SUPPORTING UTILITIES PROTECTION

Every Navy AIS facility is dependent upon supporting utilities, such as electric power and air conditioning, and may have to depend on communication circuits, water supplies, and elevators for its operation. Not all commands are self-sufficient; they contract some or all of these utilities from civil sources. In using these utilities, AIS technical managers should consider the probability of occurrence and the effects of breakdowns, sabotage, vandalism, fire, and flooding. These effects can then be related to the needs of the AIS facility as established by the risk analysis.

We have selected electrical power to illustrate support utility protection. Variations of a normal waveform in the electric power supply can affect the operation of AIS hardware. The AIS hardware rectifies the alternating current, filters, and voltage; regulates the resulting direct current; and applies it to the AIS circuitry. The filtering and regulation cannot be expected to eliminate voltage variations beyond a reasonable range. If line voltage is 90 percent or less of nominal for more than 4 milliseconds, or 120 percent or more of nominal for more than 16 milliseconds, excessive fluctuations can be expected in the dc voltage applied to the hardware circuitry. This power fluctuation causes unpredictable results on hardware, logic, and data transfer. These power line fluctuations, referred to as transients, are usually caused by inclement weather.

Internally generated transients depend on the configuration of power distribution inside the AIS facility. The effects of internal transients can be minimized by isolating the AIS hardware from other facility loads. Ideally, the computer area power distribution panels should be connected directly to the primary feeders and should not share step-down transformers with other high-load equipment.

The risk analysis should include a complete power transient and failure study. It should also carefully consider the projected growth in particularly sensitive applications (such as real-time or teleprocessing) in projecting future loss potential.

In some cases it may be economically feasible to connect the AIS facility to more than one utility feeder via a transfer switch. If one feeder fails, the facility's load may be transferred to the alternate feeder. This technique is of greater value if the two feeders connect to different power substations.

If the AIS facility is in a remote area, an uninterrupted power supply (UPS) is usually required as a backup power source. The UPS system can be manually or automatically controlled from prime power sources or from the AIS computer site. The typical UPS consists of a solid-state rectifier that keeps batteries charged and drives a solid-state inverter. The inverter synthesizes alternating current for the computer. A simplified block diagram is shown in figure 4-8.

Depending on the ampere-hour capacity of the battery (or batteries), the UPS can support its load for a maximum of 45 minutes without the prime power source. At the same time, it will filter out transients. To provide extra capacity to protect against a failure of the UPS, a static transfer switch can be inserted between the UPS and the computer, as shown in figure 4-9. The control circuitry for the static switch can sense an overcurrent condition and switch the load to the prime power source without causing noticeable transients.

Figure 4-8.-Simplified block diagram of an uninterruptedpower supply (UPS).

Figure 4-9.-UPS with transfer switch.

If the facility's current needs exceed its UPS capacity, it may be economically feasible to use multiple, independent UPS units, as shown in figure 4-10. Since each unit has its own disconnect switch, it can be switched offline if it fails.

Finally, if the risk analysis shows a major loss from power outages lasting 30 to 45 minutes or beyond, an onsite generator can be installed, as shown in figure 4-11. The prime mover may be a diesel motor or a turbine. When the external power fails, UPS takes over and the control unit starts the prime mover automatically. The prime mover brings the generator up to speed. At this point, the UPS switches over to the generator. Barring hardware failures, the system supports the connected load as long as there is fuel for the prime mover. Note that the generator must be large enough to support other essential loads, such as air conditioning or minimum lighting, as well as the UPS load.

Figure 4-10.-Multiple, independent UPS units.

Figure 4-11.-UPS with onsite generation.

When this configuration is used, maintain a close communication liaison with the power plant source to ensure the generator is coming up to normal speed for the switchover from UPS. The UPS system takes over automatically, and the change in power source may not be noticed in the AIS facility. However, when the UPS system changes over to the generator, it may require a manual power panel setting in the AIS facility by the AIS technical manager.