ELECTRIC POWER PROCESSING, DISTRIBUTION, MANAGEMENT AND ENERGY STORAGE Robert J. Giudici NASA/Marshall Space Flight Center Power distribution subsystems are required for three elements of the SPS program: (1) orbiting satellite, (2) ground rectenna, and (3) Electric Orbiting Transfer Vehicle (EOTV). Power distribution subsystems receive electrical power from the energy conversion subsystem and provide the power busses, rotary power transfer devices, switchgear, power processing, energy storage, and power management required to deliver regulated power to the load. The grounding, electromagnetic interference control, high voltage plasma interactions, electric thruster interactions, and spacecraft charging of the SPS and the EOTV are also included as part of the power distribution subsystem design. The satellite power distribution subsystem (PDS) is essential but incurs weight and power loss penalties, representing cost, to the SPS. The design approach must be: (1) to define feasible PDS concepts that can accommodate unprecedented power and voltage levels, (2) to perform system level trade/optimization studies, (3) to select a PDS that minimizes the SPS penalties, and then (4) to develop the high performance components needed to implement the subsystem. This paper will consider the preliminary SPS concepts developed by Rockwell and Boeing from the standpoints of reducing subsystem requirements and of minimizing SPS penalties. Performance improvements and projections will be addressed and technology developments will be suggested. The significance of system level studies to the selection of a PDS design can be reviewed with the aid of Figure 1. The solar photovoltaic/sandwich concept, wherein solar cells are connected back to back with solid state direct current (d-c) to radio frequency (r-f) converters, requires no switchgear or power processors. The solar cells are merely connected via short interconnect busses to the solid state amplifiers and the result is an ideal negligible PDS. None of the other system concepts shown in Figure 1 achieve this simplicity because the power source is necessarily located at a distance of several kilometers from the antenna, and for solar concepts, the power source must be pointed at the sun while the antenna maintains earth pointing. Therefore, power must be transferred across a rotary joint and must be transmitted at high voltage to maintain reasonable conductor size and power loss. It also follows that the PDS will require switchgear for the collection and management of power received from multiple sources and for redistribution to individual r-f converters. The photovoltaic/klystron system design, indicated by heavy lines in Figure 1, is the present reference SPS concept, although it requires a complex PDS. The klystron r-f converter requires bulk power at multiple voltages ranging from 40 kV to 8kV and a small percentage of power at voltages as low as 20 volts. All of the bulk power could be supplied from solar array sections operating at the proper voltage but system studies conducted by Rockwell and Boeing have employed power processors for 20 to 80% of the power due to the heavy conductors required at lower voltages and the added circuit complexity of managing a multivoltage system.
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