for High Power Solar Electric Propulsion 929084
This paper summarizes an evaluation of high-power power processing units (PPUs) for multimegawatt (MMW) solar electric propulsion (SEP) vehicles using advanced ion thrusters. Significant economies of scale are possible for PPUs used to supply power to ion thrusters operating at 0.1 to 1.5 MWe per thruster. For example, for a high-power ion thruster operating at a specific impulse (Isp) of 10,000 lbf-s/lbm, the PPU specific mass (including radiators for waste-heat rejection) is 1.76 kg/kWe and the electrical efficiency is 0.955 for an SEP vehicle employing photovoltaic solar cells with a DC output of 500 V. It was found that the PPU specific mass was strongly sensitive to variations in the ion thruster's power per thruster and moderately sensitive to variations in the thruster's screen voltage due to varying the Isp of the thruster. For example, the PPU specific mass, for a system consisting of 8 operating and one spare PPU combined with 8 operating and 2 spare thrusters, is 2.63, 2.12, 1.76, 1.69, and 1.67 kg/kWe for krypton-ion thrusters operating at an Isp of 5,000, 7,500, 10,000, 12,500, and 15,000 lbf-s/lbm, respectively.
Each PPU consists of a DC-to-DC converter to increase the voltage from the 500 V DC of the photovoltaic power system to the 5 to 13 kV DC required by the ion thrusters. The converters consist of a DC-to-AC inverter using MCTs and a three-phase wye transformer. MCTs and filters are used for the AC-to-DC conversion to the low (100 V range) power required by the thrusters, and silicon rectifiers and filters are used for the high (kV range) power required by the thrusters. The PPUs are then combined with contactors to provide PPU input and output isolation, and to allow switching between operating and spare (redundant) PPUs and thrusters, as needed.