Inversion efficiency was optimistically projected as 95 percent. The inverter outputs would then be transformed to the transmission voltage and routed to the transmission lines within the primary transformation/switch yard. It is noted that, if high voltage de transmission is selected, dc/dc conversion at the SPS site and dc/ac inversion within the utility system would be required. The two conversion steps will make it difficult for the de approach to surpass the efficiency of the ac system. The power conversion subsystem receives relatively low voltage de power from the de switch yard of Figure 8-1, which centralizes and controls the de power received from the rectenna/power grid. The output voltage may be in the order of several hundred volts; therefore, large buses and switch gear will be required. This yard also provides essential backup protection for the rectenna and fault isolation/power rerouting capabilities for the receiving site and ground system. The efficiency of 99. 8 percent indicated for the de switch yard also includes the power required to energize the receiving site and to support the control centers. 8.4 SYSTEM SAFETY 8. 4.1 SAFETY ZONE CONTROL SUBSYSTEM An important point concerning microwave power transmission is the effect that microwave exposure may have on man and other life forms in the vicinity of the rectenna. Diverse opinions exist as to what, if any, biologically adverse effects are prevalent as a result of high power density microwave transmission. In the United States the current microwave exposure standard for man is 10 mW/cm2. In other countries, (for example, the U.S.S.R.) the exposure level is duration dependent and is much lower at 1 mW/cm2 for 6 s. Over an 8 h exposure period, the standard drops to 0.01 mW/cm2. Based on the present United States standard, no safety zone around the rectenna would be required. If the United States standard was lowered to 0.1 mW/cm2, a safety zone representing an ellipse with a minor axis of 16. 2 km and a major axis of 21.1 km would be required around the rectenna. Since microwave exposure standards are somewhat loose in the United States, further studies should be made and more precise standards adopted with these results factored into the overall design of the SPS.
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