Fig. 4-90. Moving Large D.C. Currents 4.11.3 AC Versus DC Trade Figure 4-91 shows how the total efficiencies and specific masses of the two candidates were compared. Note that the generators and transformers require cooling. This is because their inefficiencies dictate that a certain amount of heat energy will be produced (if a generator is 98% efficient it must dissipate 2% of the energy which enters it). Maximum generator and transformer operating temperatures are forecasted to be 400K to 41 OK (260°F to 278°F). Radiant heat dissipation at such temperatures is only approximately l.SkW/m^ (0.12 kW/fP). Consequently the surface area of the generators and transformers are not sufficient to dissipate their waste heat, and additional radiation area is required. The specific mass of these radiators (mass per power input to generator or transformers) is included in Figure 4-91. Note the penalty on structural mass associated with d.c. distribution, and that there is also an efficiency (I^R) loss. A.C. distribution was selected on the basis of its slightly lower overall mass and slightly higher efficiency. Note, however, that if a higher voltage transmitter (e.g., 40,000 v.d.c.) were involved that the trade would probably switch to d.c. as the preferred approach. Fig. 4-91. A.C. Versus D.C. Distribution
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