SPS POWER MANAGEMENT WORKSHOP SUMMARY J. Richard Williams Associate Dean of Engineering Georgia Institute of Technology Atlanta, Georgia 30332 The economic practicality of the SPS is greatly affected by the power distribution and management subsystem which directs the electric power output from the solar array modules to the microwave antenna. The efficiency of the power distribution and processing subsystem is also critical through its impact on total SPS weight. The technical feasibility of the SPS will depend in part on the technology readiness of techniques, components and equipment to reliably distribute, process and interrupt hundreds of megawatts of power at tens of thousands of kilovolts. The problems of heat dissipation and prevention of breakdowns due to corona discharge or arc-overs are much more severe in the space environment, because of the absence of the insulating and thermal transfer properties of air. The total weight of the satellite power system is projected to be between 35-50 million kg for a 5 GW system. This corresponds to a specific power density of around 5 or 6 kg/KW of power delivered to the antenna. However present aerospace power processing technology corresponds to a power density of 10-15 kg/KW. Thus the power processing alone, using present technology, weighs more than the total projected system. In addition, present technology will not perform the functions required. Therefore a major effort must be made in power processing technology development to make future power satellite systems technically feasible and economically viable. One major concern is the successful realization of high power kilovolt protection switches which are wired to protect the transmission tubes within microseconds of normally occurring arcs. Considerable work remains to be done on switchgear, power electronic devices, power transmission elements, and rotary joints. The geostationery orbit plasma environment presents special hazards to spacecraft designers because of the presence of a dense, high temperature plasma associated with the plasma sheet. Plasma sheet electrons may charge the satellite to high voltages of the order of 10 KV which might cause arcing, shock hazards, and changes in reflective or thermal control surfaces. An associated problem with spacecraft charging is that the ambient space plasma and photoelectrons may enter the solar cell array and form a parasitic load. Both laboratory and flight tests of specific solar cell arrays operating at high voltages will be necessary to determine the extent of this problem and assess corrective measures. The space plasma interaction can have a major impact on the power distribution system. The Marshall Space Flight Center contracted with Rice University for a small study of the space plasma effects on an early Rockwell International SPS design. This study recommended several design modifications and concluded that, with these modifications, SPS operation at GEO was probably possible. However, the study stipulated that laboratory and flight testing of specific solar cell arrays operating at high voltages are necessary for a definitive conclusion. A power management subsystem is required to provide monitoring of electrical power system parameters, the state and performance of the power distribution network, the operation of power processing components, energy storage and thermal control equipment. It will also take corrective action in case of out-of-tolerance or malfunctions and protect power system elements against destructive overloads and ensure safe access for maintenance operators. The working group on power distribution and management was directed by Arthur Schoenfeld of TRW.
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