tion transformers. These tests will be specified for
connected for three-phase service. Nonflammable
large transformers only (50 MVA and larger) or
liquid or epoxy insulation is preferred. However,
where reduced insulation is provided on trans-
high tire point liquid or oil insulation may be
former windings rated 115 kV and above. Optional
provided if they are approved by the HOST/REQ
NEMA impulse tests include either a nominal test
b. Three-phase pad-mount transformers, delta-
sequence consisting of a reduced full wave, two
chopped waves, and one full wave or a combined
wye connected, are preferred in lieu of single-
test sequence which interposes two or more front-
of-wave tests between the reduced full wave and
by a single three-phase transformer and where the
the two chopped wave tests on the nominal test
upon failure of any winding and the load can be
interrupted during such repair period.
8-6. Amorphous Metal-Core Transformers.
c. Four single-phase units may be provided
Amorphous metal-core transformers will be used
where such spare capacity is required for 100
only where it can be demonstrated that the life-
percent spare transformer capacity on site. Multi-
cycle cost of the installation is less than or equal
ple installation of like substations for two or more
to standard transformers. It has been reported by
blocks of load at the same general installation
some studies that amorphous metal-core trans-
shall employ three-phase transformers in parallel
formers can save energy by reducing no-load losses
or through secondary bus tie.
d. If pole mounting is required, radial brackets
by 60 to 70 percent. However, since the percent
impedance is decreased, the short-circuit current
or cluster mounting is preferred in lieu of cros-
will increase proportionally. Therefore, the electri-
sarm or platform mounting for sizes 3 to 75 kVA.
e. Avoid locating transformers adjacent to build-
cal distribution equipment associated with amor-
ing front entrances in landscaped areas. Prefer
phous metal-core transformers must be increased
in size to withstand the increased short-circuit
pad-mounting at the rear or side area where
current. The resulting energy conservation may be
f. Polychlorinated Byphenyls (PCB) contami-
offset by the increased withstand ratings.
nated transformers shall be disposed of in accord-
8-7. Transformers at Air Force Installations.
ance with the latest requirements of the Environ-
a. Single phase, self-cooled, with provisions to
mental Protection Agency (EPA) as they are
increase capacity if externally cooled, delta-wye if
removed from service.