Internally generated transients depend on theconfiguration of power distribution inside the AISfacility. The effects of internal transients can beminimized by isolating the AIS hardware from otherfacility loads. Ideally, the computer area powerdistribution panels should be connected directly to theprimary feeders and should not share step-downtransformers with other high-load equipment.The risk analysis should include a complete powertransient and failure study. It should also carefullyconsider the projected growth in particularly sensitiveapplications (such as real-time or teleprocessing) inprojecting future loss potential.In some cases it may be economically feasible toconnect the AIS facility to more than one utility feedervia a transfer switch. If one feeder fails, the facility’sload may be transferred to the alternate feeder. Thistechnique is of greater value if the two feeders connectto different power substations.If the AIS facility is in a remote area, anuninterrupted power supply (UPS) is usually requiredas a backup power source. The UPS system can bemanually or automatically controlled from prime powersources or from the AIS computer site. The typical UPSconsists of a solid-state rectifier that keeps batteriescharged and drives a solid-state inverter. The invertersynthesizes alternating current for the computer. Asimplified block diagram is shown in figure 4-8.Depending on the ampere-hour capacity of thebattery (or batteries), the UPS can support its load for amaximum of 45 minutes without the prime powersource. At the same time, it will filter out transients. Toprovide extra capacity to protect against a failure of theUPS, a static transfer switch can be inserted betweenthe UPS and the computer, as shown in figure 4-9. Thecontrol circuitry for the static switch can sense anovercurrent condition and switch the load to the primepower source without causing noticeable transients.Figure 4-8.—Simplified block diagram of an uninterruptedpower supply (UPS).Figure 4-9.—UPS with transfer switch.4-22If the facility’s current needs exceed its UPScapacity, it may be economically feasible to usemultiple, independent UPS units, as shown in figure4-10. Since each unit has its own disconnect switch, itcan be switched offline if it fails.Finally, if the risk analysis shows a major loss frompower outages lasting 30 to 45 minutes or beyond, anonsite generator can be installed, as shown in figure4-11. The prime mover may be a diesel motor or aturbine. When the external power fails, UPS takes overand the control unit starts the prime moverautomatically. The prime mover brings the generatorup to speed. At this point, the UPS switches over to thegenerator. Barring hardware failures, the systemsupports the connected load as long as there is fuel forthe prime mover. Note that the generator must be largeenough to support other essential loads, such as airconditioning or minimum lighting, as well as the UPSload.Figure 4-10.—Multiple, independent UPS units.
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