Fig. 4-26. Electrical Resistivity of Metals For a lead length of 1.5 cm the thickness will be 0.02 cm. The fundamental satellite power unit will consist of a string of 161 converters connected in series (Ref. 6). In line with Boeing's design of a 20-m square panel this unit will form three horizontal rows approximately 18-m in width, as shown in Figure 4-27, leaving a 1-m wide strip on either side for the tapered busbars connecting these converter strings. Twenty-one such units, positioned vertically as shown, would cover the 20-m square panel. Fig. 4-27 SPS Electrical Pane! Design Maximum output voltage, anticipated for the year 1995, would not exceed 150 V which is sufficiently low to prevent electrical insulation problems at the maximum busbar temperature of 1000K. Furthermore, operation in this electrical configuration will limit the current through any converter to 1290 A. For the converter design shown in Figure 4-21, the cross-sectional dimensions of the leads connecting converters are 0.7 cm thick x 15 cm wide. Solid multiple-strand copper leads were compared to sodium filled 0.1-cm thick stainless steel leads with respect to their resistance-to-weight ratio. These ratios were 6.1 (10'9) £2/gm and 2.9 (10'^)J2/gm, respectively. Clearly copper is the better lead material. The busbar resistance per converter was calculated by summing the individual contributions of the electrodes and leads. By flashing copper on the molybdenum emitters and then fusion brazing on the copper leads, contact resistive losses can be minimized and, therefore, have been neglected in this analysis. Based on the foregoing resistance calculation, the voltage loss per converter with 1290 A current is 0.13 V which represents a 14 percent loss in power for the expected 0.93 V output per converter in 1995. In summary, a panel will deliver the following output: Assuming one converter fails, the maximum power loss is 5 percent of the panel output or, for a 4 GW module, 0.004 percent of its output. If, however, converter failure occurs from an internal electrical short, only the output of a single converter is lost to the panel.
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