1700 MW. With a nominal 20 GW being developed, the system transmission efficiency is 92 percent. Switching and other control equipment will operate at 95-99 percent. Technology Status Other than the previously mentioned unknowns regarding the effect of the plasma environment on system operation, the principal technology issues are the methods of power switching and power transfer across the rotating joint. The conductor bus system does not appear to present a major obstacle although the concept of using thin, flat conductors needs additional research and concept verification. The switch problem stems from the operation at extremely high power levels and the characteristic of the d.c„ current not passing through zero (as with alternating current) at the switching point. Ground switching systems utilize a variety of techniques to extinguish arcs during contact opening with the most popular techniques being oil cooling and magnetic blowout. The latter places a magnetic field across the superheated ionized path to lengthen the arc distance and displace it to a cooler region. Neither technique is particularly attractive for this application. Solid-state technology offers the best potential, but currently available switches are limited to a few hundred amps at a few thousand volts. However, developmental programs are underway, for d.c. ground system applications, which have switched currents of 2000 amps at voltage levels above 100 KV. The basis for the switching technique is to momentarily shunt the current to a resistive bank while the interrupter switch is activated. This and other similar techniques should provide the basic technology around which a device for specific SPS application can be tailored. The joint design employs conventional slip rings for power transfer. If the current density across the gap is maintained at less than 300 amps per square inch, the slip ring approach should be (electrically) satisfactory. Mechanically and thermally the joint presents a severe design problem. It is proposed that the complete joint system stand alone as a technology item and that new power transfer technologies such as liquid metals be addressed as components of the joint technology program. Finally, the analysis of electromagnetic field effects can be properly categorized as a technology item. Although the basic field mechanisms are understood, the rigorous analysis of the effects will require extensive math modeling and simulation. Of interest are effects which cause a redistribution of current within the conductor from either its own field or proximity fields of other conductors. Nonuniform current densities will cause additional resistive losses and localized heating. Mechanical stress and force interactions between closely spaced conductors need to be analyzed in some detail to inusre that the distribution system
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