MIL-HDBK-1004/2A
5.5.4.2 Over 600 Volts. For load circuits over 600 V, use criteria in MIL-
HDBK-1004/3 to select from the following:
a) either oilless, metal-clad, medium-voltage circuit breakers,
preferably of the vacuum circuit breaker type; or
b) grounded, metal-tank type, frame-mounted circuit breakers, with
open disconnecting switches. Circuit breakers may be oil, gas, vacuum, or
compressed air type. Oil-insulated type shall be installed only in outdoor
locations and only for voltages exceeding 35 kV.
5.5.5
Substation Structures. Depending on the chosen design, the
substation structure may consist of the following:
a) flat concrete base without elevated structures, applicable for
underground supply and load circuits served by metal-clad switchgear;
b)
steel or aluminum superstructures mounted on concrete piers; or
c)
a combination of either of the aforementioned systems.
5.5.6
Transformers
5.5.6.1 Selection. Consider the types of insulation that are suitable for
the site and the system voltages (see Table 6). Select oil-insulated for
outdoor applications, except where fire safety considerations require the use
of less-flammable, liquid-insulated insulation which is not usually available
for voltages above 34.5 kilovolts. For indoor installations, see the
paragraph on transformer insulations of this section.
5.5.6.2 Cooling. Forced cooled ratings are only available on larger size
transformers; check their availability before specifying. Dependent upon
standard ratings and overload capacity calculations, specify the cooling
method from one of the following:
a)
self-cooled, type OA,
b)
one-stage forced-cooled, type FA,
c)
two-stage forced-cooled, type OA/FA,
d)
one-stage double-forced-cooled; forced-oil, forced-air, type FOA,
or
e)
triple-rated, types OA/FA/FOA/ or OA/FA/FA.
5.5.6.3 Transformer Capacity. Choose the
transformer rating by considering
the maximum load to be carried for normal
and contingency conditions and the
possibility of accepting overloading with
accelerated loss of system life.
Transformers should be sized for 10 to 25
percent more than calculated loads
to minimize future growth costs.
5.5.6.4 Fire Protection. For minimum safe distances of transformers to
buildings and other fire protection criteria, refer to MIL-HDBK-1008. Also
refer to IEEE 979, Guide for Substation Fire Protection.
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