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 MW_e per thruster. For example, for a high-power ion thruster operating at a specific impulse (I_sp) of 10,000 lb_f-s/lb_m, the PPU specific mass (including radiators for waste-heat rejection) is 1.76 kg/kW_e 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 I_sp 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/kW_e for krypton-ion thrusters operating at an I_sp of 5,000, 7,500, 10,000, 12,500, and 15,000 lb_f-s/lb_m, 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.