SPS Feasability Study SD76SA0239-2

The power distribution network for the solar array was investigated for two basic configurations. The selected configuration is shown in Figure 2.2-22 and the currents all flow in a parallel direction in each of the 21 bays. The second configuration investigated is shown in Figure 2.2-23 and the current flows in opposite directions in alternate bays. The output of the solar cell string is 22,644 vdc and the array bus is designed for an 8-percent power loss. The voltage drop in the array supply bus is 905 volts and also 905 in the return bus. The system voltage and voltage drops for the distribution network are shown in Figure 2.2-24. A parametric analysis of the conductor weight as a function of operating temperature and conductor material was performed. The use of the SPS structure as a conductor was considered; however, the resistivity of the structural aluminum alloy (2024T-3) is y = 5.7 x 10“^ ohm cm at 20°C which is two times greater than conductor grade aluminum. The required conductor cross-sectional area for eadh Vee trough at the bay closest to the antenna is shown in Figure 2.2-25. The total weight of the array conductor as a function of temperature and material is shown in Figure 2.2-26. The weight of the conductor when using the structure for carrying the current is seen to be twice as large as when pure aluminum conductors are used and is the result of the high resistivity of the structural aluminum. The total structure weight of the SPS is 2.64 x 106 kg and the total weight of the array bus at 100°C for structural type buses is seen to be approximately 7.5 x 106 kg in Figure 2.2-26. Therefore, there is not a sufficient amount of structural material to serve as the array buses. A small percentage of the structural meterial may be utilized as the distribution system, but in the study, an independent bus system was employed and pure aluminum conductors were used. Based on an independent array bus system, the weight of the conductors as a function of power loss (I^R losses in the conductors) and operating temperature was calculated and is shown in Figure 2.2-27 for one of the four modules. The curve shows the reference design point for the array conductors at an 8-percent power loss with each module having a conductor weight of approximately 0.89 x 10$ kg for a total array conductor weight of 3.58 x 10$ kg. The conductors from the array to the slip ring and from the slip ring to the antenna were sized for a 1-percent voltage drop in the supply bus and 1-percent drop in the return bus. The network diagram of Figure 2.2-24 shows these voltage drops as 215v (106v + 107v) in each conductor. The voltage at the antenna is 20,408 vdc, and the total weight of the conductors is 0.49 x 10$ kg. Although the voltage drops are approximately four times lower in the