Communications and control interfaces are required to coordinate operations and to be responsive to utility interface demands. Considering the magnitude of the system and component numbers, it is anticipated that a highly automated data management subsystem will be needed for the utility interfaces. Similar requirements may be expected between SPS receiving sites when the system is expanded. Typically, such communications are via dedicated land lines; however, in later cases RF links may be necessaiy. 8.3 POWER CONDITIONING, DISTRIBUTION, AND UTILITY INTERFACES The capacity of an SPS receiving site, rated for 5 to 10 GW, would be approximately two to three times that of the larger generating plants currently being constructed. This should be an appropriate size for the era beyond 1990. In some regions a site may be able to supply more than one utility system, while in areas with large load concentration, several sites that are linked together by a large transmission network may be needed. Such links would have requirements similar to the utility interfaces and would not affect the concepts previously described (Fig. 8-1). Practically all the power transmitted and distributed in the United States today is 60 Hz ac. Although transmission standards may change, it is not likely that distribution systems will be changed significantly before the SPS era, because of standards established with consumers and equipment suppliers. Thus, it may be expected that the bulk of SPS power will ultimately be converted and distributed to consumers as 60 Hz, ac, three phase/one phase power at 13.2 kV. Therefore, it was assumed in this study that 60 Hz, ac, three phase power would be transmitted from the SPS to the utility interfaces. However, the option to transmit high voltage (approximately 1 MV) de power is retained, and technical performance/economic trade studies of ac versus de transmission are to be performed. Typically, three to five separate transmission lines operating between 300 and 400 kV would be used for a 10 GW plant. Assuming that the utility interfaces will be within 150 mi of the receiving site, a transmission efficiency of 98 percent was projected. The projected efficiencies of transmission and of the major power handling subsystems at the receiving site are shown in Figure 8-1, Since ac transmission was selected, the power conversion and regulation subsystem would consist of multiple, static, semiconductor dc-to-ac inverters that convert the primary power to 60 Hz, ac, three phase power and regulate the ac voltage.
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