The magnitude of the short circuit currents occurring in the event of a short at the output of the switch gear (438, 000 amperes for a 5 GW ground output MPTS) is of particular concern, primarily because it is a dc circuit. In circuit breaker applications the actual interruption of current occurs at close to current zero’s. In ac circuits, the current zero occurs naturally. For a dc circuit therefore, the zero current condition must be artificially induced. One technique (Reference 5-3), indicated schematically by Figure 5-18, uses a triggered resonant circuit to provide an alternating bucking current to the dc. Units using this technique have been constructed for voltages of 20, 000 volts and interrupting currents of 20, 000 amperes (Reference 5-4). Units of this approximate size are suitable for the crowbar unit, but are for a much lower current than needed for the switch gear unit. To reduce the current level at the switch gear, it is recommended that the solar array be sectored into a minimum of eight units and that the interface use a minimum of eight sets of independent conductors and slip rings. The effect on the switch gear design would be to limit the individual maximum short circuit current to approximately 55, 000 amperes, a value that should be attainable in a switch gear development program. 5.7 POWER DISTRIBUTION COST AND WEIGHT A cost and weight estimate for a 9 GW power input which approximates a 5 GW ground power output system requirement, is given in Figures 5-19 and 5-20. Cost estimates assume: a. 85 percent learning curve b. Labor costs equal to materials cost c. 1975 dollars The auxiliary power portion is relatively expensive and represents the equipment needed to provide the lower level voltages for operating the electronic equipment. To convert to a parametric cost basis, the overall cost can be assumed to vary as the square root of the total power. The cost of the conductors will vary directly as the power, but because the conductors represent a small proportion of the total cost, it is well within the accuracy of the estimate to vary this cost as the square root of the total power.
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