Vernon Shields Urban Systems Project Office IV.D.2 GRID INTERFACE Introduction The grid interface system collects electrical energy in the form of direct current. Several billion rectenna subelements, transmit this power to DC-AC inverters and necessary safeguards and controls are provided for operation with the utility grid. In some instances it may be economically attractive to use high voltage DC power transmission. In this case power for AC loads would be provided by inverters located near the load center. A rectenna subelement consists of a half-wave dipole, diode rectifiers and filter. These subelements are positioned normal to the incoming microwave beam and are interconnected serially in strips. These strips and a reflecting screen are supported in a rigid attitude by a structure fixed to the earth. The overall rectenna shape is elliptical if located at any point on the earth other than the equator. For the location chosen in this study (40° N latitude) the rectenna has a major axis of 14.4 kilometers and a minor axis of 10 kilometers. The length of series electrical interconnections of subelements is varied so that 1000 VDC is produced from each, independent of actual position within the rectenna. The power output (ampacity) of each will vary, however, with location. If the 1000 VDC is directly used as inverter input, many inverters would be incorporated to minimize conductor losses and cost. This configuration is the first option. In the second option, 250 KV DC is produced by connecting 250 of the 1000 volt circuits in series. This allows remote (centralized) inverter location because of greatly reduced power losses in high voltage transmission and fewer inverters for the same power output. The high voltage does create new insulation requirements and structural modification. The impact of both appraoches is further discussed in the following paragraphs. Grid Interface Requirements In providing power to the grid, the interface system must control output voltage levels, phase, frequency and current demands. Present grid voltages range from 66 KV to 500 KV. Presently, inverters in the range of 1-50 MW provide output thermal voltages ranging from 13.8 KV to 69 KV at 60 Hz with an average efficiency of 96 percent. These inverters may be phase-locked to each other and the grid by using reactive power from the grid. The switching, regulation and control of rectenna power to the grid will be maintained at control center(s) located near the rectenna site. An illustrative representation of this interface is given in Figure IV-D-2-1. The use of high voltage DC power transmission would involve different grid interface requirements from those discussed above. If a national DC grid system were established in the future, the compatibility of large power increments as provided by SPS would be enhanced. The method of producing the
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