Conducting structure was utilized for the lateral feed buses shown in Figure 7-15. A limited analysis to verify compatibility with structural requirements showed that the surface area provided by the baseline structure was adequate if thickness was increased. Thickness had to be increased by steps as modules wei^ progressively connected along the length of the beam. This increased the mass of the structure by a ratio of 8 to 13 for the particular beam analyzed. Overall, the combined structure/power conductor weight was reduced by a ratio of 17 to 13 as a result of eliminating separate requirements for power conduction and structural support. 7.1. 4. 2 HIGH VOLTAGE POWER TRANSMISSION While the total mass of the SPS can be reduced by transmitting at increased voltages (Fig. 7-18), the possible mass must be weighed against increased high voltage solar array/transmitter tube problems and the complexity of power conditioners. The relationship between mass reduction of power conductors and technology risk can be analyzed by considering the following power system options: 1. Operate the solar array, power line, and transmitters at 20 kV: 13.6 x 10G kg conductor mass. 2. Increase the operating voltage of the transmitters, power line, and solar array to 40 kV: 6. 8 x 106 kg conductor mass. 3. Increase the operating voltage of the solar array and power line to 400 kV, and provide power conditioners with a specific weight of 0.456 kg/kW at the transmitter tubes: 8 x 106 kg conductor and conditioning mass. 4. Operate the solar array at a low voltage, the power line at 400 kV, and provide power conditioning at each end terminal: 16 x 106 kg conductor and conditioning mass. For all options except option 4, the solar array was operated at a relatively high voltage when compared to present satellite solar arrays. This creates a considerable technology risk to the solar array development program because of the lack of analytical and test data on high voltage effects. Studies and limited vacuum tests have been conducted on small solar arrays biased at 16 kV, but the test results were inconclusive. There has been no flight test verification, and the highest voltage being considered for an advanced technology development program is the 400 V SEPS array. A solar array voltage of 20 kV was established for the baseline SPS design because this was the voltage selected for the transmitting tubes (see subsection 7. 3.4.1), and it was assumed that power regulators could be avoided.
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