for the very much larger power output of the SPS it should be technically possible to switch the direction of the power transmission from one rectenna to another within a few seconds. In practice, constraints on doing this are likely to arise within the ground segment of the system, both in the maximum rate of change of power flow than can be tolerated with an electricity grid, and in the maximum rate of change of load that is experienced by utilities. The most rapid changes in power flow that occur in a typical grid result from the sudden loss of the output of a power station. Hence, provided that the arrangement was economically attractive, an SPS might be used to provide periods of standby power supply to one or more utility grids, covering both loss of output and rapid increases in load. If the grid to which an SPS was currently delivering power contained sufficient storage and/or steady capacity to replace the SPS output at very short notice, the SPS could at the same time provide backup power to another grid, enabling several grids to share their standby and reserve capacity with several SPS delivering power to a number of different grids, this flexibility could substitute in part for the use of long distance transmission cables between the grids.
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