MIL-HDBK-1004/5
than a standby power system. Provide diesel-engine generator capacity of at
least 80 percent of the frequency conversion plant's firm capacity. Frequency
conversion plant firm capacity is the sum of the rated capacities of all
frequency conversion assemblies, with the largest unit not operating. Where
required by the activity, provide 100-percent diesel-engine generator
capacity. Provide diesel-engine generator sets with both manual and automatic
transfer modes which start automatically on loss of normal power. Where more
than one diesel-engine generator set is provided, provide units capable of
being automatically paralleled. Provide switches to permit testing of
diesel-engine generators without assuming load. The most economical
diesel-engine generator voltage is generally the input voltage to the
frequency conversion assembly.
3.3.2.3
Transformer. Because the frequency conversion assemblies are
low-voltage input devices, transformers are necessary to stepdown primary
power. No facility should depend on only one transformer, since this can
result in a complete shutdown of the 400-Hz system. Require duality of
transformers. Each transformer's rating shall be not less than 80 percent of
the frequency conversion plant's firm capacity. When transformers of the
outdoor substation type (see MIL-HDBK-1004/2) are installed adjacent to the
central plant as shown on Figure 3, they can be used to supply the central
plant's 60-Hz low-voltage switchboard (see MIL-HDBK-1004/3, Switchgear and
Relaying) as shown on Figure 4.
3.3.3
System 400-Hertz Conversion Capacity. Firm power is power which is
available even under emergency conditions. Determine the firm frequency
conversion capacity of the central plant by the loads served and a 15- to
20-percent additional capacity for future loads. Provide one extra unit for
standby (i.e., emergency use). If the requirement for the standby unit and
for future capacity necessitates more units than for the present load with
maintenance backup, incremental construction may be desirable. Such planning
is acceptable as long as future space and capacity provisions for ancillary
devices are covered fully in the first-design stage.
3.3.4
Frequency Conversion Assemblies. Ratings as shown in Table 3
provide satisfactory operation. When frequency conversion assembly
performance is combined with a properly designed distribution and utilization
system, it provides 400-Hz power to aircraft loads. This meets the
requirements of MIL-STD-704. Figure 5 shows a typical frequency conversion
assembly.
Motor Generator Units. Use standard units manufactured to support
3.3.4.1
both military and commercial airports.
a) Output voltage. The most preferable output voltage is that of
the distribution system or 4,160 volts; however, this equipment is not yet
commercially available. Normally, specify a 575-volt motor generator output,
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