A Power Electronic Building Block - Based Converter for Electric Propulsion System in Spacecrafts

High-voltage and high-power electric propulsion (EP) systems for large spacecrafts have gained much attention as they offer high exhaust speeds, low propellant consumption, extended operational duration, and increased flexibility. In such EPs a heat-source that consists of a nuclear reactor, a Brayton cycle and a high-speed turbine provide torque and speed for an electric generator that converts mechanical power on its shaft to an electric power for the spacecraft’s electric thruster. To control the output power of the generator, as well as manage the power for the electric thruster power electronics converters in different forms and shapes, and for different conversion such as ac-dc, dc-dc, and dc-ac, and ac-ac are employed. Neutral point clamped (NPC) converters are a suitable candidate for high-voltage and high-power conversion where they have been proposed for dc-ac conversion. However, their application for dc-dc conversion has not been investigated. This paper proposes the NPC converter topology for dc-dc conversion for electric propulsion in large spacecrafts. The proposed topology is in the form of a power electronic building block (PEBB) that can be configured to be used in a modular structure in different parts of the spacecraft. Two operational scenarios for the converter are discussed with different duty cycles that result in a 3-level unipolar dc voltage. It is demonstrated that both scenarios work equally well and can be utilized to synthesize a desired reference voltage. The switching scheme is constructed, and mathematical models are derived. The proposed dc-dc topology is compared with H-bridge common in spacecraft, and it is shown that the topology in this paper offers less ripple factor at lower frequencies, lower voltage, and current stress for the switches, and is amenable to higher voltages due to multilevel architecture. Furthermore, a conceptual space modulation diagram (C-SMD) is proposed that is used to synthesize a dc voltage in a space vector modulation domain.